Grounding X-Former

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boptrop

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Here is the setup: I have a 3KVa Single Phase 480 to 240 Trans. that is supplying no line to netural (Grounded conductor) loads(240 load only). This setup is in an industrial. enviroment. I have installed a grounding electrode conductor from the transformer case to the GE. My question is do I need to ground the midpoint of phase tap connection to earth? Thanks JBC

Edit: Have you ever heard the question.."what is the voltage to ground on a ungrounded (no netural connection-no line to netural loads) 480 system?"
the ans. is 480 although there is a equiptment grounding conductor with the system... there is no grounded conductor there to go to as the answer goes (the answer also cites definitions)...
I dunno if this applies but it makes me wonder..
I think grounding the midpoint would be a good idea but is not req. so i will leave it as it is ungrounded cause i will look like a fool changing it (the job is over) :(

2nd Edit:
"I think grounding the midpoint would be a good idea but is not req ."
Grounded system contains a grounded conductor in the spirit of 250.
The X-former supplies a magnetic materal shaker.

I just need a consistent answer & explination this is too much debate for me.
Sence This is Mike Holts forum and he is recognised world wide and I have spent a small fourtine (to me) on his books I think he ought to share his thoughts.???

[ February 15, 2005, 09:12 PM: Message edited by: boptrop ]
 
Re: Grounding X-Former

250.20 Alternating-Current Circuits and Systems to Be Grounded. Alternating-current circuits and systems shall be grounded as provided for in 250.20(A), (B), (C), or (D). Other circuits and systems shall be permitted to be grounded. If such systems are grounded, they shall comply with the applicable provisions of this article.
?
(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.
?
Click to expand...
There are many ?shall be? s in 250.20 and no direct exceptions unless the application is directly covered in 250.21 or 250.22. As you have described it so far, yours doesn?t appear to be. Therefore, since you can ground the system ?? so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts,? you must.
 
Re: Grounding X-Former

Bob,
I would tend to read it the other way. The system in question is a 240 volt system and not a 120/240 volt system. The code wording requires "alternating-current systems of 50 volts to 1000" to be grounded systems where the system can be grounded so that the maximum voltage to ground does not exceed 150 volts. This is possible and required for 120/240 volt systems, but I don't think that it applies to 240 volt systems. I don't think that the fact that the transformer can be connected as a 120/240 volt system shoud enter into this.
Don
 
Re: Grounding X-Former

Don,

If there were no center tap on the transformer I would agree with you. But as he has described it, there is, so I believe 250.20(B)(1) applies.

Edit add: My take on 250.20(B)(1) is that is says "If it can be, it shall be unless 250.21 or 22 applies."

[ February 14, 2005, 10:49 AM: Message edited by: rbalex ]
 
Re: Grounding X-Former

I hesitate to reply b/c I do not have my code book handy to check refferences, but I think since this is two-wire in/out application you can use the primary OCPD to protect the secondary. Therefore the the secondary is not required to be grounded but is permitted. Not trying to confuse the issue, maybe someline like Charlie B or Jim Dungar could clear it up.
 
Re: Grounding X-Former

My take on this is: 2 wire 240V secondary ->not required to be grounded. The presence of an un-wired (possible) center tap does not change the system to 3 wire.

That said, personally I would ground the center tap of the transformer to prevent all of the "non-normal" troubleshooting issues of un-grounded systems.
 
Re: Grounding X-Former

Plaintiff?s Attorney: Can you ground this system with this transformer so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts?

[ February 14, 2005, 04:17 PM: Message edited by: rbalex ]
 
Re: Grounding X-Former

The rest of the cross-examination(I need to go home :D ):


Plaintiff?s Attorney:Your honor the Defendant is non-responsive, please have him answer "Yes" or "No."

Plaintiff?s Attorney: Does Section 250.20 say: ?Alternating-current circuits and systems shall be grounded as provided for in 250.20(A), (B), (C), or (D).??

Plaintiff?s Attorney: And does Section 250.20(B)(1), effectively say, ?Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems 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.??

Plaintiff?s Attorney: And are there any exceptions in Section 250.20 or its Sub-Sections?

Plaintiff?s Attorney: Are there any other Sections referenced within Section 250.20 or its Sub-Sections that would alter the requirements of 250.20(B)(1)?

Plaintiff?s Attorney: Is this application consistent with either Section 250.21 or 250.22?

Plaintiff?s Attorney: I rest my case.

[ February 14, 2005, 06:06 PM: Message edited by: rbalex ]
 
Re: Grounding X-Former

Originally posted by rbalex:
[QB] ...
Plaintiff?s Attorney:Your honor the Defendant is non-responsive, please have him answer "Yes" or "No."
...
Click to expand...
Ed, I didn't mean you were unresponsive :eek:
 
Re: Grounding X-Former

In this situation,unless there is some good reason not to,- KISS.
Treat it as a common 120/240V. 1-phase 3-wire secondary of a plain jane solidly grounded, separately derived system.
Gain all the benefits of, & the familiarity of,
a plain jane system like we are used to.
I agree with many that that is intention of NEC.
 
Re: Grounding X-Former

boptrop, is this some sort of operation where interuption to service would cause a threat to human life or vital process control. I noticed you mentioned industrial enviroment. If that is the case it is not unusual to have a ungrounded system.
 
Re: Grounding X-Former

Originally posted by Ed MacLaren:
My lawyer told me to keep my mouth shut. :D :D

Ed
Click to expand...
LOL, I realized what I had done on the way home last night and could only get a quick post off to apologize.

Dereck,

I think boptrop clarified in his edited original that the only reason he didn't ground it was simply an oversight.
 
Re: Grounding X-Former

I do not believe that 250.20(B) applies in this particular instance of a 3KVA transformer as it is not part of a "premises wiring system" as defined in Art 100, unless a branch circuit has been created (i.e. installation is not covered by 670.4(B)).
 
Re: Grounding X-Former

Jim, I think that's a stretch. However, the "system" under discussion is not premises wiring but grounding - they may be related but they are not the same.
 
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