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Generators --- Grounding

I seem to recall someone posting about grounding generators a while back on the list. Something about some generators should be grounded, and some should not. From my limited experience with Electricity (mostly house power), the ground is there for safety. Should a hot leg touch the housing it will make the housing live and anyone touching the lamp risks getting shocked. I'm certain there are deeper reasons, but this one is sufficient for me. --

class="Body" Steven Gladstone

class="Body" Cinematographer - Gladstone Films

class="Body" Cinematography Mailing List - Listmum

class="Body" New York, U.S.A.


But think about that for a minute: The only reason you'd be shocked by touching the shorted housing is that you're grounded--and so is the source! If the source isn't grounded, you won't be shocked, since you can't complete the circuit with your body.

A commercial power grid is grounded (and so are all of the end users) for at least two reasons that I can think of: First, it prevents unbalanced potentials throughout a wide spread grid; second, it provides an outlet for lightning strikes on any part of the grid. There are probably other reasons I don't understand, also.

It seems to me that the local nature of a portable generator and its grid makes it highly improbable that lightning will strike it (and if it does, it's so small the whole thing will be fried anyway, grounded or not); and not grounding it is really safer for the personnel handling it. That's my opinion, anyway.

class="Body" Wade K. Ramsey, DP Dept. of Cinema & Video

class="Body" Production Bob Jones University

class="Body" Greenville, SC 29614


Wade. Grounding is necessary for electricity to work. In your house the white return wire is connected to the water main (the ground), which is also what is connected to the green wire. Electricity seeks the path of least resistance. If it has a choice between going to a neutral (return- the white wire in American homes), a ground (a safety measure to prevent the metal encasements of the electrical apparatus from becoming "HOT"), or you. Most likely it will choose the others, as you are relatively resistant. HOWEVER should those fail, or not be in existence, then since the ground you walk on has an electrical potential of 0, and the potential (positive or negative) of the electricity is 110, or 220. It may seek to jump to the ground and complete the circuit. That's how I look at it any way. Whether or not the source is grounded has nothing to do with it. If the potential exists (difference between voltage states - not sure that this is a technical term), then electricity will look to flow, and it will look for the easiest path. That may be through you if you happen to be around, because the human body is more conductive than air.

As for grounding a generator, I don't know the reason for it, except it seems to make sense to me. Then again I'm not a gaffer, and didn't do any big Genny jobs when I was an electric.

class="Body" Steven Gladstone

class="Body" Cinematographer - Gladstone Films

class="Body" Cinematography Mailing List - Listmum

class="Body" New York, U.S.A.


I believe both Steven and Richard are still thinking in terms of a commercial power grid. Yes, electricity always seeks the path of least resistance. But the primary reason it is seeking ground is that the source at the power plant is grounded, making ground a return, in effect! As I said before, they have good reasons to do that on a massive power grid, but why on a small, localized "grid?" The reason we now have a green wire in commercial wiring is because of the danger of your becoming the path to the grounded neutral when a housing becomes hot. The green wire is a better conductor than you are.

I'm sure both of you have, at some time or another, worked on a hot tie-in. You know that so long as you are not grounded (standing on a wooden box or whatever) you can touch a hot leg and not be shocked, so long as you don't touch a return at the same time. Why is that? You have a zero potential and it may be 220. Still, there is no danger, so long as you are careful (PLEASE DON'T ANYONE TRY THIS--IT IS ONE OF THOSE EXPEDIENCES FRAUGHT WITH THE POSSIBILITY OF DEATH IF YOU ARE CARELESS!) The 220 isn't seeking zero potential, it is seeking a path home to complete the circuit. You don't complete the circuit if you are not grounded.

So on a portable genny setup, my contention is that if it is not grounded, there is no path home except the return wire. You can be standing in a puddle and grab the hot lead without being shocked (but I still wouldn't try it! Some zealous electrician may have grounded the genny when you weren't looking! Or the tongue of the genny's trailer is sitting on a damp surface, at least partially grounding it.)

Current flows only when there is a complete circuit. When a system is grounded the earth becomes part of the circuit. When it isn't, it isn't.

My non-expert opinion. I'm still going to talk to an experienced electrician.

class="Body" Wade K. Ramsey, DP

class="Body" Dept. of Cinema & Video Production

class="Body" Bob Jones University

class="Body" Greenville, SC 29614


I finally called an experienced, licensed electrician about this grounding business. He agreed with me that on small, localized generator circuits there is no good reason to ground the system--it only poses a potential for shocking personnel when a housing shorts out. There is no lightning problem, as there is on large commerical

grids--a lightning strike would fry everything anyway, so you don't have the concern about big voltage surges on one part of your circuit as compared to another part, as you do on huge commercial power grids.

In fact, he pointed out that when he was helping to install a 3 phase 480v Delta supply for some large cooking ovens they elected not to ground that system, even tho' it is on the commercial supply, because with such a high voltage, to make ground potentially part of the circuit would be too dangerous to personnel in case of a short. They did install warning lights to indicate any leakage, tho'. That's an usual type situation--very heavy current and voltage being supplied to equipment in a large kitchen, where frequently the floors get wet and lots of personnel are always working around it.

I imagine that many electricians working with location gennies for filming have had the grounding thing so imbedded in their thinking that they just assume any and all systems have to be grounded. For connections to commercial power, yes. For small portable systems, no--safer for personnel.

class="Body" Wade K. Ramsey, DP

class="Body" Dept. of Cinema & Video Production

class="Body" Bob Jones University

class="Body" Greenville, SC 29614


You would not believe how many people work on Set Lighting crews who do not understand electricity - who have learned procedures by rote and who do not even care to learn the theory.

Quoting verbatim from the letter distributed byLocal 728 entitled: Basic Electrical Safety Precautions for Motion Picture and Television Off Studio Lot Location Productions This letter is dated August 01, 2000 and is issued by the City of Los Angeles Dept. of Building and Safety and is signed by Rob't England, Chief Electrical Inspector, City of Los Angeles

I am unable right now to turn it into text - scanner/OCR problems, but I can email anyone a jpg of the letter if they want it.

Here, then, the salient points from part A : Grounding

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).

Generator Grounding Connections (When Required) Interior water pipes, interieor metal fixtures, metal frames of buildings, and building grounding electrode system SHALL NOT BE USED as a grounding connection for mobile generators supplying power exclusively to location production systems.

When mobile generators supply power to location production systems IN ADDITION TO the building's electrical system, the generator's grounding connection SHALL BE BONDED TO THE MAIN BUILDING GOUNDING ELECTRODE SYSTEM AT THE SERVICE.

Multiple generators shall have their grounding connections bonded to each other when located within 20 feet of each other or when one supplies equipment which might possibly come within 20 feet of equipment suplied by the other(s)

Notes: Upper case words are underlined in the letter. A grip arm pounded into the sand does NOT satisfy article 250-83 (c) or (d)

Well, gentlemen, ladies, and grips, that's the law as it applies to our industry. If any one has any questions as to why they have written it this way, I would be glad to speculate as to their reasoning.

class="Body" Mark H. Weingartner

class="Body" Lighting and VFX for Motion Pictures


Not to start the whole California/Canada thing again, but up here, we are required by law (the provincial electrical code) to bond our generators directly to an earth ground. We are supplied with 10' ground rods that must be sunk into the ground to a depth of 6' at least, and spaced 10' apart.

When supplying power to any set or location with an existing power grid, we are required to bond to its earth ground.

Regardless of what we believe to be safe or adequate grounding, our electrical inspectors (who inspect each and every location shoot in the Greater Vancouver area) decide when we've satisfied the letter of the law.

I must say that I have seen amazingly few electrocutions in the time that I have worked in film. Our equipment is quite strictly regulated and maintained, and it seems to me that more often than not, when people have difficulty with electricity on a film set, it has more to do with the faulty wiring of rented props/set dec or simply existing location wiring.

My 2c eh...

class="Body" Phil Klapwyk

class="Body" Gaffer - Vancouver


 

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