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Overcurrent Protection Required For Zig-Zag?

Overcurrent Protection Required For Zig-Zag?

Overcurrent Protection Required For Zig-Zag?

Hello Everyone,

I am in the process of designing a 480V three-phase, 3-Wire + Ground industrial distribution system that will be fed from a 2500 KVA pad mount transformer with a 480V Delta secondary (The delta secondary is being used to create a 30 degree phase shift from a different pad mount transformer with a wye secondary). The ground reference on the delta transformer will be created by connecting a zig-zag grounding transformer and a continuously rated 50A Neutral Grounding Resistor. A ground fault monitoring system will be used in conjunction with the NGR to create a 'Ground Fault' Alarm for the plant to use to create a controlled shutdown scheme. Our intent is to connect the zig-zag directly to the secondary of the pad mount transformer with NO switching or overcurrent devices of any kind between the pad mount transformer and the zig-zag. However, it's been brought up by a team member that NEC Article 450.5 requires overcurrent protection for the zig-zag transformer, which I have never done in the past.

Question: Is overcurrent protection required between the pad mount transformer secondary and the zig-zag transformer?

Thanks in advance for your advice and assistance.


Galatians 2:20

RE: Overcurrent Protection Required For Zig-Zag?

It could be worth adding a Fuse Disconnecting switch at the zigzag transformer terminals.
Fuses will provide quick isolation in case of zigzag transformer faults and the disconnector will help during maintenance.
Do you intend to shutdown the pad mount transformer in case of zigzag transformer outage for a short time (for whatsoever reason)??

RE: Overcurrent Protection Required For Zig-Zag?

First I wonder why this thread is here, in IEEE coed issues.

Second I would say a Hoppner connector transformer does not require an over current.

RE: Overcurrent Protection Required For Zig-Zag?

Adding a fused disconnect at the zigzag is precisely what I was NOT wanting to do, nor do I think it's advisable. If that switch is turned OFF or if one or more fuses blow the derived neutral ground is no longer present. One alternative, I suppose, would be to feed the zigzag through an appropriately rated branch circuit breaker (to provide overcurrent protection for the zigzag) fitted with internal auxiliary contacts and then use the aux contacts to trip the MAIN circuit breaker if the branch circuit breaker tripped or was turned OFF?

As for why this question was asked in here in IEEE code issues... it seemed to be the best fit. If you can suggest a more appropriate forum I'd be happy to move it over to that one.


Galatians 2:20

RE: Overcurrent Protection Required For Zig-Zag?

Maybe electrical engineering discussions?

As per code, most of the zig-zag transformers I have worked with only followed IEEE, and not to any codes.

The most zig-zags that I have seen recently are on solar farms, as to keep a ground reference when the feeder breaker opened. I guess this condition causes problems for the inverters.

The transformer that I mentioned is a wye/delta, with a zig-zag internal on the delta side.

RE: Overcurrent Protection Required For Zig-Zag?

If you need to meet the NEC, I would say yes. See 450.5 Grounding Autotransformers Covered in some detail there. There are changes in the 2020 edition.

RE: Overcurrent Protection Required For Zig-Zag?

I have seen utilities in Middle East connect the ZigZag transformer directly to the LV delta winding terminals of the main transformer. No breaker, No fuses and not even Disconnetor.
This way, the zigzag transformer is treated as a part of main transformer with 87T of main transformer including the zigzag as well in its zone.

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