Step-up/step-down grounding of section between transformers

[White Lion]

A 120/240V single-phase service is stepped up to 1000 volts for the 1/2 mile run to the shop, then stepped back down to 120/240V with the x0 connected to ground. Now considering only the portion between the two transformers, can someone explain if and why you would need to ground one of the lines? Is there a danger leaving it ungrounded? Does either Canada or USA have an article in the code dealing with this situation?

 

[electrofelon]

A 120/240V single-phase service is stepped up to 1000 volts for the 1/2 mile run to the shop, then stepped back down to 120/240V with the x0 connected to ground. Now considering only the portion between the two transformers, can someone explain if and why you would need to ground one of the lines? Is there a danger leaving it ungrounded? Does either Canada or USA have an article in the code dealing with this situation?

That system is not required to be grounded. Its a bit convoluted, but start at 250.30 which sends you to some other articles. IF it is ungrounded, you need ground detectors per 250.21(B). In practice you should just ground it. Use exception #2 in 250.30(A)(1) then you only need two conductors. Then you would not need ground detectors. If you were over 1KV, then another advantage of grounding would be the grounded conductor would only need to be rated 600V. All that said, I do not see a danger in leaving it ungrounded, its just generally cheaper and easier to ground.

 

[electrofelon]

Lion, What kind of conductor are you using for 1KV? Also, what about the transformers, did you have to custom order them?

 

[White Lion]

Lion, What kind of conductor are you using for 1KV? Also, what about the transformers, did you have to custom order them?

Buried RWU90 conductors. I don't know where the transformers came from since I took over the job from another electricien after the second time that one of the underground wires fried. I want to make sure when I leave the job everything is safe.

 

[electrofelon]

Ahhh ok you are from Canada. That conductor is a Canadian specification. Just FYI my references with NEC. I am not familiar with Canadian codes and specs.

 

[LarryFine]

The least insulation voltage stress would be if you ground the center tap of the secondary of the first transformer.

 

[kwired]

The least insulation voltage stress would be if you ground the center tap of the secondary of the first transformer.

You would then need to run a third conductor is another consideration.

 

[White Lion]

You would then need to run a third conductor is another consideration.

Please explain what purpose a third conductor would serve. I cannot imagine a single possible use for a third conductor.

I would absolutely ground the center tap if that was a possibility, but there is no center tap.

 

[electrofelon]

The least insulation voltage stress would be if you ground the center tap of the secondary of the first transformer.

While I agree, and in addition to what Kwired said, it is unlikely such a transformer has a center tap.

 

[electrofelon]

Please explain what purpose a third conductor would serve. I cannot imagine a single possible use for a third conductor.

I would absolutely ground the center tap if that was a possibility, but there is no center tap.

In addition to the two circuit conductors, one needs an EGC/supply side bonding jumper between the two transformers. Imagine a fault at the second transformer between the primary winding and the case. How would that fault clear without a bonding jumper back the the source which is the first transformer? that exception I noted in post #2 allows the grounded conductor to be both a grounded circuit conductor and a fault clearing conductor.

 

[White Lion]

In addition to the two circuit conductors, one needs an EGC/supply side bonding jumper between the two transformers. Imagine a fault at the second transformer between the primary winding and the case. How would that fault clear without a bonding jumper back the the source which is the first transformer? that exception I noted in post #2 allows the grounded conductor to be both a grounded circuit conductor and a fault clearing conductor.

I follow you, but the case of the second transformer is already bonded to the secondary x0 of the second transformer, and connected to a second grounding electrode. Is that not sufficient?

 

[electrofelon]

I follow you, but the case of the second transformer is already bonded to the secondary x0 of the second transformer, and connected to a second grounding electrode. Is that not sufficient?

No it is not. We need a low impedance path back to the source to clear a fault. A fault cant get there through dirt, too much resistance.

 

[LarryFine]

You would then need to run a third conductor is another consideration.

Why? A neutral need not be run with the lines if the load doesn't need it.

Unless you mean an EGC. Does the 2-wire feeder eliminate that need?

 

[winnie]

Essentially by using a two wire feed with one feed grounded, the grounded conductor also serves the purpose of the EGC.

The center tap grounding on the second transformer does not serve to provide bonding for its primary (1000V) conductors.

If you use a center tap, on the step up transformer, then you have to run an EGC.

OP states that the underground wires have fried twice. I'd worry about poor installation causing cable installation damage.

-Jon

 

[electrofelon]

Essentially by using a two wire feed with one feed grounded, the grounded conductor also serves the purpose of the EGC.

Yeah its just like re-bonding the neutral to ground at remote structures that we used to be able to do before the 2005 (2008?) NEC. Fortunately they still allow this use of that idea

 

[kwired]

Why? A neutral need not be run with the lines if the load doesn't need it.

Unless you mean an EGC. Does the 2-wire feeder eliminate that need?

What winnie and EF said. Using grounded conductor that normally is carrying current isn't ideal, but just grounding a center tap and depending on earth to carry any ground fault current is even worse idea, at least until voltage reaches a certain level that enough current will flow to operate overcurrent devices or lower current level ground fault detection is utilized.