Series ratings application for utilities
Series ratings application for utilities
(OP)
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).






RE: Series ratings application for utilities
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
RE: Series ratings application for utilities
RE: Series ratings application for utilities
RE: Series ratings application for utilities
Assuming the upstream CB is rated for fault current.
Jim
RE: Series ratings application for utilities
RE: Series ratings application for utilities
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.
RE: Series ratings application for utilities
RE: Series ratings application for utilities
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.
RE: Series ratings application for utilities