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Odd Amperage

Published : 7th August 2007

I experienced a strange situation last week with a 12K HMI. It was connected to a 500amp 3phase genny, properly balanced with tungsten heads, but both of the hot legs were pulling 100-125amps-- each. The reason we noticed the problem was that the 220v bates breaker on the distro box kept tripping. We ran the ballast directly to banded and it ran fine all day.

Why would it be pulling over 200amps instead of the expected ~60amps per leg?

Paul Marschall, DP


If it was running on a magnetic ballast, this could be an indication that some of the big scary capacitors are shot. If it was running on an electronic ballast, there may be some magic juju in the box that is not right.

I am assuming that you checked to make sure that you were making 208v across the two hot legs in question - one thing that happens with power regulating ballasts is that if the line voltage is low, they will draw more amperage than if the line voltage is right or high.

Mark Weingartner
LA based
(many years since I have had to fix an HMI )


Paul Marschall wrote:

>>I experienced a strange situation last week with a 12K HMI. It was >>connected to a 500amp 3phase genny, properly balanced with >>tungsten heads, but both of the hot legs were pulling 100-125amps-- >>each.

Was the light still pulling over 200amps after you ran it directly to the banded?

Did anyone measure voltage on the light side of breaker/distro?

What Mark Weingartner said: I would suspect a voltage drop -- perhaps poor contacts in the breaker/distro as well.

Brian Heller
IA 600 DP


Brian and Mark raise interesting issues and offer good things to check.

Another possibility is that the bulb is old. If so, a check with a colour temp meter may indicate a very warm or very cool shift in degrees Kelvin. As HMI bulbs age, the gap between the anode and cathode increases and requires more voltage to maintain the arc.

Next, record the serial number of the bulb and check with the rental house, most houses record the number when the bulb is purchased and you can find out how long its been in circulation.

This is a possibility but to have such an increase in amperage as you describe is kind of spooky...

Remember that old HMI bulbs can explode while being struck or running. I check the colour temperature regularly and if a bulb shifts its CT output significantly, or you see a magenta or green shift, its time to put the spare in and send the original back to the rental house to be tested. The added damage to a fixture can be quite pricey if the bulb does manage to explode.

Andrew Gordon
Gaffer
Regina, Saskatchewan
Canada


Andrew Gordon wrote:

>>Remember that old HMI bulbs can explode while being struck or >>running.

I didn't know they could explode... I thought it was Xenon bulbs that where under pressure not HMIs. What makes them explode and how often does it happen?

In 15 years I have never even heard of one exploding, though plenty have failed completely... smoke pouring out of the head.

Piotr Jagninski
Gaffer / New York City


>>I didn't know they could explode... I thought it was Xenon bulbs that >>where under pressure not HMIs. What makes them explode and how >>often does it happen?

HMIs operate under extreme pressure of at least 10 atmospheres up to 20. One correction is that any HMI lamp at any age can explode. It's a rare occurrence and hence why you have never seen it, but when it happens, look out!

From my new book, "I'm not getting paid enough anyway, so why am I here" -if you want to sabotage a shoot take a sharp blade and make a vertical "S" shape scratch in a lamp vertically. Then sit back (far enough) because the lamp will explode about ten minutes after striking.

Ok the book isn't real but doing that to a lamp will demonstrate explosions.

Walter Graff
NYC


Walter Graf writes:

>>One correction is that any HMI lamp at any age can explode. It's a rare >>occurrence and hence why you have never seen it, but when it >>happens, look out!

I was working on still photo when a 250w halogen modelling light in a Chimera spontaneously popped.

Milliseconds later, this Chimera acquired a new usefulness, a stars-in-sky gobo.

Nobody was hit by the molten glass.

Cliff Hancuff
Washington, DC


Walter wrote:

>>HMIs operate under extreme pressure of at least 10 atmospheres up >>to 20.
>>One correction is that any HMI lamp at any age can explode.


That's for sure. But you don't have to see it. You'll hear it. If it's in a Fresnel, it will usually take the lens with it.

Be sure the wire lens guards are in place. They are there for a very good reason, and as Walter says, it's not necessarily age related.

Brian "prefers not to have HMIs behind the camera" Heller
IA 600 DP


Being that we are on the subject of exploding globes,

I have a question for you all.

Aside from cracks in the globe it self, is there any other reason small tungsten units like mighty moles can have a globe literally melt and fall out of the light?

On the show I am working on right now, last Thursday, we had a 2k Mighty pointed almost straight down for about 45 minutes, until the globe started to sputter and then literally melt out of the harness. Aside from an imperfection in the globe, is there any other reasons for the possibility of this happening? I know that Fresnels are not suppose to be pointed straight down, but does that apply to open face tungsten units as well ?

Jeremy Schonwald
Gaffer/BB Electric LA


>>we had a 2k Mighty pointed almost straight down for about 45 minutes, >>until the globe started to sputter and then literally melt out of the >>harness.

Two words : Heat rises.

What better way to melt somethin' than to provide a bowl to collect heat that rises?

Cliff Hancuff
Washington, DC


Walter Graff wrote:

>>HMIs operate under extreme pressure of at least 10 atmospheres up >>to 20.

Just so I completely understand... are you saying that "while they are operating" they are under extreme pressure or is this also the case when the bulb is cold (the light is off and the bulb has cooled down)? I know that Xenon bulbs are under sufficient internal pressure (even when cold) that these can explode when simply being changed (i.e. use protective glasses when changing the bulb and handle them with great care).

Piotr Jagninski
Gaffer / New York City


>>Being that we are on the subject of exploding globes, I have a question >>for you all. Aside from cracks in the globe it self, is there any other >>reason small tungsten units like mighty moles can have a globe >>literally melt and fall out of the light?

Any fingerprints, dirt, or even a bug on the globe will provide a dark spot for heat to focus and accumulate thus melting the globe. The fixture doesn't have to be pointed straight down either. The pressure from the contact springs on the softened globe and gravity will do the rest.

Brian Heller
IA 600 DP


Out on the skinny limbs of geek'dom lives the belief that fingerprints and dirt on these bulbs cause problems because these bulbs operate near the melting point of their glass ... fingerprints/dirt introduce an impurity that lowers this glass' "pop" point.

Cliff "Geek, But Only A Tourist There" Hancuff
Washington, DC


>>I know that Fresnels are not suppose to be pointed straight down, but >>does that apply to open face tungsten units as well ?

We had an 18k electronic ballast implode on us last week which I've seen several times prior, but never seen an HMI globe go. Usually when an incandescent globe is ready to violently explode it will glow off colour for several seconds giving you a chance to turn it off or at least run away :)

Erik Messerschmidt
CLT, LA


Hi Erik

When HMI globes first came into general use (wow am I really that old)we treated them with extreme caution when handling them. Now you wouldn't take any more precautions than handling a tungsten globe. Guess this means they are pretty robust.

I have had a double ended 6k HMI Frez globe explode, it was lighting an exterior set, a large explosion, glass from the lens and globe projected out through the safety wire, even the special effects guys were impressed. I was side on to it and was amazed how much glass, small fragments I guess, made it past the safety wire.

The remains of the globe consisted of only the metal connectors with a half inch or so of the support glass. For a while I considered running the larger HMI lamps with a half stop wire if they were pointed at an actor.

I never had another HMI globe explode. I have seen them dropped and broken, a cold 12k globe, it just broke into about three pieces as though it was unpressurized.

Working in extremely dusty conditions recently I had a 18k globe form a large blister on top of the globe due to fine dust becoming impregnated into the glass. I suspect the globe was cooling down when dust got into the head.

Graham Rutherford
Gaffer
Australia


Correct me if I am wrong, but I understand that an HMI bulb is roughly 4 atmospheres of pressure and a Xenon is roughly 7 atmospheres.

Either bulb, hot or cold, will pack a wollop if they go off. I know of a certain rental house with a hole in its concrete floor from a dropped Xenon bulb...

I have seen ( or heard ) three HMI bulbs go off in my career. The 4K par was the loudest and messiest. The 18K was a close second (suprisingly not taking out the Fresnel) and the 1.2 par the third loudest.

The closest calls I have ever had are two 4K Fresnel double ended bulbs whose colour temperatures started exhibiting some very distinct colour shifts combined with a low output. Once opened up, both bulbs had huge blisters on them not unlike warts. I still have both of them as examples to show trainees.

Having to answer the question of "why did it blow up?" from producers who have to pay the bill is most unpleasant and should be avoided, if at all possible.

Andrew Gordon
Gaffer
Regina, Saskatchewan
Canada


>>On the show I am working on right now, last Thursday, we had a 2k >>Mighty pointed almost straight down for about 45 minutes, until the >>globe started to sputter and then literally melt out of the harness.

You tilted the lamp too much. For smaller lamps it doesn't matter, but from 2k on, it's best to not tilt them down more then 45 degrees (the maximum angle should be noted somewhere on the casing). If you tilt them down more, the heat can not dissipate enough, and lamps and reflectors will melt.

You can tilt them down more, but will have to switch them off frequently
to let the lamps cool down.

Cheers

Martin Heffels
/filmmaker/DP/editor/
Maastricht, the Netherlands


>>I have had a double ended 6k HMI Frez globe explode, it was lighting >>an exterior set, a large explosion, glass from the lens and globe >>projected out through the safety wire, even the special effects guys >>were impressed.

I'm not saying this doesn't happen anymore at all, just that I've never seen it with any HMI globes. I have seen tungsten globes go violently so I try to minimize how many open faced units get hung overhead and make sure that they are properly maintained but it sounds like your double ended 6k was perhaps a bit past it's time or had, like you said, been contaminated with dust. Double ended globes can often fall victim to hairline fractures, particularly near the ends. They should be inspected everyday, especially if they are bumping around in trucks.

I don't know the exact science of it, but I would suspect that when a globe "explodes" it's because of a pressure change due to a leak in the glass envelope. As the heated gas escapes from the globe the pressure and temperature change cause the already superheated quartz glass to fracture, much like a burst helium balloon. High heat resistant glass is already extremely brittle, so it doesn't take much to tear it apart. This would explain why a cold globe simply breaks apart with little fanfare.

Perhaps someone with a better physics background could elaborate.

Erik Messerschmidt
CLT, LA


>>Its just poor heat dissipation. usually the sputtering your hearing is a >>momentary ground fault in the head caused by melted wiring or a hole >>in the globe just before it goes.

No sure about quartz, but glass becomes conductive at molten temperatures.

Clive Mitchell
http:/www.bigclive.com




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