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Selectivity tests on CB

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

RE: Selectivity tests on CB

Possible? Yes. Practicable? Maybe. Cost-effective? Unlikely.

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

(OP)
End customer want to be sure about selectivity. This switch board contain some safe shutdown control relay logic for one plant. Switchgear are not from same manufacurer. Some is Siemens made and some is Merlin Gerin, and this decision is maded by end customer based on failure history. Switchboard have DC 220V part and AC (400Amps incoming) 400/230V 50cps part of wiring. For DC (low current, maybe few amps total) part I have only conceptual idea how to test this but for AC side I am not sure how to accoplish this test.
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

You'll need to know the maximum fault current that the smaller breaker could possibly see and the use a primary current injection test set to provide that current. If the maximum fault current is above the instantaneous trip point of the larger breaker they'll both trip, not much need for the test. If the maximum fault current is well below the instantaneous trip of the larger breaker you'll just get the smaller breaker, again not much to gain from the test. If it's close you'll get far more value for money spent by reducing the available fault current.

RE: Selectivity tests on CB

I suggest you look in to renting a reasonably large loadbank with fairly fine control of the load and which is capable of increasing the load in user-defined pre-determined steps.

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.

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