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Thread: Fused vs. Non-Fused

  1. #11
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    Quote Originally Posted by electrofelon View Post
    But I was speaking to the low sccr rating of non fused switches. What I was getting at is if you use a circuit breaker interconnection, a non fused switch does not series rate with it. Sure likely not an issue with a residential service, it doesn't take a very large service to get over 10k. Of course if one is ignoring the sccr rating, it's a moot point
    If the breaker has a sufficient rating what difference does the rating of the switch make? Unfused switches don't need a high rating because you won't be using them to interrupt fault current.

  2. #12
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    Quote Originally Posted by ggunn View Post
    If the breaker has a sufficient rating what difference does the rating of the switch make? Unfused switches don't need a high rating because you won't be using them to interrupt fault current.
    They need a high SCCR (carrying, not interrupting rating like AIC) so that they can withstand a downstream fault until the upstream breaker opens.
    As simple as that.

  3. #13
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    Quote Originally Posted by GoldDigger View Post
    They need a high SCCR (carrying, not interrupting rating like AIC) so that they can withstand a downstream fault until the upstream breaker opens.
    As simple as that.
    Higher than the conductors?

  4. #14
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    Quote Originally Posted by ggunn View Post
    Higher than the conductors?
    Quite possibly.

    The switch has to withstand mechanical forces from high current and have low enough contact resistance at the contacts to avoid thermal or arcing problems.
    The conductors have no reason for localized hot spots and if they melt down in raceway it may not do serious damage. But their thermal mass is low enough that the sort of very short term current that the SCCR represents may not get them seriously hot anyway.
    The mechanical forces on them will be contained by the cable jacket or raceway. Or the tie downs if individual cables in a tray.

  5. #15
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    I dont know why the SCCR on these is so low, and I doubt one is going to explode at 20k, but nonetheless those are the ratings and the rules. Why is a fused switch rated so much higher? (I understand the difference between SCCR and AIC, but just saying a fused switch still has the mechanical side of things to handle just like a non fused)

    I would also add that, in my experience, SCCR is often ignored or the topic is simply not known. IIRC, the NEC has recently added more language in article 110 to "crack down" on this.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #16
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    The POCO info they have in their handbook shows the 75KVA single phase 120/240 transformer @16,000 -- & 100KVA @ 21,900 -- not really a common situation to have these large transformers in residential areas & if so the lateral usually drops the AFC significantly -- so you wouldn't have to worry with most MDP OCPD or bracing
    CircuitRyder --- Unfortunately not all good ideas are code enforceable.

  7. #17
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    Quote Originally Posted by mwm1752 View Post
    The POCO info they have in their handbook shows the 75KVA single phase 120/240 transformer @16,000 -- & 100KVA @ 21,900 -- not really a common situation to have these large transformers in residential areas & if so the lateral usually drops the AFC significantly -- so you wouldn't have to worry with most MDP OCPD or bracing
    I agree that fault current values are generally pretty low for residential, however I didnt notice anything in this thread being about specifically about residential.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  8. #18
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    No fused disconnect no matter th size is only rated to 10ka

    Sent from my SM-N920V using Tapatalk

  9. #19
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    Quote Originally Posted by electrofelon View Post
    I dont know why the SCCR on these is so low, and I doubt one is going to explode at 20k, but nonetheless those are the ratings and the rules. Why is a fused switch rated so much higher? (I understand the difference between SCCR and AIC, but just saying a fused switch still has the mechanical side of things to handle just like a non fused)

    I would also add that, in my experience, SCCR is often ignored or the topic is simply not known. IIRC, the NEC has recently added more language in article 110 to "crack down" on this.
    I think it is more of an unwillingness to pay for testing at higher fault currents, in combination with various breaker model families, than anything else.

    Given that the unfused switches are exactly the same enclosure and switching mechanism as the fused counterpart units, I also do not believe there would be any serious failure at high fault currents, provided a breaker upstream is properly rated.

  10. #20
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    Quote Originally Posted by electrofelon View Post
    I agree that fault current values are generally pretty low for residential, however I didnt notice anything in this thread being about specifically about residential.

    actually I got off on the wrong track as pv systems supply electricity & I would think the disconnect for pv fault currents would be rated per pv input power not utility -- so what is the max fault current on< 1 meg system???
    now I have to look in the book
    CircuitRyder --- Unfortunately not all good ideas are code enforceable.

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