Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 3 replies
  • Subscribers 37 subscribers
  • Views 739 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

MCB Vs MCCB instantaneous Pick up

Nick Parker

Let's say we have a MCB and MCCB with 16A rating. MCB will trip based on the curve type, say if B curve is used, it trips between 3-5 times the 16A. Therefore, the instantaneous pickup would be anywhere between 48-80A.

For MCCB, at what current the pick up woul be? (any thumbrule?.

This is to decide to use MCB or MCCB for the final circuit as the fault current would be lower for the final circuit and pondering over if the MCCB could clear for the end of line fault.

  • 0

    The more complex MCCBs allow you to adjust both the near instant and normal pcik up values, and in some cases the time delay as well. Otherwise, browse the maker's data.

    But the useful range for real applications often ends up being   fairly similar to something between the B and C type breakers. This is not too surprising, as the sort of cable requirements for voltage drop mean that if you set the instant trip to a lot more say - 20 times the nominal current, then you would never operate (or you need a massively oversized supply). ( e.g.  the same cable resistance that gives 5% voltage drop at nominal full load sort of maps to a 20:1 ratio of short circuit to nominal current and a 10% drop to 10:1 ratio etc.) Equally co-ordination with the inrush performance of fuses, breakers etc down stream often  means that a ratio  much less than 3:1 is likely to lead to uncertainly about what will trip first.

    And then when Zs is a bit uncertain, there may be an extra 'shunt trip' - usually a mains voltage or 24V solenoid add-on that forces a trip, that may be wired to be operated by an earth leakage monitor, or indeed any number of other alarm devices.
    Mike.

  • 0

    You will often find that there will be no adjustability in the smaller frame, lower current sizes of T/M MCCBs, with the Im release fixed, at say 500A and thus no distinction between the Zs max at 0.1s and 5s disconnection. So whilst your 16A type C MCB will require only 80A to operate in 0.1s, the 16A MCCB will require 500 and thus a much lower Zs.

    With say a 63A MCCB with a fixed Im of 500 upfront of a 16A type C MCB, selectivity may still be difficult to achieve. I did read somewhere in Hagers technical bumf however, that whilst selectivity is desirable, it isn’t necessary for circuits up to 63A (not BS7671). 

  • 0

    Somehow things are much simpler with HBC fuses!

       - Andy.

Community Rules & Guidelines