3 phase fault?

[Jay Glaus]

Curious if anyone has dealt with this before. I think I have a fault somewhere in my 3 phase service.

I have a 23B at the drive in connected to two hot legs of my 3 phase. Worked fine for 9 years. However… It just started not lighting the bulb when one of the other projectors wasn’t on, which never happened before. I read the diag package and the lamp power supply log is saying a limit was reached and the voltage was 277.4 v. When I read back before this problem when everything worked fine, the voltage was 240.2. I took a reading of my 3 phase each leg to ground in the panel and got 138v 154v and 129v. Each leg of this 3 phase to ground should be approximately 120 ish to ground.

Im thinking I might need to call the power company. Anyone experience this before?

[Paul H. Rayton]

I know it's possible to lose one phase (of the 3), and certain devices will keep working, some will be at a reduced voltage (light bulbs are dim), and some devices are off -- it all depends on which pair of phases they've connected to. So, yes, it sounds to me like it's time to call your power co. (Years ago, I had such a situation with the invitational premier of the movie, "Hot Shots!", on all 3 screens of my cinema in L.A. Everyone had to leave and go over to the Fox lot to see the show, since the local DWP couldn't estimate restored full power for ~3 or 4 hours... Some fun!)

[Jay Glaus]

Yeah I’ve been down that road before. I think the utility co might have 2 transformers going bad and making 1 leg somewhat high and another leg very high.

[Jack Ondracek]

Something like this happened to me 3 months ago. Thanks to an errant chipmunk, we lost a fuse on the high side of one of the transformers, and my voltages went all over the place. By the time we got it fixed, I lost a microphone processor, 2 computers and $2,800 worth of parts in one of the heat pumps that cool my booth. My 32B was very unhappy, but made it through at reduced lamp wattage. The chipmunk didn't fare so well.

First-off, 277 volts is crazy... turn everything off! Beyond that, your line-to-neutral voltages are out of balance. If you have a Delta configuration (sounds like it... 2 transformers... blanks on every 3rd slot in your breaker box that isn't running a 240 line out?) , you'll have a "wild" leg that will be high, but it should be higher than 157 volts. The other two should be closer to 120. Line-to-line should be 240, with small variations allowed... maybe a few volts. More than that and something is out of balance, and that could be either you or the power company.

A Wye configuration would normally be 110/208, with the same line-to-neutral voltage on all 3 legs. Doesn't sound like that's you, though.

Your power company could be helpful. Up here, unfortunately, they don't believe anyone else knows anything. If you're having any trouble convincing them, you might need to hire an electrician to take measurements and intervene.

[Steve Guttag]

I'm, more or less, on the same page as Jack. Though it is 120/208 for a typical Wye in a commercial application. Hi-leg systems will have two legs that can often be used as 120V circuits and you'll see every third beaker slot blanked off. Regardless, the leg-to-leg should be balanced, no matter which two you measure. If your breaker panel is supplying 120V to normal outlets and you don't have every 3rd breaker blanked off (except for 220ish circuits), then you definitely have the potential to damage a lot of stuff there. Most 120V equipment can safely deal with 100-130V but you are taking your chances above 125V. Most 220V stuff can safely deal with 200-240 without issue too. I think the big clue is how far out of balance your power is. If it is a WYE system and you picked the two highest legs to supply power to the projector, I would have expected a true RMS meter to show about 252-253 volts...which is above where you'd want to be, regardless what the projector is telling you. I would definitely call the power company.

[Leo Enticknap]

The chipmunk didn't fare so well.

Reminds me of an incident involving an Irem rectifier back in England in the '90s. I was working for a small chain of arthouses, and got a call from a projectionist at another site who was having increasing difficulty getting a lamp to strike, and reported a nasty smell in the booth when she finally got it lit after half a dozen zaps. The theater in question was a 1920s building, ridden with damp and dry rot, and which hadn't been maintained well for at least a half century. Long story short: when the rectifier was finally opened up, a deceased and cremated rat was discovered inside, shorting two of the three input phases. The ultimate variable resistor, I guess (though maybe Ted Bundy has a better claim to that title?), except that the ohm reading changed in only one direction...