240-120V step transformer grounding

[Clodbuster]

Hi everyone,

First post on the site but I've lurked for a long time and learned from you all. Thanks for that and take it easy on the new guy for his first question all right?!

So what I've got is a transformer installation that I don't believe is grounded correctly. The work electrician installed a new Acme General Purpose Transformer, No. T 2-53013-4S, 3 kVA. Windings are 240x480 Primary and 120/240 Secondary. It's purpose is to step down 240V single phase power to 120V to allow installation of a convenience GFCI receptacle at some distance (1175 ft) from the nearest panel. I sized the conductors for voltage drop and selected the transformer and the electrician installed it. When I checked it out I found weird voltages from line-ground (22V) and neutral-ground (40V) on the receptacle, but the line-neutral checks out at 120V. I think the grounding is incorrect as the receptacle ground (EGC) on the secondary side is shared with the EGC on the primary side. Also, there is no neutral-ground bond on the secondary side.

I've been looking at the code requirements and I talked to the electrician to see how he wired it. I think I have figured out how to fix but want to check myself before taking this to the electrician to get my ducks in a row so to speak. Here's some wiring diagrams of how it was done and how my interpretation of the code says it needs to be done. If anyone has any disagreements or suggestions I would appreciate hearing them.


I'm in WA state and familiar with NEC 2008.

Thanks.

 

[augie47]

Appreciate your coming out of the shadows ("lurking") and jumping in with us.

I believe your key words are "also there is no neutral ground bond on the secondary side".
Your '08 Code reference will be found in 250.30 especially 250.30(A)(1).



You do need a grounded-grounding bond jumper as well as an electrode.

(My computer did not allow me to view the images)

 

[Smart $]

...
(My computer did not allow me to view the images)

Same here.

 

[Clodbuster]

Hopefully this wiring diagram is a little more viewable for you all.

 

[Smart $]

Hopefully this wiring diagram is a little more viewable for you all.

Looks good to me, with one exception.... you'll need two ground rods (unless you can verify the one is less than 25 ohms to ground).

 

[hillbilly1]

AA also needs to be connected to F in the revised drawing, but your on the right track.

 

[augie47]

AA also needs to be connected to F in the revised drawing, but your on the right track.

Agree.

(So far we have discussed grounding....do we need to discuss over-current protection ? )

 

[Clodbuster]

OK, so I see I was on track for adding a neutral-ground bond and grounding electrodes. Also, point taken on using two electrodes - with <10" of precipitation and sandy soil here we tend to have high resistance grounds.

However, I want to make sure I understand the principle behind connecting AA to F. I thought I needed to drop this connection so as to isolate the primary and secondary sides. Is it is necessary in order to avoid the possibility of a potential difference across the two grounding systems?
At first glance it looks like connecting AA and F with the neutral-ground bond could result in a potential between the secondary's X2 and the originating panel.

As for over current protection, that all looks good on this install. I would be curious to hear how and where you would implement it for a similar circuit though for future reference.

Thanks.

 

[augie47]

The transformer case is connected to the primary EGC in case there is a fault from the primary wiring to the case.
The same thing needs to happen with the secondary windings.
The interconnection will cause no problems. If you had a building with a 480Y/277 service and a 208Y/120 service both would share electrodes in the same manner,

 

[G._S._Ohm]

. . .weird voltages. . .

I didn't think these voltages were possible at all, but if you have the equivalent of a lumped capacitance to ground at approximately the 44v point on the 120v winding you can get these readings, depending on the input resistance and input capacitance of your AC meter and the value of transformer capacitance to the ground.

 

[Clodbuster]

So to close out the issue, today we rewired the transformer installation and made the following changes:
1) Added two grounding electrodes and a conductor connecting them to the transformer enclosure and system at AA.
2) Kept the primary-secondary EGC bond between F and AA.

However, when we initially connected AA to W, (the neutral) and then checked the receptacle voltages the weird voltages were eliminated but the polarity was backwards (120V "neutral" to ground and 0 "hot" to ground). We wired AA to R instead and that corrected it. Also checked for amperage on the bonded ground and found nothing.

That wraps it up nicely. Thanks for the help and explanations on this.

 

[G._S._Ohm]

It's a trivial test to check the current through the ground lead and it may prove/disprove the reason for the weird voltages.

I guess I don't like to leave a single stone unturned, because sooner or later you may see another set of weird voltages.

 

[hillbilly1]

So to close out the issue, today we rewired the transformer installation and made the following changes:
1) Added two grounding electrodes and a conductor connecting them to the transformer enclosure and system at AA.
2) Kept the primary-secondary EGC bond between F and AA.

However, when we initially connected AA to W, (the neutral) and then checked the receptacle voltages the weird voltages were eliminated but the polarity was backwards (120V "neutral" to ground and 0 "hot" to ground). We wired AA to R instead and that corrected it. Also checked for amperage on the bonded ground and found nothing.

That wraps it up nicely. Thanks for the help and explanations on this.

Your receptacle was wired backwards. (Not wired per your diagram) The neutral or grounded conductor can be from either output side of the transformer, it does not matter electrically, but once you bond that side it becomes the grounded conductor.