Selectivity tests on CB
Selectivity tests on CB
(OP)
Hi there..
Tell me will be possibile to conduct live selectivity tests on MCB in distribution boards? Typical nominal current of MCB is up to 40 Amps and main feeder is with CB of 400 Amps. With this test I must assure that my MCB will trip before main CB in case of control circuit inside cabinet fails. And what kind of measurement equipment setup do I need for that?
Thanx in advance for all help
Tell me will be possibile to conduct live selectivity tests on MCB in distribution boards? Typical nominal current of MCB is up to 40 Amps and main feeder is with CB of 400 Amps. With this test I must assure that my MCB will trip before main CB in case of control circuit inside cabinet fails. And what kind of measurement equipment setup do I need for that?
Thanx in advance for all help






RE: Selectivity tests on CB
Why do you want to do this? There are published coordination tables available if both breakers are from the same manufacturer. It is very unlikely that a set of circumstances exist where a 40A MCB will not maintain full selectivity with a 400A MCCB. The exception might be if the 400A breaker is equipped with earth fault protection and the MCB-protected circuit has a fault to earth.
RE: Selectivity tests on CB
I now that is possibile to compare I-t charts for both CB and MCB and based on this to make selectivity report but in this case I must make actual test.
Thanx for support...
RE: Selectivity tests on CB
RE: Selectivity tests on CB
If you exceed 800A load current then you are into the instantaneous trip region of a 40A Type D breaker which has the highest mag trip threshold of the common IEC 60898 MCBs. If the 400A breaker holds in while the MCB clears at 800A then I would think you're OK - you don't need to test all the way up to then ultimate breaking capacity of the MCCB, which would demand equipment which you won't have direct access to. I guess the region of greatest interest to your client is where both breakers are on their thermal curves, although with such a large ratio of breaker sizes this testing seems like gross overkill.
Check whether the MCCB has an earth fault (the 'G' of LSIG) element. These often cause mis-coordination in installations where outgoing loads are not provided with E/F protection and the incoming breaker is: the E/F protection on the incomer usually wins the race and takes the whole board out before the breaker feeding the faulty load clears. It is bad design, or bad configuration, but it is by no means uncommon on medium-sized installations where an MCCB is used as an incoming device.