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!

 

[badservo]

It would help to know where, what voltage, and any other information you have.

 

[ScottyUK]

Most NGRs are short-term rated, so an uncleared fault on the B phase will kill the NGR. This will probably not be a good thing.

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!

 

[crthompson]

The NGR I am using is rated for continuous operation. This application is for 3 different 3 phase voltages. 480vac, 600vac, and 1000vac. All voltages are 60 cycle. This vacuuum circuit breaker will be used as a feeder in underground mining applications. I have CTs for overcurrent but the fuses are required for short-circuit protection. All of MSHA's documents only require "short-circuit protection". They do not specify how this is accomplished. When I brought this before them, they didn't want to let me use only 2 fuses to accomplish this. They could not however provide me with any information on why. They then informed me that if I could find a similar application already in use in the industry that it would help my case. I am not acutally using a circuit breaker. I am combining a protection relay with fuses and a vacuum contactor to accomplish the equivalent.

 

[rbalex]

Since you cite MSHA, I'm assuming a US installation. Have you reviewed NEC Section 430.225(C)(1)(b)? It applies to all motors over 600V.

 

[crthompson]

This is for a US application. I have reviewed that section of the NEC but this vacuum circuit breaker will be feeding motors and "other" equipment such as welders and things of the like. Would that classify it as a feeder circuit instead? In section 240.1 the informational note references the overcurrent section 110.10 for protection against fault currents. Does this cover me for circuits of not more than 600vac? If so, what about the 1000vac circuits?

 

[jraef]

All motor circuits are required to have protection in all 3 phases. Even if you think it qualifies as a feeder circuit, all that means is that the motor circuit requirements are then going to be satisfied again somewhere down stream.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[crthompson]

I have just found a related article in the NEC. Section 240-20 Ungrounded Conductors.(a) states: "Overcurrent Device Required. A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit."

Would this allow me to eliminate the B phase fuse as long as I have overcurrent protection for all 3 phases?

 

[ScottyUK]

If it were acceptable then likely the major manufacturers would have implemented it as a cost saving, but I haven't seen it on US-origin equipment I've been involved with.

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!

 

[crthompson]

The first reason I want to remove the fuse is because I can provide the protection I need with two fuses(if code allows). The second reason is because the fuses I am using are so expensive. The third reason is the downtime involved in replacing the "extra" fuse. I don't see the need to replace two fuses when one fuse could be replaced all while keeping the protection required. The customers this application will be geared toward actually don't want fuses, but the the vacuum contactor technology available doesn't meet the interrupting ratings for short-circuit current. That being said, the best idea I could come up with was to eliminate the B phase fuse...

 

[crthompson]

I think you don't see this in the industry is because of the neutral grounding resistor. MSHA requires a continuous duty 25 Amp current limiting resistor from a neutral to ground. So even if I were to get a ground fault, it wouldn't blow the fuse of the phase it occurred on, no matter the duration.

 

[jraef]

I have just found a related article in the NEC. Section 240-20 Ungrounded Conductors.(a) states: "Overcurrent Device Required. A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit."

Would this allow me to eliminate the B phase fuse as long as I have overcurrent protection for all 3 phases?

I don't understand how you are getting the EXACT OPPOSITE of what is stated here!

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

 

[waross]

You may be confused concerning the terms:
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
--------------------
"Why not the best?"
Jimmy Carter

 

[crthompson]

240-20 a states: "A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit." This clearly states that an overcurrent trip unit can be used instead of fuses. Then it clearly states that CTs and an overcurrent relay can be used on each ungrounded conductor to accomplish the same. I do have ground fault protection and in the event that any phase goes to ground the breaker would open. If two phases get together or go to ground then the speed of the fuses will open the fault before the contactor opens. Then after the phase fault is cleared the ground fault protection will open the contactor (after high fault currents have been cleared)

 

[LionelHutz]

Well, do what you want but I wouldn't accept it because it's a really, really dumb thing to do. Think about this. Phase A has a ground fault upstream of this "circuit breaker" and phase B has a ground fault downstream of this "circuit breaker". Which fuse is clearing this fault? Oh yeah, the vacuum bottle "fuse"...

On another note, how exactly are you going to allow for a maintenance lock-out with a vacuum contactor?

 

[asco1]

Overcurrent Protection is to protect the Conductors & the Devices they feed during normal operation. In this case, this is the job of your circuit breaker.

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.

 

[crthompson]

The phase A ground fault would be opened by the ground fault protection. The primary of the supply transformer will have all 3 phases fused and in the event of a secondary phase to phase fault, the primary fuses would clear the fault.

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!!

 

[asco1]

If there is a short on B-Phase, any available current will pass through the B-Phase of your contactor. If the avaible current is above 4500A, the contactor will fail.

 

[crthompson]

If there is a short from B phase to ground it will be current limited to 15 or 25A. If B phase goes to phase A, the phase A fuse will blow clearing the fault. If B phase goes to phase C, the phase C fuse will blow clearing the fault. If phase A & C fault then at least one if not both fuses will blow, clearing the fault.

 

[LionelHutz]

You have no case to strengthen. If there is a phase A ground fault upstream and a phase B ground fault downstream of this contraption then the upstream fuses are the only thing in the circuit that can protect the contactor. Your use of the combination of fuses and contactor is implying that you already checked the fault numbers and know that the main fuses won't protect the contactor. The contactor will fail if the main fuses won't protect the contactor and there is no branch fuse.

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.