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#97334 - 02/17/06 06:30 PM 250.21  
Dnkldorf  Offline
Joined: Dec 2004
Posts: 1,064
nowhere usa
I have a new customer that has a large warehouse, import/export business.

I came to find out they have a ungrounded 480V system, with an switchgear, with indicator lights.

I don't know what the prior occupants were, nor what they manufactured, but right now, it is a warehouse...some offices and lights..

So does this system stay grandfathered, or would it be best to change it to a grounded system?


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#97335 - 02/17/06 10:02 PM Re: 250.21  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
IMHO, this is not an issue of 'grandfathering'; 480V ungrounded deltas can be compliant with current code.

The questions are:

Does it offer benefit that outweighs the risks?

If the ground detector lights indicate a problem, is there someone keeping an eye on things?

How expensive would it be to retrofit a grounded service?

IMHO if they don't need the continuity of service benefits of an ungrounded system, and if they aren't _required_ to have an ungrounded system (eg cranes over combustible materials), then I'd push for a grounded system.


#97336 - 02/19/06 10:50 PM Re: 250.21  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
Consider installing a zigzag xfrmr to establish a referrence neutral and dropping in a GFCI MAIN breaker.

Delta pretty much requires a knowledgeable player on site to jump on faults. Since this occupant is not a manufacturer there is scant chance of that.

However if the customer is using 480 solely for lighting and A/C they probably won't bite.

The danger is if their maintenance man plays with the high bays and screws up. The significance of the indicator lights has to be driven home. I'd wire in an alarm contact on each branch of the indicators: stop working on the system -- call electrician immediately.


#97337 - 02/20/06 08:40 AM Re: 250.21  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Is there some reasonably inexpensive way to make the ground indicator lights _much_ more annoying, eg with a bell, or wired to a shunt trip on the main?

Prior to adding the expense of adding a transformer, how well would the system tolerate being corner grounded?


#97338 - 02/20/06 02:32 PM Re: 250.21  
sandsnow  Offline
Joined: Sep 2004
Posts: 167
Irvine, CA, USA
For a good article on corner grounded systems see this data bulletin by SQ-D:

Corner-Grounded Delta (Grounded B Phase) Systems
Class 2700
Publication # 2700DB0202

I'm assuming this is utility supplied. Will the utility convert their XFMR to a Grounded B-Phase??? If not, that's that. I'm guessing they would push to supply you with a wye system. They might want to get rid of their old ungrounded delta systems. Maybe offer some incentives??

Converting to corner grounded could be more trouble than it's worth.
Are there any fuses in B-Phase (other than overload for motors)??? They all have to come out. Breakers need to replace them or single phase fusible equip. Your local inspector "MAY" let you slug the fuses.
Higher than normal fault currents on first grounded fault.
You need a main bonding jumper in the service.
All B Phase conductors now need to be identified as white.

The offices and lights probably operate off a grounded systems via XFMRs so no problem. Is any equipment supplied directly from the ungrounded system??
That's what you need to look at.

Good Luck and please post an update.

Larry LeVoir
City of Irvine, CA

#97339 - 02/20/06 03:45 PM Re: 250.21  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
Breakers need to replace them or single phase fusible equip. Your local inspector "MAY" let you slug the fuses.

They make dummy fuses for this purpose. (scroll down to see the dummy fuse)


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