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Thread: shunt trip breaker wiring

  1. #11
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    If you need to be that sure, then use a contactor instead, wired so it drops out under emergency conditions.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  2. #12
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    My experience is with 120/208 and the control voltage is derived from the same circuit the breaker protects, usually 20A. So, not only does that handle the requirement that the shunt trip coil not be continuously energized, it also provides some assurance that the control voltage will be present, unlike if it were provided by another circuit.

    If the control circuit must be such that the reliability is compromised by factors such as separate OCP, you probably shouldn't be using a shunt trip breaker. Use a contactor wired to latch through the EMO button. This also gives the advantage of using LV control if desired.

    -Hal

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    Quote Originally Posted by LarryFine View Post
    If you need to be that sure, then use a contactor instead, wired so it drops out under emergency conditions.
    Agreed, so ...

    Kitchen equipment being left on after the shut down button is depressed is ... no big deal?

    A gas station E-Stop does not work because the shunt trip control circuit dropped out (yes I've seen people use shunt trips for this)?

    Personally I generally don't like to use shunt trip breakers for e-stops or critical got to shut it down equipment.

  4. #14
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    Quote Originally Posted by oldsparky52 View Post
    Agreed, so ...

    Kitchen equipment being left on after the shut down button is depressed is ... no big deal?
    To me, it is a big deal. Unless there are already shunt-trip breakers installed (I wire both existing and new systems), I usually install contactors for appliances.

    Failing to energize is less of a hazard than failing to de-energize to me.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by LarryFine View Post
    To me, it is a big deal. Unless there are already shunt-trip breakers installed (I wire both existing and new systems), I usually install contactors for appliances.

    Failing to energize is less of a hazard than failing to de-energize to me.
    +1 (when I was working )

  6. #16
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    Quote Originally Posted by oldsparky52 View Post
    Agreed, so ...

    Kitchen equipment being left on after the shut down button is depressed is ... no big deal?

    A gas station E-Stop does not work because the shunt trip control circuit dropped out (yes I've seen people use shunt trips for this)?

    Personally I generally don't like to use shunt trip breakers for e-stops or critical got to shut it down equipment.
    When I worked for a contractor, my techs would take the power for the shunt (if we used one) from the device being shutdown. If the power failed, no worries, because the equipment was off already.

    Usually I had a strong preference for contactors that had to be energized into the "on" position. As observed elsewhere here, not coming on when wanted is better than not going off when needed.

  7. #17
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    Quote Originally Posted by gadfly56 View Post
    As observed elsewhere here, not coming on when wanted is better than not going off when needed.
    Fire Pumps being one of the many things I would hate to have wired that didn't come on.... I think we all like our systems to work properly, so we don't sink our reputation when we turn on a light switch and the door bell goes off in front of the wrong employer/customer.

  8. #18
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    Quote Originally Posted by hbiss View Post
    As long as the control voltage is 120 I see no reason the control wiring can't be run in the same conduit as the current carrying conductors. There is nothing that says they must either.

    -Hal
    Or any other class 1 control circuit is acceptable to run in same conduit.

    Quote Originally Posted by oldsparky52 View Post
    Larry, how would you provide OCP for the control wiring if the shunt trip breaker has a 60-amp rating for OCP?

    Then, do you make any attempt to prevent the control wiring circuit from becoming de-energized and useless for operating the shunt trip?
    Good questions. supplemental overcurrent is simple, but can leave you with no power to operate the shunt trip. If you happen to have a control circuit supplied by the main circuit in some way hopefully you can use that control circuit. Should the OCPD for said control circuit be open, presumably you have no control and know there is a problem with something. Outside of that considerations become more complex.

    I connected a shunt trip breaker for a CT scanner once, was like a 150 or 200 amp breaker, 480 volts. Can't recall anymore what shunt trip unit volts was, probably 120 volts though. I think shunt trip supply came from CT control unit, so it either was derived from the supply circuit or at least was same circuit that controlled things, so if control power was lost the CT wasn't operating anyway and there would have been no need to actuate the shunt trip in some emergency situation.

    725.43 does have informational note that tells you a few places where a 14 AWG or larger control circuit conductor may be allowed more than a 15 amp OCPD. I am most familiar with motor control circuits - there you can have 14 AWG control circuit conductors on a 45A OCPD in some cases, or increase the control conductor to 10 AWG and it can be on 90 amp OCPD and would be compliant.

    For shunt tripping cooking appliance circuits under an exhaust hood - I think you often see the exhaust hood control circuit being the shunt trip supply circuit. If the hood is not working you may not have shunt trip functionality, but with no exhaust hood they generally won't be cooking either, too many vapors that need to be extracted from the space will occur and they will take action to fix things.
    I live for today, I'm just a day behind.

  9. #19
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    I believe a 480v breaker's trip coil will operate on 120 to 480 volts, expecting the trip current to be momentary.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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