HMI & Generating Grounding
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Published : 7th April 2005
I've come across a question that I can't find a satisfactory answer to, and am too embarrassed to ask elsewhere... An HMI won't strike without a proper ground. Is this because a proper ground is part of the functioning of the process? Or is a proper ground required as a safety feature due to the high voltage required for striking? In fine, is this a necessary element of the process or a safety precaution? Any explanation regarding similarities/differences with magnetic/electronic ballasts is of appreciated.
Please, no primers on how a ballast works, since these are readily available ... such as at
(apologies to the owner of this document since I haven't received permission for the link) even if I don't fully understand them...yet!
As always, thanks to all for shared expertise and experience.
>An HMI won't strike without a proper ground. Is this because a proper >ground is part of the functioning of the process? Or is a proper ground >required as a safety feature due to the high voltage required for striking?
The ground is there for safety only...it is by definition an equipment grounding conductor and as such is not ever supposed to carry any current - it is only there to protect us from malfunctions. That doesn't mean that you won't frequently read a potential (voltage) between the equipment chassis (which is electrically connected via the head cable to the lamp housing) and your power system ground, but once you hook up your ground, that voltage will go away . The ground conductor is not supposed to carry any current, however.
Note to readers of the NEC or other commercial wiring books etc. What we call a neutral conductor is sometimes called a ground conductor in commercial and house wiring...this is different from the equipment grounding conductor.
Stephen Olsen writes :
>I've come across a question that I can't find a satisfactory answer to, >and am too embarrassed to ask elsewhere... An HMI won't strike >without a proper ground.
Hmm, that's not an immediately obvious scenario. The only parallel I can draw is the requirement of a standard fluorescent tube to have a ground plane in it's vicinity to assist it's striking, since it allows the discharge to "wipe" along the tube due to the vicinity of the outer conductive plane.
Is this HMI lamp being used without a conventional fixture or is there some scenario where a fixture is being used without an earth connection?
What's the power source for the light? Even if it doesn't have a
connection directly to the general mass of ground, it's own earth system should be extended out to the exposed metal parts of a fixture.
If the lamp is being used in an unusual situation and no other fix is possible, then it might be helpful to add an "antenna" of wire in the vicinity of the lamp itself to act as a virtual ground plane. This could be connected to one terminal of the lamp using a high value resistor for safety.
HMIs and MSRs will work without a ground. We do it all the time in our workshop for test purposes BUT...
Ballasts and lamp head igniters contain filters which are connected to the ground terminal hence also to the casing of the lamp head. These filters are to prevent the high voltage spikes (up to 75kV) generated in the igniter from damaging anything in the ballast. This is particularly important for electronic ballasts where spikes can punch through the (expensive) power transistors if they have nowhere else to go.
Also, consider what happens if the high voltage during ignition arcs to the casing of the lamp head during ignition. This is common if the HT cable is old and gets too near a panel, if it is damp, if the lamp is reluctant to hot restrike etc. The high voltage appears on anything connected to the earthing system including the guy who has his hand on the lighting stand.
There is also the question of the interference generated in an electronic ballast. No earth can mean no filtering of this either so upsetting the sound man.
So I recommend not to run discharge equipment without a ground.
I recommend not running anything fed with mains power and having a metal case...... without an earth...... unless you want to die!!!!!!!
... continuing on...
>An HMI won't strike without a proper ground. Is this because a proper >ground is part of the functioning of the process?
The term "proper ground" is a misnomer because we in the movie business rarely ground our equipment to code. Technically all equipment including generators should be grounded to a sunken 8' copper rod. In LA we rarely ground our generators, while in NY it is common practice.
I often argue with NY best boys about whether or not grounding a generator is actually necessary as the neutral is bonded to the generator chassis.
What are people's thoughts.
Erik "path of least resistance" Messerschmidt
DP â€¢ Gaffer
>The term "proper ground" is a misnomer because we in the movie >business rarely ground our equipment to code. Technically all >equipment including generators should be grounded to a sunken 8' >copper rod.
No, the code does not require a reference ground on all portable generators, although it does require it in some cases. Some of this depends on whether the generator is connected to a permanently installed electrical system in a building (which includes trailers... they have to have a ground rod and bonded neutral).
Some of it depends on whether the generator is truck-mounted or portable. Portable generators are considered "through frame" grounded and can float legally without a ground rod.
The complete discussion can be found in the NEC in section 250-6. It is possible that the provisions of section 530 can also apply to remote locations the way they apply to studios, and this depends on the local inspector.
Oh, yes, and all the rules change above 600V.... but that's another section of the NEC altogether.
I can see both sides of the debate. However, we always ground our plants here in Saskatchewan. The only time I ever floated the ground was when we worked on a damp salt flat at a potash mine. The electrician we consulted told us that as long as the electricity stayed within the system, we'd be ok. Grounding in that scenario, had there been a ground fault, would have simply electrified the salt around the ground rod.
There are several reasons for grounding:
1/. Some of the plants we use won't start unless the unit is grounded.
2/. It generates some good PR for the department and the show. Safety
3/. It makes sense to do it and it doesn't take a lot of extra time to make
it work. I find that our equipment seems to have fewer gremlins if we take the time to ground properly.
A point that I find to be as important as grounding is bonding the ground of the plant to the ground of the house/location/building that we are working in.
We have had some troubles with HD cameras becoming "tingly" and with sound carts hearing ground loops if we neglect this step.
I am not trying to sound glib but if there ever is an accident with a subsequent OHS investigation, I owe it to my employer to be able to prove that we took all safety measures possible.
I have had OHS inspectors look at our distribution and approve. One even commented that if the construction industry took as much time and care regarding electricity that we do, they'd likely have fewer claims.
Oh yes, I now check every generator to make sure that they do indeed have a bonded neutral and ground. I found one last year that didn't and depending on load, it developed potential between the neutral and ground of between 5 to 65v. Enough to cause you to stop and think...We bonded the white and green at the camlock outputs and the problem went away.
For what it's worth...
Erik Messerschmidt writes :
>I often argue with NY best boys about whether or not grounding a >generator is actually necessary as the neutral is bonded to the >generator chassis. What are people's thoughts.
Imagine a generator sitting in the back of a truck or on it's own rubber wheeled trailer. A cable gets damaged and is in a damp location, so it makes good electrical connection to the general mass of ground. You walk up to the generator truck and put your hand on the chassis or door handle. The current now has a route back from ground to the generator chassis....You!
Here in the UK where rain is pretty common, it is absolutely essential to provide grounding from the generators chassis to terra firma. This could be a simple earth spike or in some cases, just a metal plate that the truck is parked on. Absolutely anything that makes a better connection between the ground and truck than a human.
Fortunately earth leakage circuit breakers ELCB's or GFI's reduce the amount of fault current required to trip out. (So forgive them the inconvenience of the odd nuisance trip.)
>A cable gets damaged and is in a damp location, so it makes good >electrical connection to the general mass of ground. You walk up to the >generator truck and put your hand on the chassis or door handle.
Actually, the resistance of the ground is so high that it would never allow current to travel any significant distance. Otherwise any time a cable that had a slice in it touched the ground we would all get shocked!
The only way this situation might really occur would be if the damaged cable was sitting in the same puddle of water that the electrician was standing in, and they were fairly close together. Water has greater resistance than copper, so the current can only travel a limited distance at a hazardous level. Also, a person wouldn't even have to touch the generator to get shocked. All they would have to do is become a conductor by being grounded themselves, and then touching the puddle of water. A grounded generator is irrelevant in this situation.
One of the biggest reasons why you shouldn't ground a generator is that unless you have a ground resistance meter, you have no idea if the ground is properly set. Without this, there's no way to tell if the rod is long enough for the soil type you're driving it in, and you have no idea if the water pipe or street sign or whatever that you're trying to clamp onto actually goes deep enough into the ground, or if it turns into PVC pipe a couple of feet down. The resistance of an improperly set ground can cause the path of least resistance to no longer be the generators alternator, but the electrician who is touching the faulty light.
Remember, a proper ground is really only a back up neutral. Even if a light has a ground fault, a person touching it will still get shocked (unless the circuit is on a GFI). The key is to make the electrician as high a resistance as possible when compared to the ground so the electricity will flow mainly to it's proper place. The only time you ground a generator is if you are mixing power sources, such as multiple generators, or house power and genny power. Then you make sure the ground is shared between the two sources. But this is really only needed if the distro or lights from the two sources come within 25 feet or so (I'm not sure what the actual NEC requirement is) of each other. And by the way, don't ever count on the circuit breaker tripping. I've seen several ground faults occur where the breaker never switched off!
>Imagine a generator... on it's own rubber wheeled trailer. A cable gets >damaged and is in a damp location, so it makes good electrical >connection to the general mass of ground.
Yes, I've had this happen when I was an electrician. I was leaning on a Cinemobile (wonder where they are today?) and felt a tingle and sure enough there was voltage from the chassis to ground.
An electrical distribution system should have two or three hot legs, depending on the phasing of the system, a neutral and a ground. Some devices use the two hot legs (208-240v in the US) with no neutral so the case ground is necessary for safety. I used to ground the genni after the above incident although an 8 foot long grounding rod may be impractical for a temporary setup. Is that what is called for in LA where inspections often happen on set?
The possibility also exists to utilize an existing ground in a building or to use other grounds such as fire hydrants.
>Yes, I've had this happen when I was an electrician. I was leaning on a >Cinemobile (wonder where they are today?) and felt a tingle and sure >enough there was voltage from the chassis to ground.
I reread the original post and realized I had misinterpreted it. My situation was that the genni was leaking voltage thru the frame of the vehicle it was mounted on. The genni frame was electrified and I was the conduit to the ground.
Jim Sofranko writes :
>I was leaning on a Cinemobile (wonder where they are today?)
Stu Segall (Silk Stalkings, Renegade) in San Diego bought a few of these rigs and uses them today. I don't know how many were made, but they're holding together.
I believe we discussed this a few years back and I sent some people the letter from LA electrical code - (check archives? ) according to LA, it is ILLEGAL to ground a generator being used for motion picture use - in fact you must make sure that the safety chains are not touching the ground.
The only exception to this is if you are powering equipment that is hanging on a metal structure or grid where you are concurrently powering your equipment from "house power" in which case the gennie should be grounded also.
A C stand arm pounded into the sand is not a ground.
Craig Kief writes :
>Also, a person wouldn't even have to touch the generator to get >shocked. All they would have to do is become a conductor by being >grounded themselves, and then touching the puddle of water.
Very true. Also the chassis of the generator truck or trailer can become energized through leakage from the alternator. The leakage might not be enough to trip the breaker or even a GFCI since it can be sensed by the GFCI as a part of the load, and not a ground fault, but it can give the unwary a nasty surprise.
>One of the biggest reasons why you shouldn't ground a generator is >that unless you have a ground resistance meter, you have no idea if the >ground is properly set.
This is why the NEC goes into great detail specifying ground rod length and soil type for proper grounding. In most cases it is more than what is necessary, but I've never seen an electrical inspector with a GRM.
>The resistance of an improperly set ground can cause the path of least >resistance to no longer be the generators alternator, but the electrician >who is touching the faulty light.
Electricians aren't the only ones who are susceptible to electric shock.
It's been my painful experience that miswired or misrepaired lights and extensions are the greatest on set electrical hazard.
IA 600 DP
Should I tear the plastic case off my double insulated power drill ands run an earth wire back to the mains plug, as the manufacturer has failed to connect an earth.
Comparing what the supply authority does or what happens on a building site is not applicable to a mobile generator we use in film.
Our mains supply is 240volt... Commercial supply 415 volts three phase... 240 volts to neutral. A modern mobile film generator has no connection between the alternator and the outside casing... the outside casing is isolated from ground on rubber tyres. Any electrical connections are at least double insulated. Establishing an effective earth can be impossible .. dry sand for example. The safest you can be is with your system earth isolated back to the generator.
I would suggest this is the reason the Electrician in Canada said "Don't earth the generator" the ground was both wet and conductive. To get an electric shock you must get across active and neutral.
Don't earth the generator... bond the generator to grids and bond different supply sources.. back to the generator.
The earthing of metal components of Lights, ballasts and electrical equipment is vitally important to prevent electric shock or more importantly getting hung up (you become the conductor) when using mains supplies.
(yes I have grabbed the active and stuck my hand in the swimming pool)
Andrew Gordon wrote :
>We have had some troubles with HD cameras becoming "tingly" and >with sound carts hearing ground loops if we neglect this step.
I just finished a pilot and we had this exact thing happen only our tingly happened to be Fifty volts between the shield of the SDI line and ground. Yes you read right, that's 5 0 Volts.
We fixed it by running a "Drain" wire from the SDI Shield back to the Genny Via the Electrics lunchbox at the DIT station.
Eric Fletcher SOC
Steadicam/"A" Camera Operator
Los Angeles, CA USA
With earth potential voltages, voltage between a piece of equipment and earth relative to the generating source, you are getting into a phenomenon common with multi phase supplies. These voltages aren't hazardous and although they will give you a unpleasant boot there is no current. A television station where I worked had a 100 volts between technical earth and mains earth.
Some attention needs to be paid that associated equipment running on mains volts(say...your video camera and the monitor that is joined to the camera by the video cable with its copper earth braid) is plugged into the same supply, not one into the gene. and the other into the house and also that they are on the same phase.
The generator loading (balance of amps drawn per phase) if it is multi phase and a large capacity needs to be kept relatively even so that excessive out of phase neutral current isn't drawn.
When I started in lighting you pretty much needed an electrical background of some sort. Live hook-ups were common, calculating loadings and voltage drops second nature. Now I find some of the lighting technicians electrical knowledge either non-existent or basic. I say to them "If you get across 240 volts it will sit you on your backside, get across three phase it will blow bits off you" I walked on to a tank set where a 240 Volt underwater tungsten par plugged into a wall socket had exploded whilst submergedâ€¦the techs had been in the water at the time, hanging onto a nicely earthed scaffold tower.
It didn't take out the circuit breaker ..and one of them had eventually got out of the water and unplugged it.. they were all offering up suggestions as to why they weren't electrocuted.
I suggested that if you are really- really dumb your body resistance if much higher than normal people.
Graham (no electrical flash burns) Rutherford
Craig Kief writes :
>Water has greater resistance than copper, so the current can only travel >a limited distance at a hazardous level.
Pure clean water has a very high resistance, but if there are impurities in it (like dirt and mud) then it conducts very well.
Here's a description of a rather nasty fatal incident involving a fairground operative.
A ride was being stripped down on a wet night and the generator was running to provide light for the pull-down. As they finished work, one of the operatives was sent to turn off the generator, but after a while it was still running and he hadn't returned. A fellow worker went to find what was wrong and found the guy still with his hand on the handle of the generator truck. Although they managed to resuscitate him at that point, he died on the way to hospital. Incidentally he was wearing rubber boots at the time, but they simply don't guarantee insulation from ground when wet.
In general here in the UK it is standard practice to ground generators to their chassis and to terra firma via an earth rod or plate. This is purely to provide a consistent fault path and ensure that no potential can exist between adjacent ground planes or metalwork. It doesn't replace the need for an earth connection to bond all connected equipment's metalwork to the same potential.
Yes earth electrodes are a pain in the ass to install, but they do serve a very important function. As far as an ungrounded chassis generator goes, this scenario has it's own drawbacks. It only takes a leakage to chassis of 30mA to cause electro-paralysis of the diaphragm if the current passes from chassis to ground through an operative. This level of current will clamp their hand to metal and cause asphyxiation through the inability to breathe. That's why most earth leakage detection devices are designed to trip at about 30mA.
To put things into perspective, 30mA can light a 3.5W lamp on 110V or 7W lamp on 240V. That's a small night-light.
Yes Clive what you say is all true....
We are also 240 Volts and if I was using some dodgy old gene.. I would drive earth stakes and bond it to every metal object in sight... but because I know the gene I am using and know it is regularly tested for insulation resistanceâ€¦I prefer to M.E.N. (mains earth -neutral)the generator supply, the Gen. set is fitted with a R.C.D(residual Current device) as well..
Lamp stands etc are metal and get an earth reference from the lamp head then touch the ground ... if you happen to pull any current active to earth or neutral to earth you trip the R.C.D. The R.C.D's are regularly tested and tagged. As you know a R.C.D. will not trip from current draw active to neutral.
All our power distribution boards must be E.L.B.(earth leakage breaker) protected as well.
Cheapest life insurance you can get.
I was told that in the U.K. you don't run three phase power(Three actives, Neutral and earth) onto Film Stages....... but run single phase for the safety aspect
Is this true?
A 30ma encounter is God's way of telling you your day rate isn't so high after all...even though the producer thinks it is... my question is, what about the Honda 30's or the little putt-putts that always seem to have a PA in charge of... Should they not be grounded? Is there NEC code for them?
I trust my local owner/operator to keep things safe with the big generator, it's the little generators without an experienced operator that scare me.
Graham Rutherford writes :
>I was told that in the U.K. you don't run three phase power(Three >actives, Neutral and earth) onto Film Stages....but run single phase for >the safety aspect
There are many location generators that have a single phase high current output, but this isn't particularly standard.
The use of three phases has no significant safety issues over single phase, since it would require a very odd fault situation to allow 415V to appear between adjacent metalwork. (Two detached earth bonds and two live to case faults simultaneously.)
Probably the biggest risk associated with a three phase supply in the film industry is the chance that a neutral could become detached or get damaged and cause large scale lamp damage due to the highly variable loads on different phases. The use of non-interlocked Cam-lock style
single-pole connectors can make the detached neutral much easier to achieve.
>I suggested that if you are really - really dumb your body resistance if >much higher than normal people.
I think I hurt myself laughing.
back in LA
Dan LaBorde writes :
>what about the Honda 30's or the little putt-putts that always seem to >have a PA in charge of... Should they not be grounded?
Use Hondas that are small enough, and their plastic housings will keep your PA's safe and sound.
Marin County, CA
Generator grounding is a very complex issue.
I have driven copper earth stakes down ten feet and still not established an effective earth... so we cheated and poured salt water around the earth stake for the electrical Inspector's test.
Even if you establish a reasonable earth at the generator and run out a length of mains cable... and again measure to earth .. you will need to drive another earth stake.
Personally I never earth generators because of the difficulty of getting a good earth... Even when you can establish a earth bond there are still good and bad points for both.
So I am doing a travelling shot on a Student Film. They have an old portable generator, noisy as hell for power and someoneâ€™s pick-up towing a steel car trailer. I put a thick piece of high density foam in a plastic box I found and stick the generator in on top of the foam (mainly to reduce the noise and vibration).I secure the whole lot in the back of the pick-up ...there is no way any part of the generator can come in contact with the pick-up.... it is effectively electrically isolated.
I then plug in the old Tungsten lamp that's sitting on a wooden apple box (because I don't have a turtle) on the steel car trailer. Unbeknown to me, the lamp's earth wire went open circuit several years ago and someone yanked the mains input lead so the active has come off the input side of the switch and is now touching the metal case. I kneel down put my hand on the metal case of the lamp to steady it and switch it on... dam doesn't work... unplug it.. not the globe.. get my trusty meter ..huh no earth ..still should work .. oh my God active to case short... lucky.
Same Scenario but this time I have connected the generator earth to the pick-up and to be extra safe bonded the pick-up so it is earthed to the trailer. I kneel down beside the lamp, I have my cute little short pants on, so I am making a good contact with the metal trailer. I put my hand on the metal case of the lamp and do one of my better wrap dance impressions. I am now a conductor between the live case of the lamp and the earthed trailer. Maybe I should have clipped an earth from the lamp to the trailer before I switched it on... unlikely.
Dan Drasin writes :
>Use Hondas that are small enough, and their plastic housings will >keep your PA's safe and sound.
To toughly quote :- "The self employed joiner was electrocuted while using a small generator in the back of his pickup truck to run the charger for his cordless drill. The cable got pinched between the truck and a wall causing his electrocution when he touched the truck."
(Current to earth via wall... Current back to generator via joiner and truck chassis.)
Shit really does happen, allow for it.
Graham Rutherford writes :
>I then plug in the old Tungsten lamp that's sitting on a wooden apple >box (because I don't have a turtle) on the steel car trailer.
This is where this lists short quotes give problems. Five lines is too little for scenario's like this.
OK, so your live fixture is sitting on the ground making a potential difference between the ground and generator, but the generator is sitting on a foam base to acoustically and electrically isolate it from the truck. You go to check the diesel or turn off the generator and bridge between ground and the generators chassis directly. BZZZK.
Even if you're having difficulty getting a decent earth in the vicinity, it's still useful to at least make an effort, since any path other than you is a bonus. The earthing requirement is not nearly so critical with an earth leakage breaker, since it's quite easy to "leak" 30mA back to source.
Yeah, you poured salty water on the electrode did you? I think the technical term for that is "pissing on the electrode."
Sorry if this thread is boring the pants off the less electrically
biased members of the list. It's better to bore the pants off you than to blow them off you. (Ooer!)
Clive Mitchell wrote :
>Even if you're having difficulty getting a decent earth in the vicinity, it's >still useful to at least make an effort, since any path other than you is a >bonus.
I remember this discussion from a few years ago. I always felt it was left somewhat unresolved. It seemed to be stated that in LA it was illegal to ground gennys (via Mark W and others) and its often- even demanded- that they be grounded out here in NYC. In fact several gennys I've seen here have prominent signs saying "this unit must be grounded by qualified personal" or words to that effect.
I'm still trying to understand how having a partial ground or a less than absolutely "code" ground in the case of generators would cause problems vs no ground. I always try to ground to fire hydrants as it seemed to me the best bet found on most city locations. Fences, and pipes of unknown destination were often suspect at best. Assuming the neutral is bonded to the ground as is the case with almost every genny I've encountered out East, I'd like to know the theory/logic of running or not running a ground line to fire hydrant or some such. How I'm I going to be less protected by such a ground? Or stated differently, why would such a ground be considered illegal in LA? Under what conditions would such a not-to-code ground be dangerous.
Further, if the genny is not grounded what happens when there is a short? I often here those of the "don't ground the genny camp" saying the genny *is* in fact grounded via the truck/genny chassis- which made no sense to me.
John Roche writes :
>I remember this discussion from a few years ago. I always felt it was left >somewhat unresolved.
This is due to a number of misconceptions and to some confusion of terms, e.g., mechanical ground and earth ground; as well as the different nomenclature used in different parts of the world and a greater emphasis on electrical safety where 240 vac systems are used.
>It seemed to be stated that in LA it was illegal to ground gennys (via >Mark W and others) and its often- even demanded- that they be >grounded out here in NYC.
The grounding requirement in NYC probably has to do with the very strong possibility that the generator and house lines might be accidentally cross connected. Keep in mind that the real purpose of connecting an electrical system to an earth grounding rod (electrode) is to dissipate a lightning strike or voltage surge or a high voltage cross connect. (BTW, I have never heard of an electrical inspector in NYC checking the ground on a generator. I have seen a couple of inspectors checking out ground lines run to parking meters etc., and walk away muttering to themselves and shaking their heads. So I'm curious: who's doing the demanding?)
>In fact several gennys I've seen here have prominent signs saying "this >unit must be grounded by qualified personal" or words to that effect.
That really doesn't mean much. It's probably meant as a dodge for the rental company if something goes wrong -- They can say: "It wasn't properly grounded". "Improper use voids warranty."
>I'm still trying to understand how having a partial ground or a less than >absolutely "code" ground in the case of generators would cause >problems vs no ground.
Could you be more specific as to what these problems might be?
>I always try to ground to fire hydrants as it seemed to me the best bet >found on most city locations. Fences, and pipes of unknown >destination were often suspect at best.
The only way to determine if you have a good earth ground is to test it with a ground resistance meter. Keep in mind that on a potable generator a ground rod, even a proper ground rod, is not going to prevent you from getting a shock or even electrocuted. People are electrocuted every day in homes with properly grounded electrical systems.
>Assuming the neutral is bonded to the ground as is the case with >almost every genny I've encountered out East, I'd like to know the >theory/logic of running or not running a ground line to fire hydrant or >some such.
You should read the relevant sections of the National Electric Code. Grounding electrodes (rods) are only required if the generator is a separately derived system. The NEC in Article 100 defines a Separately Derived System as : a premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.
Better yet, go to www.imsasafety.org and follow their prompts to generator grounding. It has several diagrams which will make that definition a lot clearer and I believe it has a very clear explanation of grounding/not grounding generators.
>How I'm I going to be less protected by such a ground? Or stated >differently, why would such a ground be considered illegal in LA?
Just guessing here. Possibly because it is a violation of the NEC. Or it has become an issue with the inspectors. Or the insurance companies. Is it serious? Not in the grand scheme of electrical violations.
>Under what conditions would such a not-to-code ground be >dangerous.
A lightning strike or contact with overhead high voltage wires or cross connection with another electrical system. Imagine your truck mounted generator back feeding Madison Square Garden or vice versa.
>Further, if the genny is not grounded what happens when there is a >short?
A lot depends on where the short occurs. Are you talking about a direct short between a hot leg on the genny and the vehicle chassis -- the neutral in other words? The generator and the distro system should have circuit breakers which would trip in the event of such a short. However, if you are asking about personal protection in the event that some piece of equipment that is not properly connected to the neutral and does not have a mechanical ground becomes energized, that is another matter entirely, and is slowly being addressed by requiring GFI/GFCIs. And IMHO, way too slowly.
>I often here those of the "don't ground the genny camp" saying the >genny *is* in fact grounded via the truck/genny chassis- which made >no sense to me.
It's grounded in the sense that truck chassis is connected to the
neutral/"mechanical ground" of the generator, but obviously not to a ground electrode.
There is a companion volume to the NEC. I think it's called the NEC Explained, it may help in getting through some of the technical terminology used in the NEC.
For best results do exactly what the inspectors require.
IA 600 DP
>...why would such a ground be considered illegal in LA? Under what >conditions would such a not-to-code ground be dangerous. Further, if >the genny is not grounded what happens when there is a short?
Look at it this way:
If you are isolated from ground, you can grab a 240 volt conductor and nothing will happen to you as long as you don't also touch any other conductor (I have drilled holes in live buss bars (not in the film industry, mind you)
If your genny is not grounded and you have a light with a fault so that the hot conductor hits the inside of the case and electrifies the case, you can still touch it without getting a shock because that 110 volts with respect to genny neutral is not with respect to earth ground... so even if you are grounded, there is no path from the light to the genny ground (as long as the genny is not grounded and the tow chains are not touching the ground.
One problem with this is that you have no way of knowing that you have a hot fixture...and since no short circuit is formed between the hot and ground, no breaker or fuse pops so you have a potentially dangerous situation if you touch the light and then touch the generator.
Now, this is only the case if the ground conductor on the light is no good, because if the light's hot wire grounds to the case, the
equipment grounding conductor makes a short and the power distro breaker/fuse will pop - thereby protecting you.
If the genny is grounded, and if the equipment grounding conductor from the light is faulty, you become the path to ground if you are grounded to the earth.
In the situation where the genny is not grounded, when working with lights that have equipment grounding conductors (anything except zip corded china balls and suchlike) you need TWO faults before you get a shock - you need the case of the light to be energized at the same time that the grounding conductor is not working.
Thus, I believe, is the rationale behind keeping the genny ground and neutral floating with respect to earth ground.
> why would such a ground be considered illegal in LA?
The legend I heard was that here in Los Angeles, electricians (film and private) were striking unlabeled gas and sprinkler lines with their stakes because they weren't notifying dig safe before earthing. Therefore, after analysing their necessity, and consulting with the NEC requirements, generator grounding became illegal.
BTW, The only time you are really protected against a ground fault is if the distro system is equipped with a GFI disconnect such as a Shock Block. In my experience however, GFIs can be more trouble then their worth. I did a job a while back where I ordered two GFI shock blocks for a rain sequence. Once our distro boxes got wet and leaked a little power to ground the breakers were throwing faster then Pedro Martinez, making it impossible to shoot. We disconnected the GFIs and I told the crew to touch on stand at a time. Now I only use GFI's if the talent is immersed in water and running DC is impossible.
It reminds me of a mentor electrician who taught me to "stay off my knees if I wanted to keep my caps."
Los Angeles, CA
Graham Rutherford wrote :
>Now I find some of the lighting technicians electrical knowledge either >non-existent or basic.
Hmm, well, nowadays with the employers having to offer a safe working-environment in Aus, you are only allowed to hire qualified electricians as gaffer. In case something goes wrong, insurance won't pay if you were not-qualified (yes, I know, the rules are bent all the time).
(I better go and get my basic electricians license quick....)
"better to let them do it imperfectly than to do it perfectly yourself, for it
is their country - and your time is short" T.E. Lawrence
>if this thread is boring the pants off the less electrically biased >members of the list. It's better to bore the pants off you than to blow >them off you. (Ooer!)
Or blowing the voltage regulator or the matrix, you could get a beautiful two color invoice with bold black numbers at the end and a smiley tech in front of you with a very mad producer.
It happens, I know.
Miguel Del Valle Prieto
>Sorry if this thread is boring the pants off the less electrically biased >members of the list. It's better to bore the pants off you than to blow >them off you. (Ooer!)
This is one of the most interesting threads for the poor blowed or not spark! props to the list!
How can I measure the ground-resistance, or how should I decide if its better to ground the plant or not? Does a grounding rod driven in the earth have a greater resistance than a human body wearing working boots? What about steel stands? Do I go better if I ground the on set distro-boxes than the genny?
Better off asking
Miguel Del Valle Prieto writes :
>Or blowing the voltage regulator or the matrix
AVR's (Automatic Voltage Regulators) can be temperamental beasts at the best of times. It's maybe a good idea to keep a spare. (Yes they ARE expensive).
Matrix? Is your generator the ONE?
Daniel Pauselius writes :
>how can I measure the ground-resistance, or how should I decide if its >better to ground the plant or not?
The proper way to test ground electrode resistance is rather complicated and involves an array of test electrodes. Unless you're putting in a permanent installation it's hard to justify the time involved. (Maybe this is why some of the American states mentioned seem to dissuade the practice.) The metal stands provide a form of grounding wherever they sink into the ground, and the metallic bits of an earthed distro box do the same. But these are poor earths and will just set up voltage gradients in the surrounding earth if something live leaks to earth. (tingle time)
At the very least, consider the local generator earth electrode as just to provide a modest level of potential match between the generator and the earth it's sitting on, and provide the minimalist continuity required to trip an RCD (GFI). Try to think of it as equipotential bonding between the bit above and below the rubber tyres on the truck. If the ground is damp then it should act as a good return path for a fault on the location itself.
Again, your local electrical scenario might vary. It's become very clear in this long thread that there's no global standard. I'm guessing that the locals that discourage generator grounding just got sick of the hassle of trying to cover every possible scenario. I wonder if these rules were made before or after the advent of earth leakage protection.
Of course...I assume that no one on the list has bypassed the earth leakage trip in desperation have they? (Don't reply!)
>...you can still touch it without getting a shock because that 110 volts >with respect to genny neutral is not with respect to earth ground...
Are you suggesting that a live conductor from a genni will not short out to earth ground if the genni is not earth grounded?
Yes, Yes, Yes, the generator active has no reference to ground (ie) the dirt you are standing on....unless you establish one...So you can grab the active from the generator supply and grab onto the copper earth stake you have driven into the ground (the same earth you are standing on) and ...nothing...until you connect your earth/neutral bond from the generator to the ground(the dirt you are standing on) via the good old earth stake.. there is no circuit. In other words to get a belt you must get across active and neutral This relies on the generator.. if the earth/ neutral is bonded to the vehicle chassis, being insulated from the ground by the rubber tyres...so maybe you would be better off parking it on a rubber insulating mat.
This is why...there is no provision under (S.A.A.30.3)of the Australian electrical code (our voltage is 240 Volts twice the dielectric breakdown capability of the U.S.)to earth mobile generator supplies unless you are connecting to an existing system..(a domestic or industrial supply fed from the Electricity supply Authority's power grid) I don't earth generators ...as I have said before.
The Electricity Authority runs earths to ground (ie) the earth we walk on from various power poles using copper earth straps, mainly to shunt lightning induced spikes to earth.
The lightning doesn't need to strike the wires to induce the voltage....Your house switchboard similarly has neutral earth bonding and is earthed to an earth stake driven into the dirt you walk on.. because your house is connected by the power lines to the electricity power supply authority's power grid.
I'll take my chances on the genny getting struck by lightning.
>I always try to ground to fire hydrants as it seemed to me the best bet >found on most city locations. Fences, and pipes of unknown destination >were often suspect at best.
I lost track of who this quote is from, but I'm wondering how exactly you're grounding to fire hydrants. Most likely, they aren't a very good ground at all. Unless you're grinding off the paint to bare metal at the point of attachment and you're sure that the bottom of the hydrant head isn't coated or painted or has a gasket where the head is bolted to the main pipe, I'm pretty sure you won't be getting any kind of ground out of attaching to the hydrant. But if anyone has actually tested this method and it works, I'd love to know about it.
> Matrix? Is your generator the ONE?
Jajaja, sorry I am translating from Spanish, Should I say: exciter?
Miguel Del Valle Prieto
I think it is absolutely wrong to ground a freestanding generator that is it's own derived power and not connected to or used in association with a utility power, and I believe the reason why some municipalities require such grounding is because of lawyers and not the reality of what a earth ground if for.
Basic Electrical Safety Precautions for Motion Picture and Television Off Studio Lot Location Productions August 01, 2000, City of Los Angeles Dept. of Building and Safety:
"Generators, Truck or Trailer Mounted Generators mounted on trucks or trailers shall be completely insulated from earth by means of rubber tires, rubber mats around metal stairways and rubber mats under any type of lift-gate or jacking device. Metal supports for trailers shall be insulated by means of wooden blocks. Safety tow chains shall be secured so as to not touch the ground. If complete insulation is not possible, a grounding electrode system shall be installed per the California Electrical Code, Article 250-83 (c) or (d)."
class="style6" >I think it is absolutely wrong to ground a freestanding generator that is >it's own derived power and not connected to or used in association with >a utility power, and I believe the reason why some municipalities >require such grounding is because of lawyers and not the reality of what >a earth ground if for.
That would explain the warning tag on a neighbourâ€™s stroller: "Remove child before folding stroller."
For a complete explanation of the rationale for not grounding/grounding (earthing) generators, go to :
For a complete explanation of grounding, and how it is measured, get a copy of "Getting Down to Earth" put out by the Biddle Company, who manufacture electrical measuring equipment, including Ground Resistance Meters.
Brian "Hello Walter" Heller
Miguel Del Valle Prieto writes :
class="style9">>> Matrix? Is your generator the ONE?
class="style9">>Jajaja, sorry I am translating from Spanish, Should I say: exciter?
Sorry Miguel, that was a joke based on the film called "The Matrix". It obviously didn't translate well.
Walter Graff writes :
>Generators, Truck or Trailer Mounted Generators mounted on trucks or >trailers shall be completely insulated from earth by means of rubber >tires, rubber mats around metal stairways and rubber mats under any >type of lift-gate or jacking device.
Now THAT makes perfect sense. Instead of the bonding of chassis to ground, they are preventing the risk of operator shock from terra to chassis by the use of rubber mats as is often used in power plant to reduce the risk of operator shock from panel wiring to concrete floor.
But how many of you even bother with a rubber mat? Even one at the most likely point for an operative to board or touch a trucks chassis?
Ah well, there's nothing quite like a close shave to bring reality home.
Clive Mitchell Writes :
class="style9">>Now THAT makes perfect sense. Instead of the bonding of chassis to >ground, they are preventing the risk of operator shock from terra to >chassis by the use of rubber mats as is often used in power plants"
But how does the operator get a shock if you have not established a potential difference between the ground (dirt you are standing on, and the generator)as you haven't referenced the generator to ground. If on the highly unlikely chance the active fell off and livened up the outside of the generator to 110 volt /240 v. you still wouldn't get a belt unless you simultaneously touched the outside of the generator and generator neutral.
Most domestic and industrial electrical deaths here occur when the person gets between active and ground (earth). They are making contact with a good ground, say a copper water pipe,(sitting on it , leaning against it) and grab onto an active.
With modern footwear, sneakers, with their heavy insulation it is possible to grab a active with 240 volts on it and not even realize it is live until you touch an earth, say a copper water pipe etc.
class="style9">>Most domestic and industrial electrical deaths here occur when the >person gets between active and ground (earth).
Because the power plant is grounded to earth, a portable generator isn't so their is no potential, but to be safe you want to make sure you have good insulation of both yourself and your generator. Once again more of a lawyers clause than reality.
class="style9">>Most domestic and industrial electrical deaths here occur when the >person gets between active and ground (earth...
Sorry Walter I was referring to power derived from the Supply authority power grid.
I don't ground generators.. as I said a while ago... to my way of thinking it is the same as ripping the plastic case from your double insulated power drill(which has no earth) and fitting a metal case so you can earth it.
class="style9">>For a complete explanation of the rationale for not >grounding/grounding (earthing) generators
I don't get it. Maybe I'm missing something obvious here.
The article seems to indicate that there is no ground required if the neutral is not switched but that application seems to be primarily for backup generator power for an existing electrical service. So if you lose your utility-provided power in a storm, you turn a transfer switch to utilize genni power in the wiring of an existing premise. This has all to do with bonding of grounds and such.
But at the bottom of the article it states "If the generator neutral is grounded then the generator can only be used with a transfer switch that transfers the neutral, or as a stand alone generator for a carnival or special event, and then ground rods are required."
My interpretation of that is a stand alone generator, not integrated into the wiring of an existing premise, needs a ground rod.
class="style9">>My interpretation of that is a stand alone generator, not integrated into >the wiring of an existing premise, needs a ground rod.
If you are running a carnival. While some of the shoots I have worked on are almost carnival-like in atmosphere, we don't ground our gennies. Or look at it this way, next time you are in NY and walk by the library when they are having the fashion shows, you'll notice more than 100k of power generators on the 41st street side, none grounded, but rather well insulated from the ground.
Jim Sofranko writes:
class="style9">>But at the bottom of the article it states "If the generator neutral is >grounded then the generator can only be used with a transfer switch >that transfers the neutral, or as a stand alone generator for a carnival or >special event, and then ground rods are required."
The rationale for requiring a ground rod in these instances is the difficulty in keeping every ride or attraction totally isolated from the ground, and the length of time the installation would be in place increasing the likelihood of a lightning strike.
I long ago gave up trying to convince people that grounding a generator was not a good idea, but I've given up.
Most of the belts I've gotten, have been on stages. Now I carry a "Volt
Tick" in my kit.
IA 600 DP
Mark H. Weingartner wrote:
class="style9">>so even if you are grounded, there is no path from the light to the genny >ground (as long as the genny is not grounded and the tow chains are >not touching the ground.
In your statement, partially quoted above, I think you were saying that a person cannot be shocked touching a hot from a generator if the generator is not grounded. The reason, you stated, is that there is no complete circuit to the neutral. You seemed to be suggesting that by "grounding" the generator to the earth one would be completing a circuit to the bonded ground/neutral via the earth.
Did I get that correctly? Because if I did I have to disagree. The idea that one needs a completed circuit for electricity (electrons) to move is not true. One only needs to have a potential difference (measured in volts) between point "A" (let's use an example, say, the generator) and point "B" (let's say the earth). Potential difference happens when point "A" has electrons which are closer together than point "B". Electrons hate being anywhere near each other and will try to move as far apart as possible. We could say that point "A" (the generator) is stuffed with electrons compared to point "B". We are not talking total electrons here (obviously the earth has more electrons than the generator) rather we are speaking of the spacing of the electrons in either place. To oversimplify a bit, if the generator is stuffing electrons into the "hot" cable and you touch it, you likewise will become stuffed with electrons. If you then touch the ground (earth) and the ground's electrons are farther apart than they are in you, the electrons in you will go to the ground. You WILL be shocked. It doesn't matter if the generator is "grounded" or not.
In an A/C generator (where the electrons reverse direction periodically), the generator goes between being an electron stuffing device (where you become stuffed with electrons which then go into the earth) and an electron sucking device (where the generator pulls electrons out of the "hot cable" which then pulls electrons out of you whereby you then pull electrons out of the earth. The electrons being pulled by you out of the earth will continue their journey toward the generator along the "hot" cable. In either case (stuffing or sucking) you are being shocked.
The case where all the above is not true is when you are talking about chemical-based generation of electricity (i.e., batteries). With batteries you DO need a completed circuit, without the completed circuit you cannot continue the chemical reaction which stuffs electrons, so no electrons are stuffed and no one gets shocked.
Gaffer / New York City
class="style9">>... you'll notice more than 100k of power generators on the 41st street >side, none grounded, but rather well insulated from the ground.
So you are suggesting that there is no electrical potential from a hot leg of an ungrounded generator to the earth ground?
I'm telling that not a single generator operator I know grounds a gennie. Potential happens when there is some sort of return. No ground, no potential.
Jim Sofranko wrote :
class="style9">>So you are suggesting that there is no electrical potential from a hot leg >of an ungrounded generator to the earth ground?
I know you were not addressing me but I'm answering anyway.
Electrical potential is all about spacing of electrons from each other and the fact that electrons repel each other. To illustrate in an oversimplified manner: In a vacuum, if you have five electrons in a box A and you connect them to box B (which has no electrons in it) via a tube (which has no electrons in it), two of the electrons will go to box B, one electron will stay in the middle of the tube and two will remain in box A. The electrons will now be as far apart as possible... remember electrons repel each other.
Likewise if you have a generator (an electron stuffing device) and you connect its "hot" to the earth, electrons will go to the earth. This is assuming that the earth has electrons that are farther apart than the electrons being stuffed into the "hot" by the generator.
In an A/C generator the flow of electrons switches periodically (in the USA 120 times per second). When the switch from our previous example takes place the generator starts stuffing electrons away from the "hot". This makes the hot lose electrons and the hot (still connected to the earth) now has a larger spacing of electrons than the earth. So, electrons from the earth now flow toward the generator via the "hot". Disconnect the generator from the earth in either situation and you stop the flow of electrons and "the lights go out".
You do not need a complete circuit or "return" for electrons to flow.
Now for some perhaps confusing after-thoughts:
In real life, when a three phase generator (it has three separate "hots") is stuffing electrons away from the "hot" of our previous examples, it is actually stuffing them up the "neutral".
IN THE GENERATOR ITSELF all the hots are connected together at a point.
The point where all the hots converge is called the "wye" connection. The wye connection is where the neutral wire going out of the generator is connected. In the generator itself, the wye connection IS the neutral. Having thought about this for a while, I find this is an incredibly complex concept to wrap my head around. Three phase systems are no simple matter. I do not mention these "after-thoughts" to you to illustrate anything.
I don't think I could explain the above via the written word. I would need diagrams and even then I would probably not fully understand what I was telling you. But the point I guess I am trying to make is that when the generator is stuffing electrons away from the "hot" it is actually stuffing them up the "neutral" which means they may be actually being stuffed up another hot via the wye connection.
Gaffer / New York City
class="style9">>In either case (stuffing or sucking) you are being shocked.
That's along the lines of what I was thinking. I used the carnival example because that is the example to which the NEC (National Electric Code) refers. Article 525 is entitled 'Carnivals, Circuses, Fairs and Similar Events'. And as Walter mentioned many of our shoots may qualify under such a designation.
Article 527 is entitled 'Temporary Installations' but refers mainly to construction sites notably where GFI's are specified.
Article 530 is entitled 'Motion Picture and Television Studios and Similar Locations' but this seems to be mostly addressing studios and labs with house power.
Walter mentioned that the larger location power supply companies do not ground their generators for live events. But in my reading of the code, especially Article 525 noted above, it would seem as though generators should be grounded. I talked to a gaffer who works on many of the biggest commercials and shows in town. He grounds his genni's. He said Feature Systems requests that you ground their genni. Perhaps it has to do with the design of the genni?
I suppose there is a reason that NEC Article 250 on grounding is the largest Article in the code.
I think it is the most misunderstood thing in the film business.
So, Piotr, do you ground the generators on location?
class="style9">>He grounds his genny's. He said Feature Systems requests that you >ground their genny. Perhaps it has to do with the design of the genny?
Since they are using these gennies often within systems where potential connection to municipal power systems exist, they should.
class="style9">>Since they are using these gennies often within systems where >potential connection to municipal power systems exist, they should.
Wouldn't that be true with Fashion Week in Bryant Park as well?
But the more practical example of what I'm thinking about is an electrician raising a 4K Par on a reflector stand on a wet lawn. Isn't there a potential for any errant AC off the head to travel down the stand to the ground? Or if it's a rubber-wheeled stand then down through the electrician to the ground?
Jim Sofranko wrote :
class="style9">> So, Piotr, do you ground the generators on location?
I let that decision rest with my Best Boy unless I am hiring a best boy who has less electrical experience than I.
When I work as a Best Boy, I usually ground the generator. I always ground the generator when it is raining.
Here in NYC we now use distribution systems which have ground wires which (in theory) are connected to the exteriors of everything we electricians can touch. This means that if something goes wrong inside a light (or the generator for that matter) we have some kind of backup path for the electricity other than through us. Electricity takes the easiest (not shortest) path to where it is trying to go. In most cases it will travel down the ground wire rather than through us. When it is raining the formula gets a little more complicated and so I ground the generator and/or some other part(s) of the ground wire system to encourage the electricity to return along the ground wire rather than though human beings.
Gaffer / New York City
Jim Sofranko writes:
class="style9">>But the more practical example of what I'm thinking about is an >electrician raising a 4K Par on a reflector stand on a wet lawn.
Yes, there is, but only if there is no mechanical ground from the head bonded to the generator neutral. If that is the case, it won't matter whether the generator is grounded (grounding electrode) or not. The current will not travel from the ground on which the electrician is standing back to the ground into which the ground rod has been inserted.
However, the electrician won't get the full voltage. voltage will be diminished, i.e. not the full 120 or 240. It's easy enough to test for yourself. A chain link fence usually makes a decent enough ground. Just put a light bulb between a hot leg and the fence.
IA 600 DP
I really have to smile at all this discussion.
All these folks are talking about how they ground, but I'll bet many of them have no idea if the ground they use even works properly. Some folks seem to think that just because you have more electrons in a wire than the earth that the electrons are always going to want to go into the earth. Better go back to physics class, you're forgetting how much energy an electron has.
And the other sad part of the story is that according to a white paper on grounding most of the 8 to 10 foot rods used for grounding don't even meet NEC code requirements for resistance. Any of you 'grounders' ever check that. Any of you even know what it is? It seems not even the NEC really knows what to do. They say "the resistance to ground for a rod or pipe ground should be 25 Ohms or less for a single electrode." And that if the "electrode does not meet 25 Ohms, it must be supplemented by one additional electrode." Now get what they say next after telling you what the resistance should be. That write "the combination of the two electrodes does not have to meet the 25 ohm requirement! "
There are many beliefs on grounding, none wrong and none right. All have some bit of truth to them. Problem is beliefs come from many philosophies. In that white paper on grounding I mentioned the authors found that 90-95% of all facilities inspected lack an effective grounding system. In addition, none of the facilities inspected had ever tested the ground resistance of their electrode system. Sorry but sticking a rod in the ground doesn't guarantee you are properly grounded because there is more to the equation or as the NEC rule says:
"A ground system must meet NEC (National Electrical Code) Article 250 requirements. The NEC  defines "grounded" as "Connected to earth or to some connecting body that serves in place of the earth" and "effectively grounded" as "intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current carrying capacity to prevent the build up of voltages that may result in undue hazard to connected equipment or to persons."
In a study of over 100 grounding rods put in the ground at various depths a 5 foot ground actually had about the same resistance as a deeper rod (66 Ohms for a five footer and about 40 ohms for a 8 footer- still above the minimum that the NEC suggest, then dismisses). It was determined that unless you are greater than 30 feet in depth you are not meeting requirements for proper grounding. So waste your time hammering rods in the ground, the average 8 to 10 foot ground rod does not meet minimum NEC code requirements for earth resistance let alone that the time of year you put that rod in the ground makes a major difference in what you get.
So do it if you want, but I have to wonder with the 12 billion dollars each year that this industry spends on location in the US, with all those ungrounded generators, certainly after some 50 years in NY alone someone must have been electrocuted, or even shocked by an ungrounded generator, no?
class="style9">>Are you suggesting that a live conductor from a genny will not short out >to earth ground if the genny is not earth grounded?
The gennie is making electrical potential (voltage) between that hot and the genieâ€™s neutral and ground.
If the neutral and ground of the gennie are isolated from earth
ground, you will get no current flow if you touch the hot leg to the ground
(How many times have you seen bare copper on a piece of 4/0 that has been abused? It doesn't always arc to ground.)
class="style9">>In your statement, partially quoted above, I think you were saying that a >person cannot be shocked touching a hot from a generator if the >generator is not grounded.
I should have been a bit more careful and said that "it is by no means necessarily the case that a person touching a hot leg from an ungrounded generator will get shocked.
I will go on to say that I disagree with your theoretical explanation as stated regarding electron spacing - it is by no means all wrong but not exactly right - I don't have time to go back to my physics texts and put it into math where this all belongs (nor are enough of those brain cells still extant for me to do calculus as I did at university)
but more to the point, I have some considerable empirical experience to draw on regarding this - I have owned and run truck mounted generators for years.
Voltage (or electrical potential) is not an abstract...it has to be "with reference to" something. In the case of a 3 phase Y tapped gennie, the three legs have a potential of 208 with respect to each other and 120 with respect to the neutral.
If the neutral is in no way tied to earth ground, there is NOT necessarily any potential between a hot and earth ground. Take an ungrounded gennie, take one side of a light bulb and tie it to a hot leg, take the other end of the light bulb and ground it to earth ground, and send me an email if the light bulb lights up. I'll bet that it doesn't
This, by the way, is one reason why the "old school" way of checking voltage with a pigtail socket with a light bulb may be more useful sometimes than an auto ranging DVM (digital volt meter)
Mark H. Weingartner
Walter writes :
class="style9">>I really have to smile at all this discussion. .... Better go back to physics >class, you're forgetting how much energy an electron has.
It's probably true that a large percentage of grounds do not meet the NEC's requirements, and it's true that most electricians and even electrical inspectors would have trouble doing an accurate ground resistance test, and it's certainly true that the NEC is not terribly clear on the issue of grounding generators, however none of this changes the fact that even if a generator is completely isolated from any earth, you will receive a serious shock if you contact a hot leg and any earth ground.
Anyone who maintains otherwise may be in for .. well, a shock Not as strong as a direct hot -neutral shock, but substantial nonetheless.
IA 600 DP
class="style9">>Anyone who maintains otherwise may be in for .. well, a shock Not as >strong as a direct hot -neutral shock, but substantial nonetheless.
And you know this from testing this theory perhaps with a meter to see if it actually happens the way you say? I know a few folks who have and might just say you are not entirely correct.
class="style9">>The case where all the above is not true is when you are talking about >chemical-based generation of electricity (i.e., batteries
I have to disagree! What makes 'chemical' generation of electricity safer than magnetic? Nothing! [Apart from AC versus DC, which is not the issue here.]
"Neutral" is a word allocated to one wire in a typical mains distribution system. When we are talking about power utility systems then the Neutral is grounded [to terra firma] at many places, from the power station, to switch yards, to an earth stake near your meter box [I'd better qualify that last one by saying "here in Australia, using the MEN system" - Multiple Earth Neutral, but I think most other countries work the same way]. The reasons for grounding the Neutral are many, mainly to do with referencing the power to ground so that if lightening strikes an aerial cable, surge limiters can [attempt to!] prevent any conductor rising more than the specified voltage above earth.
Historical note: Early power systems often were DC and involved rooms full of batteries. With one leg earthed, this power was just as dangerous as today's AC systems.
Lets turn to local generation. With no connection to utility power systems...
If the generator's neutral is NOT connected to earth [its metal housing], then a person can touch EITHER the neutral OR active wire whilst being in contact with the ground [as in 'terra firma'], the truck/trailer/generator or even a metal light fitting and not be in danger. With one provision - that no active-to-case or neutral-to-case fault exists in any equipment connected to that generator. Or within the generator.
So, how do we know if we have a fault in a light fitting in this scenario?
Firstly, we need all the "case" wires in our system to be connected together. I use the term "case" to avoid confusion - I'm referring to the [usually] green or green/yellow wire commonly referred to as the earth wire. But there's no need for this case wire to be bonded to terra firma!
Getting back to cinematography, I see no point in grounding generators [when used in isolation of mains power systems]. If the generator is set up with a device to detect any 'live' conductor [I include the 'neutral' here] to case/chassis/ground connection, and all the normal earth wires [the 'third wire' in the equipment's power lead] are used then this should result in a safe workplace.
Perth, Western Australia.
Walter Graff wrote:
class="style9">>And you know this from testing this theory perhaps with a meter to see if >it actually happens the way you say? I know a few folks who have and >might just say you are not entirely correct.
I have tested this theory. With a meter, with a light bulb, with myself. The electricity does go from the generator (ungrounded, genny on rubber tires) to the earth. The meter says so (low-ish voltage between a hot and a crumby earth ground), the light bulb says so (it lights up) and I say so (I was shocked).
Gaffer / New York City
class="style9">>I have tested this theory. With a meter, with a light bulb, with myself. The >electricity does go from the generator (ungrounded, genny on rubber >tyres) to the earth.
Good you learned your lesson. Now try the same trick with a potential circuit involving a neutral and a hot leg and the earth ground and cause a potential along the two and see how much the earth wants that voltage.
Walter writes :
class="style9">>And you know this from testing this theory perhaps with a meter to see if >it actually happens the way you say?
class="style9">>I know a few folks who have and might just say you are not entirely >correct.
Me too. As I said, it has to be a decent ground.
I used to own a few generators. So the whole thing came about from asking electricians who were grounding generators, why they were grounding them. They kept asking for ground rods, sledge hammers, etc. Out came the NEC, usually to no avail. Especially when HMIs became popular. (We finally gave up and sent out the gear with the genny.)
Anyway, I was curious as to how they checked the ground electrode since I've never seen anyone, including inspectors, using a ground resistance meter. We actually acquired a Megger GRM, but properly testing for a good ground is a very time consuming and very involved procedure.
I suggested that if a flow could be established between a hot leg and the earth, then at least the ground would be partially effective. In performing this "test" with various objects found around the urban landscape, sometimes the test light glows and sometimes it doesn't. For some reason, chain link fences are usually pretty good -- at least for getting the light to glow.
Since as you noted, most grounds -- even properly installed grounds -- are often not effective, in the overall picture of potential hazards on a movie set, grounding gennys seems like a rather harmless diversion. And as you say, once the lawyers get involved....
IA 600 DP
class="style9">>If the neutral is in no way tied to earth ground, there is NOT necessarily >any potential between a hot and earth ground.
I forgot I had a camera car and genny out back. The bulb lights up. The meter on the genny reads 120V. A Wiggins between a hot leg and a fence post reads about 110V -- a pretty decent ground probably because the ground is very saturated with snowmelt.
class="style9">> I'll bet that it doesn't.
I'll take your bet. If you like, I can place it with a local bookie who looks a lot like Dennis Farina and is referred to in the local press as "colourful". Since he doesn't really know you, the maximum line he will cover for you is $25,000. If you have any family members living here, he'll go to $50,000. Let me know how much you'd like to lay down. Or I can put you directly in touch with him.
class="style9">>This, by the way, is one reason why the "old school" way of checking >voltage with a pigtail socket with a light bulb may be more useful >sometimes than an autoranging DVM (digital volt meter).
I've always preferred the Wiggy for that reason.
Brian "feeling lucky?" Heller
IA 600 DP
class="style9">>Take an ungrounded gennie, take one side of a light bulb and tie it to a >hotleg, take the other end of the light bulb and ground it to earth >ground, and send me an email if the light bulb lights up."
Great idea Mark. If some enterprising chap would do this, safely, when time allows and report back here to the list, perhaps we could answer this question once and for all and move on from this issue.
Brian Heller writes :
class="style9">>The bulb lights up. The meter on the genny reads 120V A Wiggins >between a hot leg and a fence post reads about 110V
That's what I'm talking about. Thanks Brian.
Randy Miller, DP in LA
I wrote :
class="style9">>The bulb lights up. The meter on the genny reads 120V A Wiggins >between a hot leg and a fence post reads about 110V
My shoot was snowed out again, so I have a little time. We just tried this with a portable generator belonging to a carpenter working next door.
The bulb did not light. This requires further exploration.
IA 600 DP
Brian Heller wrote:
class="style9">> The bulb did not light. This requires further exploration.
What was that bookies number again?
Anders "could use a little extra scratch" Uhl
Walter Graff writes :
class="style9">>Since they are using these gennies often within systems where >potential connection to municipal power systems exist, they should.
Just out of interest, could you describe this scenario further?
class="style9">> Anders "could use a little extra scratch" Uhl writes:
class="style9">>What was that bookies number again?
Unfortunately, Big Julie, will not be taking ny bets for awhile. He suffered severe electrical shock while shaving.
IA 600 DP
class="style9">>My shoot was snowed out again, so I have a little time. We just tried this >with a portable generator belonging to a carpenter working next door. >The bulb did not light. This requires further exploration.
I think you'll find it all comes down to whether any of the generator's windings are commoned to its chassis and/or its earth/ground sockets.
My bet is that the small unit is fully floating, whereas large sets will be wired with the neutral bonded to the chassis.
Do you have access to a multimeter? If so, try a resistance reading with your carpenter's unit - when its NOT running. Measure between :
- Chassis and the ground pin on the socket - may or may not be connected!
- Chassis and the active ['hot'] pin.
- Chassis and the neutral ['return'] pin.
If you get a high reading on the last two then you have a floating genny
Perth, Western Australia.
class="style9">>If the generator's neutral is NOT connected to earth [its metal housing], >then a person can touch EITHER the neutral OR active wire whilst being >in contact with the ground [as in 'terra firma']
Yeah that may be true here in Oz, but my interpretation from reading some of the responses on C.M.L. " generator grounding", is that electric current and voltage both A.C and D.C. follow a completely different set of rules in the U.S. from the Aussie stuff we are used to.
You don't need a potential difference between active and neutral/earth to ground. All you need is the active and it will create its own current path, sort of like the way lightning induces the opposite polarity to ground in its pre conductive phase I guess.
Another great advantage is, you can series a whole bunch of 100Ah. twelve volt truck batteries together to get a 120 volts or so and in addition to the obvious saving on insulation because it is D.C., it is perfectly safe to get across positive and negative.
So none of that A.C skin effect, leading and lagging voltage versus current and all that other nonsense we are stuck with.
In the U.S. system, it seems to depend whether you are stuffing or sucking electrons...
Woodward, Clive wrote:
class="style9">>I have to disagree! What makes 'chemical' generation of electricity safer >than magnetic? Nothing! [Apart from AC versus DC, which is not the >issue here.]
Chemical generation is not "safer". What I was saying is that in chemical generation (ie. batteries) you need a complete/closed circuit (from "hot" to "return", negative to positive) for someone to get a shock. To get shocked in chemical electricity generation you need to get between the "hot" and "return". With batteries, if you get between the "hot" and the earth/ground/terra-firma-that-you-stand-on then nothing happens because the chemical reaction creating the electrons in the first place will immediately stop without that complete/closed circuit.
In my original post, I was offering what I have just said above as an example OPPOSITE to my postulation that one WILL get shocked if getting between a "hot" and earth/ground/terra-firma in a magnetic generator situation. UNLIKE chemical generation, in magnetic generation you WILL get a shock when getting between the "hot" and earth/ground/terra-firma.
Gaffer / New York City
class="style9">> (ie. batteries) you need a complete/closed circuit (from "hot" to "return", >negative to positive) for someone to get a shock.
Sort of except for the positive to negative theory that we've been taught since grammar school is actually wrong. Batteries are non-electron conductors referred to in physics as ionic conductors. It is true that when you connect a lamp to a battery you form a complete circuit as the flow of charge is occurring inside the battery and the lamp. But unlike AC flow with DC flow while you have a lot of electric charge going through the battery no individual electrons flow through the battery.
The other error is that there is no flow in one direction in a wire, its actually in both directions at once. Half of the charge-flow from a battery is composed of positive atoms and the remaining portion is composed of negative atoms flowing backwards. In reality the real particle flow in a wire connected to a battery is from negative to positive. And it doesn't do it exactly the same way in a battery as it does in the wire. In the battery it goes in two opposite directions at the same time. In fact this is exactly how electricity works in your body, electrolytic capacitors, and fluorescent tubes to name a few examples of positive/negative charge.
Take a look at a piece of chrome, that 'chrome' gets attached to that metal the same way, through a loss and simultaneous gain. The differences between the flowing motion of charged particles in a battery and the way a generator pushes charge along with the fact that the earth is more like a battery itself is one reason why you don't get electrocuted when you touch a battery wire and the ground.
class="style9">> because the chemical reaction creating the electrons
Chemical reactions in batteries don't 'create' electrons just as a generator doesn't 'create' electrons.
class="style9">>With batteries, if you get between the "hot" and the earth/ground/terra >firma-that-you-stand-on then nothing happens because the chemical >reaction creating the electrons in the first place will immediately stop >without that complete/closed circuit.
Piotr Jagninski writes:
class="style9">>IN THE GENERATOR ITSELF all the hots are connected together at a >point. The point where all the hots converge is called the "wye" >connection.
On easy way to visualize three-phase power is to imagine a lawn sprinkler that's constantly rotating in one direction so it sprays in a continuous circle (imagine a lighthouse that sprays water instead of light).
The sprinkler is located at the nexus of three pie-wedge-shaped gardens of equal size, which quickly drain back into the well that feeds the sprinkler. The water drainage from each garden is like the three "hot" conductors in a three-phase system -- they carry current on a rotating basis (120 degrees apart), though their flows overlap in time.
The neutral can be thought of as the pipe from the well to the sprinkler
One can carry this crude analogy only so far, but that's the general idea.
Here's another analogy :
Single-phase AC is like paddling a canoe with a single oar. Two-phase AC would be like two oarsmen paddling the same canoe in alternating fashion. Three-phase is like having three oarsmen in one canoe, each taking his turn.
Using only one phase (or "leg") of a three-phase circuit is like putting each of the three oarsman in his own boat. That's how commercial electric power is transmitted (three-phase) and distributed locally (single-phase).
Dan "do try this at home" Drasin
Marin County, CA
Walter Graff quoted me as saying :
class="style9">>*"because the chemical reaction creating the electrons" *and then >corrected me by saying: *"Chemical reactions in batteries don't 'create' >electrons just as a generator doesn't 'create' electrons."
Quite right... I stand corrected... batteries do not create electrons. What I probably meant to say and should have said was "because the chemical reaction creating the electron flow..."
Gaffer / New York City