Recently jumped the fence and started working at a utility. I am still trying to get up to speed on rules and requirements for utilities. I am working on some projects where they are using a fuse and circuit breaker in series: the circuit breaker has too low of an interrupting rating for the application and they are claiming that the higher rated fuse is protecting it. I have not yet asked if this is a "tested combination" as I know it from the NEC; I'm not sure that they would even know what I was talking about. I snooped around the office and found an NESC. I'm not sure if it's the right section as this is my first time looking at the NESC but Section 171 for circuit breaker application states "Devices that are intended to interrupt fault current shall be capable of safely interrupting the maximum short-circuit current they are intended to interrupt, and for the circumstances under which they are designed to operate." There is no mention of a series rating here and my interpretation of that section is that each device must be fully rated. Is this incorrect? A larger question; are utilities free to use the NESC in whatever manner they please whereas non-utilities don't have a choice in following NEC requirements? Generally city or state governments adopt a cycle of the NEC as an ordinance, is this done with the NESC? Is it permissible to apply the up-over-down method in the utility world? This is for 480V service by the way (before service disconnect).
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Series ratings application for utilities 1
- Thread starter wroggent
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That quote sounds very similar to 110.9 in the NEC.
I think utilities are able to provide a risk / reward type analysis and utilize engineering judgement more than the traditional NEC would allow for non-utility work.
This is consistent with the NEC Section 240.86 (A) which allows for selection under engineering supervision
I think utilities are able to provide a risk / reward type analysis and utilize engineering judgement more than the traditional NEC would allow for non-utility work.
This is consistent with the NEC Section 240.86 (A) which allows for selection under engineering supervision
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It is also not uncommon practice in utilities, that if a CB is over-dutied and if the actual fault current is above the CB rating that the CB (as controlled by the CB relay) is blocked from tripping and the upstream CB operates.
Assuming the upstream CB is rated for fault current.
Jim
Assuming the upstream CB is rated for fault current.
Jim
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- #6
lansford - I can understand that approach. I would argue though that at some point you may need to consider a withstand and/or short time rating for the device that is being restrained depending on upstream device's trip time. Then you also have to worry about if the upstream device will actually trip when it's supposed to. That is different from my situation though, as there is nothing preventing the circuit breaker from trying to open.
CLiP
Here's a utility grade device that brags that allows the user to "Escape the cost of full rated breakers" with no discussion regarding series ratings or combination testing or even whose brand relay/breaker might be placed in series with it. The voltages are a bit higher than the OP's 480, but I think it makes the point that the series combination is an NEC deal. I suppose it might still be considered good practice to use tested combinations at the service level.
The situation Lansford describes meets NESC, since it the device is only intended to interrupt faults within it's ratings.
Yes, NESC is adopted or not by the states at their own time. I understand WA adopts all parts but Part 4, and CA has their own rules.
Here's a utility grade device that brags that allows the user to "Escape the cost of full rated breakers" with no discussion regarding series ratings or combination testing or even whose brand relay/breaker might be placed in series with it. The voltages are a bit higher than the OP's 480, but I think it makes the point that the series combination is an NEC deal. I suppose it might still be considered good practice to use tested combinations at the service level.
The situation Lansford describes meets NESC, since it the device is only intended to interrupt faults within it's ratings.
Yes, NESC is adopted or not by the states at their own time. I understand WA adopts all parts but Part 4, and CA has their own rules.
If the upstream fuse is not current limiting, then the breaker will try to interrupt at least one-half cycle of the current and may fail. If the upstream fuse is current limiting, then the combination might work, but there is an interaction between the breaker opening and the fuse interrupting. This is a physical phenomenon that occurs regardless of the governing standard. The problem is, you won't find tested series combinations of utility breakers and fuses, so you don't know if it works or not.
I've tested series combinations a number of times to UL list equipment. So far it's worked. This was always with fast current limiting fuses. (Mainly class T.) We didn't have to worry about coordination because it was feeding power electronics.
One thing we have seen is the breaker welding at high available fault currents. My conjecture is that the breaker is trying to open, but the fuse interrupts the current before the mechanism over-centers and the breaker welds shut on reclosure. This was in the 30kA peak ballpark. We have had a few field reports of this too. They have to bust the interlock to get the door open.
Fortunately we can get 200kA breakers now. There is more let-through and more damage at the fault. But now the risk is pushed upstream. We're no longer outside the rating of the breaker.
One thing we have seen is the breaker welding at high available fault currents. My conjecture is that the breaker is trying to open, but the fuse interrupts the current before the mechanism over-centers and the breaker welds shut on reclosure. This was in the 30kA peak ballpark. We have had a few field reports of this too. They have to bust the interlock to get the door open.
Fortunately we can get 200kA breakers now. There is more let-through and more damage at the fault. But now the risk is pushed upstream. We're no longer outside the rating of the breaker.
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