The Scenario:

Gymnasium, several rows of 400W MH ceiling fixtures.

One row (all on same circuit) has 10 lights fail on the same day. 2 lights on this circuit still work.

Other rows of lights on the same panel are fine. These are on different breakers.

Voltage is good on all 3 legs and at fixtures, breaker seems fine, can't find any reason for this.

Installed new ballasts and everything works.

Anybody have any guesses what happened?

You mention different breakers, but is it possible they're using a combined neutral that may have become loose? Either 3 separate phases with the common neutral or a multiwire branch circuit using both hot legs of a single phase supply and the neutral?

Certainly could have a shared neutral but I'm not sure. Either way, the voltages at the fixtures were all a solid 277V. I checked other phases at the panel and everything agreed within two volts.

There may be a bad neutral, I had thought of that too. But the consistant voltage tends to make me think that's not it.

What failed, the reactor, the bulb, or the cap?

You can bet the farm that it isn't normal end of life failure. Too coincidental for that many. Wouldn't be surprised if it recurs, since you found no cause. The last time I had a similar problem (but not near as many failed lites as you)was in a high school gym with a lower than average ceiling. The kids, when coach was not in the gym, wound the basketballs at the fixture with such force it would either short out the capacitor (which stuck out the side) or ruin the lamp. The custodian finally observed their gleeful sport one day and the riot act was soon read to them.

Any possibility of a lightning strike that day? Some fault must have been common to all that went bad.

One other weird thing i saw in a factory with rows of HID lites...a circuit contained a bad ballast that would frequently trip the breaker. They would reset the breaker but not turn off the switch to that row(which sounds normal to me). The lights would not restrke even after an hour. If the switch was turned off they would come back on after the few minute period we are all familiar with.

How would the voltage being present at the input to the ballasts keep them from being able to restrike? Isn't it just a cool down period? HID can be a real mystery at times.

What is behind the warning on HID lamps that tells you to periodically turn them off? If you leave one on forever does it sit in a state of impending explosive failure?

Is there a way to check the old ballasts to see what failed?

What failed, the reactor, the bulb, or the cap?

Ballasts. At least there were three or four that were noticibly burned by sight or smell. I assumed the rest were the same and changed both on all fixtures.

Need to check wire nuts even if you read a good voltage. If connection is loose it will heat up when in use and then could cause a floating neutral which could have damaged your fixtures. My guess is someone didn't twist their wires in one of the j-boxes. Good hunting!

Chief, a neutral dropped out somewhere! Although you may have a good nominal voltage now, you didn't for a few seconds or so, its all it takes. What Dnkldorf was going for was what part of the ballast, ie cap/starter/reactor/tranformer. It could be a clue to what happened. As you describe a stink, I imagine the transformer didn't survive an over-voltage situation. Bulb, or cap could be a transient, or for that matter, low voltage too. Either way, I would sort out what happened before you go to replace too many of them. Could get expensive.

How about the wrong voltage tap on the ballast being used?

I'd agree with the assesment that the burnt smell from the ballasts is a sign of over-voltage.

Dave

Caps don't smell to good after they explode either.


Dnk.....