Short-Circuit protection of a three phase motor with two fuses?
Short-Circuit protection of a three phase motor with two fuses?
(OP)
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!





RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
Circuit breakers are normally designed to clear fault current. Normally fuses are used to back up an HV contactor of limited breaking capacity. What are you actually dealing with here?
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If we learn from our mistakes I'm getting a great education!
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
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RE: Short-Circuit protection of a three phase motor with two fuses?
Would this allow me to eliminate the B phase fuse as long as I have overcurrent protection for all 3 phases?
RE: Short-Circuit protection of a three phase motor with two fuses?
Why are you so determined to remove the fuse?
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If we learn from our mistakes I'm getting a great education!
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
To me it clearly says EACH ungrounded conductore. If you have a 3 phase Y system, as implied by having an NGR, you have 3 ungrounded cunductors. How is it that you seem to think this means you do NOT need the fuse on the B phase then?
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Short-Circuit protection of a three phase motor with two fuses?
overload and over current.
Overload is overload protection. This is provided by the motor overload relay.
Overcurrent is short circuit protection (which may be set to also provide overload protection). Your fuses are your overcurrent protection required on each conductor. You have stated that the vacuum contactors will not break the levels of fault current available. Hence you can not provide overcurrent protection by tripping a vacuum contactor.
A simple ground fault may not be a problem. Ground faults are often not simple. Phase to phase faults often involve a ground path as well as the phase to phase fault particularly where arcs are involved. A phase to phase to ground fault will generally take out both fuses. If only one phase is fused you may be left with a 25 Amp arc to ground. That may do a lot of damage and may soon involve adjacent conductors and cables.
Bill
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"Why not the best?"
Jimmy Carter
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
On another note, how exactly are you going to allow for a maintenance lock-out with a vacuum contactor?
RE: Short-Circuit protection of a three phase motor with two fuses?
Your Conductors & your Overcurrent Protection Device (your breaker) have specific short circuit ratings. The short circuit rating is the amount of current the device can handle before the overcurrent device opens.
Your over current device should have a series rating..."Current Limit Rating When Fused" This will state the type of fuse and the maximum let through. e.g. Your Over-Current device may be 1000A. Your Current-Limiting fuse may be 1600A. This will allow the the over-current device to act first. Fuses will only blow on a dead short.
That said, To provide Short-Circuit Protection to a 3-phase device, and to all conductors, you will need to provide fuses on all phases.
RE: Short-Circuit protection of a three phase motor with two fuses?
The nominal current for this application is 600A. The vacuum contactor I am using has an interrupting rating of 4,500A. The fuse that is used to protect the contactor is a 700A fuse. The specifications I am building this to require the fuses to open first in the event of a short-circuit.
I don't want anyone to think I am just trying to cheapen a design or anything like that. Safety is my number one concern when designing this. If I can find any reason to "need" the B phase fuse, then it will be there. As of yet I see no danger in removing it. In the long run, the final decision on this will be made by MSHA but I am looking for information to strengthen my case. Thank you all for input!!
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
Besides, even if it did work, I'm sure the mine will not appreciate you taking out the main and shutting down their whole power system, compared to just shutting down this branch system.
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
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
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"Why not the best?"
Jimmy Carter
RE: Short-Circuit protection of a three phase motor with two fuses?
BTW... Thank you all so much for your input!!!
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?
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
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"Why not the best?"
Jimmy Carter
RE: Short-Circuit protection of a three phase motor with two fuses?
RE: Short-Circuit protection of a three phase motor with two fuses?