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Thread: Interrupt Rating for local Disconnecting Means 430.110

  1. #1
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    Interrupt Rating for local Disconnecting Means 430.110

    I'm a long time reader, first time poster. Hoping for some input on a little NEC mystery.

    I was perusing a manufacturer's catalog for an enclosed disconnect switch for a feeder going to a 3-ton crane motor. Out of curiosity, I tried to find the actual interrupt rating of the switch, and was surprised to find none listed.

    I grabbed my trusty 2011 NEC hand book, and found 430.110, which is titled "Ampere Rating and Interrupting Capacity". Sections A thru C are all about the amp rating, which is standard 115% motor FLA stuff. But I was surprised to find that, despite the section title, no actual requirements are listed for interrupt rating. Granted, you probably would not want to throw the switch when the motor is under load, but I would tend to think that there would be at least a minimum requirement.

    Anyone know if there is a standard or guideline on this? Is there no interrupt-capacity rating for service disconnects?

  2. #2
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    Consider both Sections 110.9 as the general statement and 430.100 (E) where the switch is actually an isolation switch rather being intended as a interrupting switch. Note: 430.110 is 600V and below; 430 Part XI appends or amends for over 600V.
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  3. #3
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    Think about it. What are you trying to interupt? With a manual switch may you be only interupting the load? With an automatic device such as a breaker wouldn't it be required that it would interupt under a fault condition?
    So, when it comes to the term "interupt" it must be defined because I don't think that you would want to be around should you be unlucky enough to try to open a "disconnect switch" under a faulted condition.

  4. #4
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    Normally you can find that info in the manufacturers catalog. For example heres a sheet from G-E's BuyLog:

    Cartridge Fuse, Type TH Switches
    —Designed for applications where safety, high performance
    and continuity of service are required.
    —UL Listed. Products in this section have been tested by UL and
    shown to meet the construction and performance requirements
    of UL Standard Number 98, Enclosed and Dead Front Switches.
    —CSA Certified.
    —Meet Federal Specification WS-865C for heavy duty switches.
    —Meet major automotive manufacturer’s and JIC electrical
    specifications.
    —Meet NEMA Enclosed Safety Switch Standard KS1 for Type HD.
    —Full cover interlocks.
    —60°C and 75°C conductor ratings.
    —Quick-make, quick-break mechanisms.
    —Suitable for use as service entrance equipment when installed
    in accordance with the National Electrical Code.
    —UL Listed ratings for fusible switches when used with
    these fuses:
    —Class H (10,000 rms amps, sym IC). Standard on 30-600 amps.
    —Class L (100,000 rms amps, sym IC). Only applicable when
    used with 800 and 1200 amp Type TC Safety Switches (e.g.
    TC72367)
    —Class J (200,000 rms amps, sym IC). See page 2-28 for field
    conversion details.
    —Class R (200,000 rms amps, sym IC). Order appropriate
    Class R fusing kit from page 2-28.
    —Safety switch product numbers with suffix “SS” are grade
    304 stainless steel. Product numbers with suffix “SS316”
    are grade 316 stainless steel.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  5. #5
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    Quote Originally Posted by templdl View Post
    So, when it comes to the term "interupt" it must be defined because I don't think that you would want to be around should you be unlucky enough to try to open a "disconnect switch" under a faulted condition.
    Well of course. Even under normal operating conditions, opening the switch while an inductive load is operating could get exciting. The local disconnect is not something you would normally use to break the circuit while it's under load (it's only real purpose is LO/TO)

    I ask only out of curiosity because I was puzzled that there wasn't a guideline on "Interrupting Capacity" in a section with that very phrase in the title.

  6. #6
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    SC Rating of disconnects

    The ratings to be concerned about with disconnects are Voltage, continuous current, HP and SC rating.

    The short circuit rating is 10 kA for unfused disconnects, and it's the fuse interrupt rating for fused-disconnect switches.

    John M

  7. #7
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    Quote Originally Posted by BringDaJuice_PE View Post
    Well of course. Even under normal operating conditions, opening the switch while an inductive load is operating could get exciting. The local disconnect is not something you would normally use to break the circuit while it's under load (it's only real purpose is LO/TO)

    I ask only out of curiosity because I was puzzled that there wasn't a guideline on "Interrupting Capacity" in a section with that very phrase in the title.
    It is obvious that you are stuck on the term "interrupting rating."
    You have to separate out the term "interrupting rating" when taking about disconnect or safety switch as it does not apply. Those devices are rated to disconnect the load that they are rated for. If a switch is HP rated it is designed to open LRA. You stated that opening a disconnect switch to open an inductive load could be exciting but that’s what happens when you open a HP rated disconnect switch on LRA. When you refer to interrupting capacity it can be interpreted as to mean opening up and clearing a fault or short circuit.
    It only would apply to a device that is required to operate automatically under a fault condition such as a TM breaker. Even a "non 'automatic" circuit breaker which is identical in construction as a standard TM breaker does not have an interrupting rating as it is not allowed by UL. Even take a look at an MCP, mag only motor circuit protector which also does not have an interrupting rating either as it would violate it UL listing as the non-automatic breaker would also.
    Then what would it be for the lowly safety of disconnect switch? None. The next lower animal in the pecking order would be a simple isolation switch which can not be opened even under load which I don't think you will find available in a 600v and below rated switch.
    A switch is just that as it has no arc chutes to clear the arc when the contacts open. If you have the opportunity to scrap out a larger breaker, other that a small residential ones, take it apart and compare it to a disconnect switch.
    A disconnect switch would have a withstand rating which is the switch's ability to withstand fault and live to tell about it. A withstand rating are never to misunderstood as being an interrupting rating. When a switch is subjected to a fault there is a tendency for the contacts to be blown apart. The withstand rating is the maximum fault that the device can withstand before it is damaged.

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