Fusing a Zig-Zag transformer

[raisinbran]

Looking to install a 4160 volt system dedicated to a 3000 HP motor. Total cable length is less than 200 feet. Existing transformer secondary is delta connected. Contractor wants to install a resistance grounded (15 ampere limit) using a zig-zag grounding transformer. Question: Do you fuse the zig-zag? Second question - if you used a wye-delta grounding transformer instead of a zig-zag, would you fuse it?
Personal preference - due to the criticality of the load, I would go ungrounded with a ground detection scheme to alert on a ground fault, but maybe I am old-fashioned.
Regards,
Raisinbran

 

[dpc]

If you must meet NEC, see Article 450.5. I would fuse it just to protect against unlikely case of an internal fault - either zig-zag or delta-wye.

You're better off with the high-resistance grounding instead of ungrounded due to reduced overvoltage risks. But 15 amps sounds high - the current is normally limited to roughly the capacitive charging current in high resistance grounding - this is usually less than 5 amps.

The delta-wye unit will work just fine for grounding and will probably be a lot easier to come by - probably cheaper as well. Since you are resistance grounding, you don't need a very large unit.

 

[RalphChristie]

According to my opinion it would depend on if the zig-zag/delta-wye transformer is included in a protective zone, like in a differential or REF zone. If not, I would fuse it. I am not familiar with the NEC, so I would not/can not comment on it.

It seems as if it is difficult to obtain zig-zag transformers in certain parts of the world - and it would be most probably a special order to obtain one. If it is true in your case, I would go for a delta-wye unit. However, a zig-zag trsf is usually smaller compared to the same size delta-wye units.

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[TestBeforeTouch]

Per NEC 450.5 (B) (2) the zig-zag autotransformer must be protected by a device that will open simultaneously all ungrounded conductors. The setting of the device cannot exceed 125% of the rating of the autotransformer's continuous rating. Therefore, a simple fused disconnect (no anti-single phasing protection) can not be used.

The NEC seems to be silent on the use of a wye delta grounding transformer, probably because the autotransformer installation is cheaper. I would recommend in this case following the same rules that apply to the grounding autotransformer.

It is interested to note that if you were to make this a 3phase 4 wire distribution system than the autotransformer must be directly connected to the ungrounded phase conductors and cannot have a switch or overcurrent protection that is indendent of the main per 450.5 A(1)