OT: Ungrounded system

[Lancie1]

Actually, Lancie1, the second ground fault will open a phase breaker or fuse.

It should, if everything goes according to plan, and the breakers are not full of metal shavings, accumulated dust and dirt, mouse dens, snakes, or insects. I know of one case (the old Diamond Shamrock plant in Sheffield, AL) where the second fault did not trip the 12kv breaker. The main tried to open (we assumed), but arced and started a meltdown. I was not there, thank God, but I saw it afterward. I could not recognize any of the electrical equipment. There was a pile of slag in the floor. The metal roof had burned off except for a few pieces around the walls. Pieces of hot metal had been blown in a circle all around the block building. The switchgear room, all breakers, busses, and the main 161/12 kv transformer were a complete loss.

I think if the breaker had been able to open, the damage would have been limited. The switchgear was old and probably in need of cleaning. The ungrounded system did not cause the damage, but the fact that the delta-delta transformers allowed a first fault with no breaker trip lead to the second fault, which was not a mild easier-controlled phase-to-ground but a worst-case phase-to-phase fault.

 

[OdinIII]

I'm going to peek my head up over the wall and say that I think I work at one of the places with an ungrounded 575 VAC system. I've been here a long time and really didn't know that this type of system was considered to be so unsafe.

Just to be clear all of our equipment is grounded but not referenced or bonded to the 575VAC system. One ground = one ground light off... Two grounds = all ground lights off.

I'm intrigued about the problems mentioned with high "leg to ground" AC and DC. I'm going to try to check this more often and see if I can find any examples of this being an obvious problem here.

We do have one brand of VFD that seems to fail too quickly and one lighting transformer that we've had trouble with failing and running hot without being current overloaded. Another brand of VFD which runs fine elsewhere trips on high buss voltage without reaching the fault SP on an intermittent basis in one application.

There's plenty of food for thought in this thread for me. I have much research to do about this subject. I had never thought about the possibility of dust or lint bypassing a disconnect due to higher than normal voltage to ground. Wow.. just when you thought you were safe!

 

[The Plc Kid]

The lighting transformer issue is likely due to harmonics. Neutral may be sized too small also.

The drive issue is likely from the ungrounded system.

 

[Lancie1]

Just to be clear all of our equipment is grounded but not referenced or bonded to the 575VAC system. One ground = one ground light off... Two grounds = all ground lights off.

Yes, I would bet that your system is not hard-grounded, and that the 575 volt transformer does not have Wye secondary windings. I am sorry and will keep you in my prayers for a safe journey.

The lighting transformer issue is likely due to harmonics. Neutral may be sized too small also.

Kid, with a delta secondary, there is no neutral. That is usually part of the problem - no good place to attach a ground wire. That is, if the lights are 480 volt or higher. Maybe Odin does have a grounded transformer feeding the lights at a lower voltage.

However, many plants (in the U.S.) now use 480 volt metal halide lighting due to the high fixture mounting heights for 1000 watt fixtures. I designed several with 480 volt lighting (not 277 volts, but the full phase-to-phase 480 volts). With 50-feet high stamping presses, 1000 watt fixtures are one of the few choices, and often require higher voltage to make the circuits practical, with reasonable wire sizes.

 

[OdinIII]

The lighting transformer issue is likely due to harmonics. Neutral may be sized too small also.

The drive issue is likely from the ungrounded system.

Harmonics is what we suspected but the power company has put recording equipment at the specific panel multiple times and they claim that there isn't any issues with the power.

I've learned through the years that sometimes people who should know things can be completely incompetent so I can't say for sure if they even checked for the correct problem.

 

[OdinIII]

Yes, I would bet that your system is not hard-grounded, and that the 575 volt transformer does not have Wye secondary windings. I am sorry and will keep you in my prayers for a safe journey.

Thanks Lancie, You are correct, we have no neutral on the 575 systems. Am I understanding you correctly that the extra hazard comes into play because of the extra voltage present that has to be switched off if a 2nd ground fault occurs? This leads to greater likelihood of deadly arc flash?

Maybe Odin does have a grounded transformer feeding the lights at a lower voltage.

The transformer mentioned is a 600 delta to 208/120 Wye.

 

[Lancie1]

Am I understanding you correctly that the extra hazard comes into play because of the extra voltage present that has to be switched off if a 2nd ground fault occurs? This leads to greater likelihood of deadly arc flash?

Yes, that is one of the problems. Read the response of DickDV for his experiences with other problems when troubleshooting and using voltmeters. Some meters will not have the insulation rating to safely measured the ungrounded voltages in a delta-type ungrounded system.

The transformer mentioned is a 600 delta to 208/120 Wye.

Hopefully then this transformer should have a hard ground point on the Wye side, making it safer to use and to troubleshoot.

It is the transformers that have Delta-Delta or Wye-Delta configurations that usually do not have a hard-ground point. Some are "corner grounded" or "resistance grounded", but both those methods have problems and can cause safety issues.