Transformer secondary grounding

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bnewport

bnewport

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We have a single phase 480V primary / 240V secondary transformer feeding only a UPS load. (2 wire + gnd) X4 is grounded at the transformer. The ground on the transformer is causing problems with the DC system and external equipment. The UPS manufacturer (engineers) want us to unground the secondary. Our company engineer says this is ok by the NEC, but the customer engineer says its a violation. Any code references to support this either way?

For those who are interested in the details: The UPS has external batteries and chargers. The UPS output feeds lighting and control panels. The DC batteries are tied to the UPS DC buss as well as a DC panel for motors and other loads. Apparently, something is causing a + and - ground fault to appear to the charger. The UPS company engineers think it might be this grounding configuration. Myself, I wonder if it is a ground fault downstream from the DC panel. Any help would be appreciated!

Bryan

[ February 01, 2005, 10:03 PM: Message edited by: bnewport ]
 
Re: Transformer secondary grounding

250.21 lists the systems that are not to be grounded. It does not look like your system is listed.
250.30 Gounding Separately Derived Systems is what you are looking for.
 
Re: Transformer secondary grounding

Bryan,
Look at 250.20. A two wire 240 volt system is not required to be a grounded system.
Don
 
Re: Transformer secondary grounding

Is the output single phase 2 wire? Sounds like it is 240/120V.

Is it the input or the output to the UPS they want you to unground?

Steve
 
Re: Transformer secondary grounding

Don-

I read 250.20. I'm not real clear on (B)(1). Is this the reason why it is not required by code?

Bryan
 
Re: Transformer secondary grounding

Don
I am looking at 250.20 NEC 2002 and I do not see it.

[ February 01, 2005, 06:19 PM: Message edited by: bob ]
 
Re: Transformer secondary grounding

(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts
(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral is used as a circuit conductor
(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor
Click to expand...
These are the only 50 to 1000 volt AC systems that require grounding. A two wire 240 volt system cannot be grounded so that the maximum voltage to ground is 150 volts or less and does not have a neutral or midpoint conductor that is used as a circuit conductor so it does not have to be a grounded system.
Don
 
Re: Transformer secondary grounding

NEC 250.20 (B)(1) states that the system shall be grounded: Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts. Since this is a single phase 2 wire 240V secondary (X2-X3 "neutral" is not brought out of XFMR), the voltage from ungrounded conductor to the grounded conductor will still be @ 240V. I dont like this from a safety standpoint, but I'm still not sure what the NEC requires. Any clarifications on this section? This is a very urgent matter. Thanks to all-

Bryan

[ February 01, 2005, 09:59 PM: Message edited by: bnewport ]
 
Re: Transformer secondary grounding

Thanks Don-
You must have posted while I was typing my reply-
That answers my question. If there was a fault in the secondary, primary current would increase and trip primary overcurrent, right?

Bryan
 
Re: Transformer secondary grounding

Okay, I'll accept all this. But first I would probably have hooked up the XFMR secondary in a conventional manner.

(That is with X2/X3 connected together & earthed & bonded, even though neutral is not being used to supply the straight 240V. 2-wire input to UPS.)

Something tells me that it would have worked fine & no one would question it.
 
Re: Transformer secondary grounding

X2 and X3 were connected and bonded when I arrived on site (after a 4 hour drive), but they were having ground fault problems on an external charger. Besides 120VDC on the DC buss, there was 120VAC from + or - to ground. It has to do with the inverter referencing L2 instead of ground. (Dont ask- I dont know why they do that) The manufacturer engineers suggested grounding X4 instead, which I did. This fixed the 120VAC problem, but apparently the ground fault remains. The engineers next suggestion was to completely remove the secondary ground. If anyone is really interested, I have a PDF of the wiring configuration. Send me PM.

Bryan

[ February 01, 2005, 09:58 PM: Message edited by: bnewport ]
 
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