Fault in an Ungrounded System 2

[ReliabilityPF]

My UPS system is ungrounded and monophase (120VAC), it seems like I have a problem in the distribution because the neutral momentarily went intentionally to ground and the voltage decrease. It seems like I have a fault or a poor isolation in the cables in some part of the system. The ground fault doesn’t alarm. The system should allow the neutral go to earth. What should I do?, should I check for a fault? the problem is that I can't shutdown any breakers, I can't affect the production. The Neutral to Ground voltage is 72Volts is this normal.

 

[peebee]

"My UPS system is ungrounded": sounds like a code violation and a safety issue to me. You should ground your system in accordance with code requirements and manufacturer's recommendations.

"the neutral momentarily went intentionally to ground and the voltage decrease.": What voltage? Where did you measure this? Please be more specific.

"It seems like I have a fault or a poor isolation in the cables in some part of the system.": Why? What makes you think so?

"The ground fault doesn’t alarm.": You wouldn't expect it to if your system is ungrounded.

"the problem is that I can't shutdown any breakers, I can't affect the production.": The problem is your system will likely fail on it's own, dropping production. You can deal with that but you can't deal with a planned shutdown?

"The Neutral to Ground voltage is 72Volts is this normal.": No, it's very high and sounds unsafe to me. Could well start leading to equipment failure too.

 

[ScottyUK]

I agree with Peebee - sounds dodgy as hell.

Better to have a few hours planned downtime than wait for either equipment failure or someone being injured or killed. Nothing quite like the latter to affect production.

 

[ReliabilityPF]

peebee
It sure is a violation of the NEC, that is wy there is ground fault detector. Configuration of this type are made for getting high reliability.
The voltage decreas in the output of the UPS. The control of a TurboGenerator trip because it has a low-voltage protection.

 

[peebee]

Violating NEC is no way to increase reliability. Matter of fact, I just about guarantee that you've decreased your reliability. You're just fooling yourself if you think otherwise.

Re: "The voltage decreas in the output of the UPS." Where? Line to line? Line to Neutral? Neutral to Ground?

Re: "The control of a TurboGenerator trip because it has a low-voltage protection." What?

 

[busbar]

POS UPSs rarely have low enough source impedance to operate branch-circuit overcurrent devices.

"...I can't shutdown any breakers..." is just a chest-beating contest between maintenance and production. It may take a nicely timed unscheduled shutdown to affect the short-sighted production-management braintrust. Try not to laugh too hard.

 

[ScottyUK]

ReliabilityPF,

Most of us seem to agree that your installation is unsafe and probably in contravention of the electrical regulations I expect you will want to address the problem. Here's some thoughts:

Is the UPS working ok apart from the (not insignificant)grounding issue? If so, you might consider a 1-1 ratio isolation transformer directly coupled to the output. Earth one end of the secondary to a good earth. This becomes your neutral. This should eliminate the grounding problem and make the installation safer by having a neutral at earth potential. Downsides are the cost of the transformer, and some slight degradation to the voltage regulation, as the transformer will have a non-zero impedance.

Not sure what the NEC says about this sort of practice as I'm on the other side of the Atlantic, but UK regulations allow (don't disallow?) it.

Scotty.

 

[nukeman]

Let's not all jump on the band wagon. Ungrounded (or floating) UPS's are common in many applications in power generation stations. Power generation stations (especially nuclear) are not committed to NEC requirements and being ungrounded is not a code violation for these plants. Being ungrounded does pose a safety concern but trained electricains should be able to work on the system safely. And yes ungrounded systems have ground alarms. It sounds like one of your loads is indeed grounded. With one load grounded the system will still function but if you get a second ground you will have a huge problem. Either your inverter will blow a fuse or circulating currents can cause other protective circuits to pickup. In either case you will probably loose your loads. Since you have a "hard" ground you should be able to trace it down using ground detection equipment such as "The Ground Hog" or "GroundBuster" by Biddle. Look on the internet for this equipment.

 

[huntercon]

I agree with nukeman - there have been plenty of advancements in GF detection & location technology over the past 10-20 years - especially in Europe. Hospitals use this sort of isolated system in many applications.

Along with the Ground Hog & the Megger unit, I would like to add Bender (

http://www.bender.org).

I have specified Bender's online ground fault location system before. It tells you when you have a 'hard' ground or developing ground, then starts a location function which isolates the ground down to circuit level. There is also another option which enables you to locate the fault down to component level.

 

[jbartos]

Suggestion: Reference:
NFPA 70-2002 National Electrical Code (NEC)
states in Article 250.4 "General Requirements for Grounding and Bonding," Section (B) Ungrounded
various requirements for an ungrounded system.