To know the reason why it does not trip the MCCB and MCB.

Discussion in 'General Electronics Chat' started by tahir mehmood, Sep 1, 2016.

  1. tahir mehmood

    Thread Starter Member

    Jun 28, 2013
    short circuit happened in of the tube lights in air handling unit and it tripped the substation feeder which is Air circuit breaker. it rating is 200AT/800AF. its setting for instantaneous setting is 400 amperes. the details are as under:

    Substation feeder breaker rating: 200 AT/800 AF.

    Substation feeder breaker setting: 200 x 2 = 800 A (Instantaneous)

    Distribution panel main breaker rating: 60 A

    Branch circuit breaker rating: 20 A

    Short circuit fault current: 1007 A

    The question is why it did not trip the branch breaker or the main breaker of the distribution panel.?
  2. MaxHeadRoom


    Jul 18, 2013
    What is the power rating of the air handling unit and lights?
    It may be a question of the Sub.Stn breaker being faster, although depending on the Kw size of the unit, I would have thought that conductor resistance would have offered some limit.
  3. tahir mehmood

    Thread Starter Member

    Jun 28, 2013
    Max it's just a 28 watts tube light where the short circuit happened. because of that the substation feeder breaker tripped, bypassing the main panel breaker and the branch breaker..
  4. profbuxton


    Feb 21, 2014
    I am going to assume the feeder breaker is not an earth leakage type. The possible way it could have been tripped when your light shorted is if the load through the breaker was already close to its set point and the light short cuircuit current caused it to trip on instantaneous. Can't really think of anything else. I guess you can check your existing load current to see what it actually is.
  5. tahir mehmood

    Thread Starter Member

    Jun 28, 2013

    max, the feeder breaker is having setting for earth fault also. the branch breaker is MCB, and panel main breaker is MCCB with out setting. so it means there is no benefit of these two breakers, as their main function is short ckt protection and overload, every time if short circuit happens it will trip the substation breaker. Max is this a design fault or what?