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Thread: Non-Fused Disconnects and 110-10

  1. #1
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    Non-Fused Disconnects and 110-10

    I have asked several manufacturer's of Non-fused disconnect switches what there AIC ratings are. Every manufacturer has indicated they do not have an AIC rating for their non-fused disconnect (safety) switches. How can a designer comply with NEC 110-10 (2002) if the component does not have an AIC listing? This same issue carries over to devices such as receptacles, starters, snap switches, etc..

  2. #2
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    Re: Non-Fused Disconnects and 110-10

    How would a device that is not intended to clear a fault nor open in case of a fault come under the requirements of 110.10?
    Jeff Cook
    Master Electrician

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    Re: Non-Fused Disconnects and 110-10

    I just checked the manufactures label in both a 30amp 240v and 100amp 600v non-fused EXO and both state that injury may occur if used on circuits where the available fault-current is above 10,000 amps. These are GE disconnects, I cannot say what other manufactures may show in their equipment with respect to application. Typically EXO's (fusible)get their fault-current ratings based on the fuses used.

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    Re: Non-Fused Disconnects and 110-10

    Looking at the UL White book, I see numerous listings for switches of all types. I also find this: "Knife switches are marked with a short-circuit current withstand rating", and this: "Switches which are marked "Suitable for use on a circuit capable of delivering not more than _______ RMS Symmetrical Amps". Look at WIQG, WIOV, WJAZ. Also, WHXS, where I found this: "Switches intended for isolating use only, are marked".

    I don't know who you have talked to with these manufacturers, but I would venture they were the wrong person to ask.


    The issue should not necessarily carry over to snap switches, receptacles, starters, etc., as these items hopefully have protection in the OC device for their branch circuit.
    Earl
    Earl

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    Re: Non-Fused Disconnects and 110-10

    Do any of the devices open automatically, that is in response to a fault and clear it? All the devices you have pointed out are either manually operated or control and not protective devices. They may have a withstand rating, that is their abiltiy to ride throught a specific value of fault, but how can they have an interrupting rating? Remember that the contact must be able to clear a fault. When it is a manually operated device will you be operaing the device at the exact instance tyhat a fault is present?
    As an example, A "fused" disconnect switch relies on its AIC rating because of the fuse and not by the switch itself. The fuse does the interrupting.
    As another example A contactor and starter must open and close the load such as a motor's full load and locked rotor current. It doesn't have a means nor is it intended to interrupt a fault.
    A combination starter incorporates "motor circuit protection" that adds either a fuse of thermal magnetic or magnetic only circuit breaker combined as a "combination" starter. Fuses do have a kaic rating as do TM breakers where mag. only (motor circuit protectors, MCP)breakers do not. In order for a combination starter to get an AIC rating the entire assembly must be testing together to achieve a given fault rating with a contactor/OLR damage standard met. This may be that if a NEMA 1 enclosed combination starter is subjected to a 65ka fault that the "combination starter" is capable of clearing the fault, very possibly destroying either the contactor or OLR or both, but yet keep all the pieces within the enclosure.

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    Re: Non-Fused Disconnects and 110-10

    From the Square D Digest:
    "Not Fusible Safety Switches
    Any brand of circuit breaker or fuse not exceeding the ampere rating of the switch may be used ahead of an unfused safety switch when there is up to 10,000A short circuit available.
    Above 10,000A - When applied on systems with greater than 10,000A short circuit available, the UL Listed short circuit current rating for SQUARE D unfused switches is based upon the switch being used in conjunction with fuses or SQUARE D circuit breakers or MAG-GARD circuit protectors."
    Just because you can, doesn't mean you should.

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    Re: Non-Fused Disconnects and 110-10

    Jim, that does apply to an "aic" *amps interrupting capacity) rating. Non-fused disconnects still don't have an AIC rating.
    However they must be able to "withstand" a fault, that is being able to hold themselves together if subjected to a fault. The upstream device is intended to prevent the non-fused disconnect switch from being subjected to a fault that exceeds its ability to withstand.

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    Re: Non-Fused Disconnects and 110-10

    Generally a non-fused disconnect switch has a 3 cycle withstand rating of 5-10 times the full load amps, with a minimum of 5 or 10k depending on the manufacturer. I have never been able to find documentation of such, just word of mouth.
    The comments regarding the AIC rating for a manually operated switch (no automatic disconnect) are correct, there will be none.

    You must be sure the withstand is adequate.
    Almost always, a fused disconnect will take on the AIC and withstand values of the installed fuses.

    One last note, unless there is a series rating, "The upstream device is intended to prevent the non-fused disconnect switch from being subjected to a fault that exceeds its ability to withstand." is not valid until proven in a testing laboratory.
    Ron

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    Re: Non-Fused Disconnects and 110-10

    templdl, My quote is taken directly from the UL Maximum Short Circuit Current Ratings from Square D's new Digest page 3-6.

    Ron, I agree with you about the series rating many of the "new generation" rotary style motor disconnects do not have the required series ratings so they are limited to the same levels as a motor controller (usually 5-10KA). The above reference includes the Square D UL Listed combinations.
    Just because you can, doesn't mean you should.

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    Re: Non-Fused Disconnects and 110-10

    It appears that the terminology "UL Maximum Short Circuit Current Ratings" is another way of saying "withstand" which has been described by Ron. Again, it can in no way be be*considered an interrupting rating, that is an ability to open and clear a fault.

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