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Thread: Interlocking as to not overload a 75kva

  1. #11
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    Quote Originally Posted by Smart $ View Post
    Depending on who you talk to, driving two relays with timers or a PLC does not interlock out the loads unless you use:
    • a mechanical relay interlock, or
    • electrically use a NC contact (auxiliary or otherwise)

    ...to completely disable the other relay while the one is in operation.
    Simple interlock in the logic works but if you must guarantee it, yes.
    Tom
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  2. #12
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    Quote Originally Posted by ptonsparky View Post
    Simple interlock in the logic works but if you must guarantee it, yes.
    Programmable electronics have been known to lose their programming from time to time... then there's no interlock until reprogrammed. During that time, you may have an irreversible equipment catastrophe... or worse. That's perhaps an extremist point of view... but it is valid.
    I will have achieved my life's goal if I die with a smile on my face.

  3. #13
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    Quote Originally Posted by Smart $ View Post
    Programmable electronics have been known to lose their programming from time to time... then there's no interlock until reprogrammed. During that time, you may have an irreversible equipment catastrophe... or worse. That's perhaps an extremist point of view... but it is valid.
    I've also seen broken mechanical interlocks, welded contacts, etc. Keep putting in redundant checks until Iwire calls ENOUGH already!! An extremists point of view but valid.
    Tom
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  4. #14
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    Quote Originally Posted by ptonsparky View Post
    I've also seen broken mechanical interlocks, welded contacts, etc. Keep putting in redundant checks until Iwire calls ENOUGH already!! An extremists point of view but valid.
    I will have achieved my life's goal if I die with a smile on my face.

  5. #15
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    Quote Originally Posted by justin13me View Post
    So I guess the question now is how do I wire this?
    I am going to point out that the NEC does not require an interlock.


    220.60 Noncoincident Loads. Where it is unlikely that two or more noncoincident loads will be in use simulta-neously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.
    So if you don't expect the loads to run at one time ....

  6. #16
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    Quote Originally Posted by iwire View Post
    I am going to point out that the NEC does not require an interlock.

    So if you don't expect the loads to run at one time ....
    Fair point. As I said earlier...
    Quote Originally Posted by Smart $ View Post
    Depending on who you talk to...
    Another possibility is to power via one OCPD. Let timer/programming handle load diversity... but if both loads are incidentally powered the OCPD will trip.
    I will have achieved my life's goal if I die with a smile on my face.

  7. #17
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    Quote Originally Posted by Smart $ View Post
    Fair point. As I said earlier...
    And I do not think it maters who you talk to.

    'Unlikely' has a different meaning then 'cannot'

    NEC = minimum, we can always choose to go further.

  8. #18
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    Quote Originally Posted by iwire View Post
    And I do not think it maters who you talk to.
    ...
    We would not be having this discussion if it didn't matter who you talk to. Granted the requirement doesn't change, but its interpretation most certainly does.
    I will have achieved my life's goal if I die with a smile on my face.

  9. #19
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    Quote Originally Posted by Smart $ View Post
    We would not be having this discussion if it didn't matter who you talk to. Granted the requirement doesn't change, but its interpretation most certainly does.
    Does unlikely have a diffrent meaning than cannot?

    If so it should not mater who you talk to.

  10. #20
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    Quote Originally Posted by iwire View Post
    Does unlikely have a diffrent meaning than cannot?
    Yes. Unlikely = not probable to happen. Cannot = not possible to happen.

    ofc that is assuming failsafe engineering and proper use of the terms in the first place.

    It is unlikely that my home will burn down today due to the failure of a FPE breaker.

    It would be impossible for my home to burn down due to electrical failure if I did not have an electrical service. I realize there are some incredibly far-fetched scenarios where this may happen, but for all intents and purposes it is so far beyond the realm of possibility as to be 0.

    eta: interlocks are not intrinsically failsafe. I dealt with interlocked equipment for many years, and while they worked, you were not allowed to turn off a breaker to say the ash screw to the bin (last eqpt in the long chain of eqpt than must run) and use its interlock to shut off the bucket elevator; if you were working on the bucket elevator, its breaker must be secured.

    would a lead/lag system work for the OP?
    Last edited by JFletcher; 03-31-17 at 03:13 PM.
    Electricians do it until it Hertz!

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