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#97206 - 02/01/06 05:22 PM System Grounding Optional  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
I am interested in the advantage or disadvantage of corner grounding a 240v or 480v system. Are there any papers out there that give direction on this subject? Also, along that same line, anything on ground indicator lights. Maybe some design professionals will comment.


George Little

2017 / 2014 NEC & Related Books and Study Guides

#97207 - 02/01/06 05:54 PM Re: System Grounding Optional  
winnie  Offline
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Joined: Sep 2003
Posts: 649
boston, ma
I had to do quite a bit of reading on this subject, but for a particular installation only, rather than for general use. So I am not remotely qualified as an expert but feel that I have a reasonably informed opinion to offer. My application was a non-NEC compliant installation for temporary power on an experimental aircraft system.:

Ungrounded systems mean that a ground fault won't shut your system down. If you get a _solid_ ground fault, then a small amount of current will flow, and you will be able to continue operation of the system. Your ground fault detection system will tell you of the fault, and you can perform an orderly shutdown, locate the fault, and repair it.

The problem with ungrounded systems is that you don't have a solid voltage reference. Because of the capacitive coupling between the system and ground, if you have an _intermittent_ ground fault, you can actually pump the voltage between the system and ground to levels that substantially _exceed_ the line voltage of the system. This can lead to a situation where a minor fault on one piece of equipment can lead to high voltages and insulation breakdown all over the place.

It is my opinion that a good ground reference and proper breaker coordination are vastly preferable to an ungrounded system.

If you actually require the continuity of service benefits of an ungrounded system, than I would suggest looking at 'high impedance grounded' systems. These are wye systems where the neutral to ground bond is made using a resistor (50-100 ohms on a 480/277V system) that provides a good ground reference but limits ground fault current flow to a low value. Depending upon how the system is designed, the current through a ground fault will be perhaps 2-10 amps, which most circuits will tolerate indefinitely. You need to use the same sort of ground fault detection used for ungrounded systems, but in the event of an intermittent fault you won't see excessive voltages. Even though this is a wye system, under the NEC an impedance grounded system cannot supply line-neutral loads, and is thus limited to the same sort of loads seen on a delta system.

The companies that sell neutral grounding resistors will have a lot to say about the dangers of ungrounded systems and the benefits of resistance grounding [Linked Image]

-Jon


#97208 - 02/01/06 07:52 PM Re: System Grounding Optional  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 825
Michigan
George
Soares Grounding book covers this quite well as several others . Ground dection lights are only used on the ungrounded Delta.
It also covers impedance neutral grounding. The ungrounded systems I;ve run into have been in Saw Mills and Large Water pumping stations. The impedance neutral systems have been in paper mills.


#97209 - 02/01/06 11:14 PM Re: System Grounding Optional  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Thanks Ernie- Excellent reference. I'd forgot about Soares.


George Little

#97210 - 02/02/06 01:53 AM Re: System Grounding Optional  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,105
Estero,Fl,usa
Corner grounding simplifies the equipment you need. There are only 2 ungrounded conductors so, with the right breakers, you are basically using single phase equipment.
The only place I have seen it is in sewer lift stations and these are usually special purpose controllers.
You certainly need to read the "qualified personel only" sitckers. It would be real easy to think you are looking at a single phase box. (As Ron White says "happened to me") [Linked Image]
The motors were a tip off.


Greg Fretwell


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