Ungrounded System Questions

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nhee

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An existing plant has a 480 V 3 ph. ungrounded service. The main service equipment in one building feeds 3 ph. power to another building, 1000 feet away (same site). The feeder is 3 wire in non-metallic conduit.

Both buildings (and all other buildings on site) are tied to an extensive 4/0 buried ground electrode system, and also have many other metallic paths (process piping) between buildings. All electrical equipment, panels, etc. are bonded to this electrode system.

The entire facility electrial system is being expanded as part of an overall plant expansion. A new utility service will be installed. Grounded delta or grounded wye supplies are being considered. The existing feeder described above was intended to remain unchanged. Re-looking at this feeder, it looks like 250.32(B)(1) will require a new EGC be run between buildings, with the phase conductors, if the new service is grounded.

Am I reading this correctly?
 
Re: Ungrounded System Questions

Originally posted by nhee:
An existing plant has a 480 V 3 ph. ungrounded service. ...

Both buildings (and all other buildings on site) are tied to an extensive 4/0 buried ground electrode system, ...

A new utility service will be installed. Grounded delta or grounded wye supplies are being considered. The existing feeder described above was intended to remain unchanged. Re-looking at this feeder, it looks like 250.32(B)(1) will require a new EGC be run between buildings, with the phase conductors, if the new service is grounded.

Am I reading this correctly?
Click to expand...
Assumptions:
1. New service comes into the same building as the existing service.

2. New feeders from the new service will be run to the separate building.

Yes, I think you are reading Art 250 correctly.


Question: How come they are going with a grounded service?

The only reason I can think of is the company doesn't want to pay for the transformer, and the only service the utility will provide is grounded.

carl
 
Re: Ungrounded System Questions

Question: How come they are going with a grounded service?


prior to reviewing the interconnecting installations described above, my answer would be "Why not?"

Although the site is industrial, there is no permananent on-site electrical maintenance staff. Without personnel on site to monitor/troubleshoot the ground detection systems and correct L-G faults if/when they occur on an ungrounded system, I was thinking the grounded design would be better. Maybe I'm overthinking that aspect.
 
Re: Ungrounded System Questions

"Ungrounded" systems have the advantage of not tripping on the first ground fault, allowing time for troubleshooting without disturbing the plant process. Ground fault current might be low also. (No such thing as an ungrounded system, the system capacitance leakage will always provide a ground fault current path, but it will be of unknown and uncontrolled impedance.)

If keeping the process running is a concern, go to a high resistance grounded system with alarms when a ground fault occurs. These are very successful and safe, but the electrical maintenance and operating staff must be trained, see 250.36.

An EGC would still be needed. There is always the chance that two ground faults will occur.

BTW, the high resistance grounding also reduces the arc energy for ground faults and keeps ground fault damage to barely visible arc marks. But the arc energy for a phase-phase fault is still there so it doesn't help reduce NFPA 70E arc flash requirements.
 
Re: Ungrounded System Questions

You don't say how the transformer is connected to the original service - Delta-Delta or Delta-Wye? AND, how is the 120VAC obtained (for office equipment) in the buildings? Was this industrial plant constructed before the 1980's?
 
Re: Ungrounded System Questions

thanks, rcwilson.

mark, The plant is early '60's vintage.

Any 120/208 systems are grounded wye, tied to the common ground electrode system.

[ June 08, 2005, 08:51 AM: Message edited by: nhee ]
 
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