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Thread: Non-fused disconnects

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    Non-fused disconnects

    I was looking for an AIC rating on a non-fused disconnect and couldn't find one.

    Then I realized a non-fused disconnect will not automatically open a fault current. That leads me to the assumption that it only needs to be able to interrupt its rated current.

    So it seems a 600A disconnect would be rated to interrupt (or open) 600 amps of current. It also seems that a non-fused disconnect would not have or need an AIC rating.

    Am I on the right track here?

    Or does a non-fused disconnect need to have a withstand rating in case there is a fault downstream? I don't see one listed, although the fused versions have AIC ratings that are dependent on the fuse installed.

    The cut sheet does mention a 12X overload rating, but that wouldn't get anywhere close to the available fault current.

    This is all because the utility company has asked for a line side service disconnect without any overcurrent protection. I believe they plan on locking this disconnect on.

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    Quote Originally Posted by steve66 View Post
    I was looking for an AIC rating on a non-fused disconnect and couldn't find one.

    Then I realized a non-fused disconnect will not automatically open a fault current. That leads me to the assumption that it only needs to be able to interrupt its rated current.

    So it seems a 600A disconnect would be rated to interrupt (or open) 600 amps of current. It also seems that a non-fused disconnect would not have or need an AIC rating.

    Am I on the right track here?

    Or does a non-fused disconnect need to have a withstand rating in case there is a fault downstream? I don't see one listed, although the fused versions have AIC ratings that are dependent on the fuse installed.

    The cut sheet does mention a 12X overload rating, but that wouldn't get anywhere close to the available fault current.

    This is all because the utility company has asked for a line side service disconnect without any overcurrent protection. I believe they plan on locking this disconnect on.
    It still has a withstand rating.Typical "safety switch" has same switch assembly as a fused safety switch, and often has same rating "when protected by fuses".

    If this is on POCO side of service point, it is not covered by NEC and POCO can specify anything they want to.
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    Quote Originally Posted by kwired View Post
    It still has a withstand rating.Typical "safety switch" has same switch assembly as a fused safety switch, and often has same rating "when protected by fuses".

    If this is on POCO side of service point, it is not covered by NEC and POCO can specify anything they want to.
    So if it is rated for 200KAIC with a certain fuse, can I assume it could withstand 200K amps when unfused?

    I don't really have that much fault current, but I do have more fault current available than the lowest 10KAIC rating it gets with a "basic" fuse.

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    Eaton's design guide says non-fused switches requiring an SCCR at >10000 amps must be protected by a suitable fuse or circuit breaker upstream and provides a table of suitable devices. I'm looking at a 2011 Eaton design guide manual, section 28.

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    Quote Originally Posted by steve66 View Post
    So if it is rated for 200KAIC with a certain fuse, can I assume it could withstand 200K amps when unfused?
    Absolutely not.

    For a non-fused version of a 'safety switch' the UL SCCR rating is only 10KA. Actually, unless rejection style fuse clips are installed, this is also the rating of fusible designs.
    However most manufacturers have series ratings between their non-fused switches and specific circuit breakers as well as upstream fuses.
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post
    Absolutely not.

    For a non-fused version of a 'safety switch' the UL SCCR rating is only 10KA. Actually, unless rejection style fuse clips are installed, this is also the rating of fusible designs.
    However most manufacturers have series ratings between their non-fused switches and specific circuit breakers as well as upstream fuses.
    This doesn't make sense to me. It's essentially the bus of the switch. So if the bus can only handle 10KA you can put in 10000000000000000KA fuses in there still going to explode at +10KA. I must be missing something

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    Quote Originally Posted by Designer69 View Post
    This doesn't make sense to me. It's essentially the bus of the switch. So if the bus can only handle 10KA you can put in 10000000000000000KA fuses in there still going to explode at +10KA. I must be missing something
    When the switch HAS fuses, the fuses will limit the fault current seen by the switch to below 10kA. Without the fuses to clear that fault, the switch reverts to the 10kA.

    As Jim said, most mfrs will series list their switch with the same fuses they might hold, and with fuses, the same "class" of fuse from one mfr will be equivalent to another, i.e. a Class J fuse from Bussman is the same as a Class J fuse from Mersen or Littlefuse, at least as far as the minimum interrupt and current limiting capabilities. But you cannot assume that, you must be able to prove it with documentation from the switch manufacturer. They will also sometimes series list it with VERY SPECIFIC circuit breakers and these will be current limiting breakers that are manufactured by the same company as the switch. That's because the series rating can only be attained by destructive testing, so Eaton may test their NF switch with their CL breaker, but they will not pay the money to test their switch with a Schneider or GE breaker.
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    Quote Originally Posted by Designer69 View Post
    This doesn't make sense to me. It's essentially the bus of the switch. So if the bus can only handle 10KA you can put in 10000000000000000KA fuses in there still going to explode at +10KA. I must be missing something
    I am as confused a you are, but I trust that Jim knows what he is talking about.

    For the higher AIC ratings, I guess its possible that the current limiting aspect of the fuses helps.

    As far as the series ratings go, I think I found Siemens:

    https://www.industry.usa.siemens.com...-rev052517.pdf

    but it seems to require the breaker on the line side: something that the utility would not allow.

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    Quote Originally Posted by Designer69 View Post
    This doesn't make sense to me. It's essentially the bus of the switch. So if the bus can only handle 10KA you can put in 10000000000000000KA fuses in there still going to explode at +10KA. I must be missing something
    It has to do with the amount of current limiting performed by the fuse, and the UL testing of the devices.
    Many, if not most, 'safety switch' designs max out at 200kA even though Class R and J fuses maybe rated higher.

    This is not new, non-fused switches have been rated like this for decades.
    And remember, if you don't install rejection style fuse clips in your fusible switch it is only listed to 10kA max. Yet, in 40 years, I don't think I have sold 40 sets of clips.
    Just because you can, doesn't mean you should.

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    Also just to be clear, the correct term for a non fused switch is "short circuit current rating" not AIC.

    There is some ambiguity in the code as to what constitutes the allowed "meter disconnect switch". The code doesn't say that it must be non fused to be a meter disconnect switch, but I have run into some utilities and AHJ's where if it is fused, it is called the service disconnect, and if it is unfused it is just a meter disconnect switch.
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