Blowing Lamps

[TupeloTechie]

Today I was doing lighting for a one act festival, and as I was bringing down the lights I see a big blue-white flash, and the next scene I have a gigantic dark spot downstage left. After the show I look at the AP Slot and about 3 or 4 lights were not working, I haven't had a chance to go up and see whats going on, but I was wondering what might have caused this and has anyone else experienced it?

[Charc]

Surge, my only guess. Was this an outside gig?

[TupeloTechie]

thats what I was kinda thinking, now that I think about it I think it also happened again on the 2nd electric, as a lamp blew, I thought it was just the flash of a camera, but I guess I was mistaken.

and it was indoor, not outdoor.

[icewolf08]

Originally Posted by charcoaldabs

Surge, my only guess. Was this an outside gig?

Maybe you can get points for a good answer.

Most likely a power spike. Least likely: all the lamps just died at exactly the same time.

[JD]

Surge is one possibility. Here are some thoughts: Were all the lights on one dimmer? Have you checked to see if that dimmer is still working? Here's what is bothering me- You said it was as you were bringing the lights down. As your lights are going down, stress on the neutral is the greatest (is on the way up too) usually around 30-50% You might want to have a qualified electrician check all of your neutral feeds in the dimmer system. If one is not tight, you could have had a momentary loss of neutral and actually over-voltaged those lamps.

[David Ashton]

First rule of fault finding is do not assume anything.
Collect the information like
1-are the lamps on the same dimmer
2 are they on the same phase
3 are they on different phases but in the same rack
4 was any other gear in the building effected
With this sort of information we can narrow down the problem and bad neutral is a common cause, but a little analysis before stripping down your dimmers could save time and money, because you then are following a logical path to the problem rather than just trying different ideas.

[TimMiller]

not to hijack the thread or anything but i havent had anyone explain to me about how the hot leg spikes up when the neutral is disconnected. Lets use a 3 phase power source for example (120/208) connected to a 24 ch dimmer rack (dimmer rack is 3 ph). I have 6 par cans plugged in one on each channel so i have 2 par cans connected to lines L1,2,3. I pull the neutral, ground takes its place as an alternate path, in this THEORETICAL EXPERIMENT (since i would never do this) lets say ground gets disconnected also, (really bad hs electrician). Wont the lights just all turn off. (I know in reality i'm sure the new path to ground would actually become the conduit, since it is designed to act like a ground and everything would work fine.) But how would the voltage go "high" and make the lamps blow up if there is nothing else to work off of, just because neutral and ground dissapears, doesnt mean the voltage would change, now if the neutral comes off and contacts another line such as par can connected to L1, neutral jumps off of bar and decideds to play with L2 then yes you would have 208 going to your poor par can no matter what and ground wont help you. Am i seeing all of this correctly or am i missing something. I have just started taking classes in AC circuits to learn more b/c you can never learn to much or know everything.

[David Ashton]

Right, disregard the earth, it should only be connected to the metal on your lights, it plays no other role.
If you lose the neutral the neutral point in your dimmer moves towards the heaviest loaded phases increasing the voltage across the lightest phase and blowing up lamps, if your lamps are at say 50% then the " neutral" is whizzing around chasing the imbalances and will eventually annihilate the dimmer.You are putting an unbalanced load onto the 3 phase.
The whole exercise is much more exciting on 240/440v

[JD]

Originally Posted by TimMiller

I pull the neutral, ground takes its place as an alternate path

The ground path should a no point be connected to the neutral (except at the service entrance) so it would not become an alternate neutral path. As "Allthings" stated above, the load imbalance would quickly take out the leg with the lowest load.

I think it was in Allentown PA in the mid 80's, but I watched as REM lost a lot of their system when the bolt holding their neutral lug snapped mid show. Guess the house electrician torqued it down a bit too much. Lost neutrals are not a pretty sight.

EDIT:
As for where the over-voltage comes from, imagine you have a single phase 120-0-120 feed. (two hots and a neutral) From one hot to neutral, you have a 200 ohm load, from the other hot to neutral you have a 40 ohm load. As long as both loads see neutral, both loads will have 120 volts across them. Now, disconnect the neutral. You now have a 240 ohm load on 240 volts, or about a volt per ohm. If at this point you looked at the junction, you would find 40 volts dropping across the 40 ohm load, and 200 volts dropping across the 200 ohm load. In other words, what was the neutral point has shifted 80 volts producing an under-voltage on the 40 ohm load and an over-voltage on the 200 ohm load. I hope this helps.

Remember, ohms = volts / amps, so the heavier the load the lower the ohmage.

[David Ashton]

One thing I forgot to mention.If you plug everything up and switch on and many of your lights all start to flicker, turn off power quickly, this is a classic "no neutral" symptom.