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Thread: Shunt Trip

  1. #1
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    Shunt Trip

    When is a shunt trip required on the outside of a building?
    Stephen M. Butler, P.E.

  2. #2
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    Quote Originally Posted by sbutler View Post
    When is a shunt trip required on the outside of a building?
    You're going to have to make your inquiry a bit more clear.
    Knowing that a shut trip is usually a breaker option are you also implying that the breaker is located outside the building?
    Or is the breaker with the shunt trip option located in a panel on the inside of the building as an actuator of some type located on the outside of the building when pushed (or?) activated the shunt trip which opens the breaker?

  3. #3
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    I will clarify. The MDP will have an 800A MCB and will be in a locked electrical room. The owner has indicated a shunt trip on the outside of the building is required for the fire department since they cant get to it to secure in case of a fire.
    Stephen M. Butler, P.E.

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    Quote Originally Posted by sbutler View Post
    I will clarify. The MDP will have an 800A MCB and will be in a locked electrical room. The owner has indicated a shunt trip on the outside of the building is required for the fire department since they cant get to it to secure in case of a fire.
    There is nothing in the NEC requiring that.

    However it is not an unusual fire dept request.

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    Quote Originally Posted by sbutler View Post
    I will clarify. The MDP will have an 800A MCB and will be in a locked electrical room. The owner has indicated a shunt trip on the outside of the building is required for the fire department since they cant get to it to secure in case of a fire.
    First, the actual shunt trip is an accessory which is added to the inside of the breaker. As such include it with the specification for the breaker. You have a choice of voltages, 120vac is commonly used. The shunt trip will be normally terminated if (2) terminal which are accessible of one of the sided of the breaker. Then you need a reliable 120vac power source which most likely will be supplied by the building power. If the main is closed and is supplying power to the building then you should have 120vac available.
    Then you need a N/O PB that is accessible on the outside of the building. It must be NEMA 3R enclosed and possibly lockable to allow for emergency access only.
    The power source in wired in series with one of the terminals of the ST and the N/O PB, back from the PB to the other terminal of the ST. Not a complicated wiring scheme at all. The shut trip in the breaker has what is called a cut-off switch with is wired in series with the ST solenoid coil which opens automatically when the breaker opens removing power from the ST coil even though the external PB may be held closed. If you didn't have the cut-off switch you would smoke the ST coil should power be still applied to the ST even after it tripped the breaker.
    I tried to be detailed enough such that you feel comfortable in using a shut trip in this manor. The key is a reliable power source in which the ST can be activated. If this isn’t good enough then there are other ways to accomplish tripping the breaker.

  6. #6
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    Be careful though, some manufactures add a stored energy backup for the shunt trip, so even if the main and utility is dead, there is still enough charge to kick your butt. The bad thing about this is the manufacture did not post any warnings anywhere on the gear.

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    Quote Originally Posted by templdl View Post
    The key is a reliable power source in which the ST can be activated.
    Years ago this was a common way in my area.

    They tapped the breaker with a pigtail socket with a 15 amp Edison base fuse in it.




    The trip for this one was a fire alarm type pull station in the lobby of the apartment building.

  8. #8
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    Quote Originally Posted by hillbilly1 View Post
    Be careful though, some manufactures add a stored energy backup for the shunt trip, so even if the main and utility is dead, there is still enough charge to kick your butt. The bad thing about this is the manufacture did not post any warnings anywhere on the gear.
    What you are refering to is a capitive discharge shunt trip option which is an external device that requires it be matched with a specific internal shunt trip. A capacitive shunt trip device sometines refered to as a CTD basically charges a capacitor which stores enough electrical power to trip a shunt trip should the system loose electrical power and the breaker is required to be tripped. But, there is a definite length of time that you can wait after a power loss to do so as the power store will eventually decay.
    Please note that an external CTD is required.
    And he's right, if the CTD is still charge it will bit you.

  9. #9
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    Quote Originally Posted by iwire View Post
    There is nothing in the NEC requiring that.

    However it is not an unusual fire dept request.
    Or data centers. I seem to recall that there is an NFPA or other standard that requires that, but could be faulty memory and was some other standard practice, like IBM used to set standards for their data center constructions that everybody just adopted back-in-the-days.

  10. #10
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    Quote Originally Posted by iwire View Post
    Years ago this was a common way in my area.

    They tapped the breaker with a pigtail socket with a 15 amp Edison base fuse in it.




    The trip for this one was a fire alarm type pull station in the lobby of the apartment building.
    As a side note it is interesting that the breaker doesn't have a nameplate nor does it have a modification tag which with tell the electrician what accessory is inside.

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