Short-Circuit protection of a three phase motor with two fuses?

[crthompson]

I am designing a vacuum circuit breaker that will be used to feed three phase motor circuits. I am using a neutral grounding resistor to limit my ground fault current to a safe level. I want to fuse phase A and phase C. I want to leave the B phase un-fused. This will provide the short-circuit protection I need for any fault. Does anyone know of any regulations that this may be in violation of? Any help would be appreciated! Thanks guys!

 

[crthompson]

We have upstream ground fault protection that will remove control power from the contactor control circuits so they can't be closed. If they have any phase to ground fault upstream it will take the entire power center offline anyway. I meant to say strengthen or weaken my case.

 

[waross]

I wouldn't want to be your lawyer when this goes sideways.
The best result that I can see is that you push this until the wording in the code is strengthened.
Bad idea! Bad idea!
The one way around this will be to corner ground the system.
Of course that introduces several other negative factors.
If the contactor was suitable for overcurrent (fault current) protection you would not need any fuses. The contactor is not suitable for overcurrent protection so you do need three fuses.
No "What ifs" no "Ya buts" "no I thinks" just three fuses.
It's the code and the code is quite clear.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crthompson]

The code states that three CTs in conjuction with an "overcurrent" relay ON ALL THREE UNGROUNDED CONDUCTORS will suffice for fuses. At this point the fuses are only there to interrupt short-circuits before the contactor has the chance to open. Two fuses accomplish this perfectly fine. Lionel did make a great point... That was something I hadn't thought of, but we protect our transformers with ground fault protection so that if any phase goes to ground upstream of the contactor the incoming power feed is removed.

BTW... Thank you all so much for your input!!!

 

[LionelHutz]

[sarcasm]Oh you will be fine. Nothing could possibly go wrong. I've ~never~ heard of an electrician bypassing or miswiring a protection relay. I've also ~never~ heard of a protection relay failing either. I'm sure no-one else here has ever any of these possible failures happen either...[/sarcasm]

 

[crthompson]

If they bypass the relay they are in violation of the law. You can tamper with anything to make it fail as far as that goes. These devices would be sold in a completed power center and fully wired and operationally tested. I am required by MSHA to use a relay that is failsafe just in case of such a failure. All of that aside I am now actually leaning toward the third fuse...

 

[waross]

I don't live in the USA and don't have a current copy of the NEC to check the wording,
but if you depend on the relay to comply with the code requirement for protection in all ungrounded conductors the contactor that it operates must be rated for the available fault current.
You may encounter bolted faults when commissioning new installations but faults on existing installations may often be arcing faults with considerably less than full available fault current. An arcing fault may be enough to destroy your contactor but not enough to ensure that the fuse blows instantly or in time to save the contactor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crthompson]

I don't see how a third fuse would prevent arcing faults...

 

[crthompson]

Thanks to everyone for your help! I have found a case where only using two fuses would compromise the protection of the circuit. If the neutral from the transformer gets grounded before passing through the neutral grounding resistor, it becomes a solid grounded system with no current limit for ground faults. I really appreciate everyone's ideas and comments!! Thanks again!