MCCB Rating Selection - Incomer - Outgoing Coordination

As far as i know, IEC does not specify nor recommend any discrimination guidelines. If you think about it, it is a user's choice, since a very good discrimination system will be more expensive than a somewhat simpler cascading one. All design is a trade-off between conflicting requirements.

A very good paper on discrimination is here:ECT 167

As far as i know, Schneider guarantees discrimination between upstream and downstream breakers which are rated 2.5:1. LSIS claims 1.6:1.

Why should there be any regulation for sizing the Incomer MCCBs and/or the Outgoing MCCBs in a particular ratio?

Imagine you have a 2500kVA, 11kV/433V Transformer feeding a PCC (Power Control Centre) and from this PCC you also have to feed a small lighting panel, say, of 100A. The incomer ACB should be of 4000A rated current and with a short circuit breaking capacity of about 60kA (Assuming 6% Impedance of the transformer). And, the outgoing feeder in this PCC for the above lighting panel should have a continuous rated current of 100A and with a breaking capacity of 60kA.

The problem with MCCBs would be that a 100A MCCB Frame Size may not have a beaking capacity of 60kA; so, you may have to go for a higher frame size MCCB of, say, 400A, with a breaking capacity of 60kA and set the over current release of this 400A MCCB to 100A to have effective over current protection. This is how it is solved.

But, if the outgoing feeder is a Switch-Fuse-Unit, you don't have any problem as even a 100A HRC fuse would have a breaking capacity of 80kA or more.

As a rule of thumb the incoming > 1.6 times the outgoing but you can use MCCB with adjustable settings & suitable KA rating.

Following steps can be taken-

1) Calculate fault level at the incomer of the MCC.

2) You can assume the same fault level at the outgoing.

3) Plot this fault current on the trip characteristic of the outgoing breaker.

4) Check the total load current on the MCC and the largest starting current of the motors starting together.

5) The incomer trip characteristic should not trip under 4) above and should also be clear of the fault current trip curve of any of the out goings. This is called discrimination between incomer and outgoing breaker. Select an incomer size which fulfills these conditions.

6) My experience suggsts that the incomer can be twice the size of the outging provided it is more than the full load current of all ougoings.

Yes, you could have. The most practical approach is to match time-current curves for back up protection, and for proper coordination. I do not have IEC reference, but I can send you some manaufactirers' literatur if you wish.

http://www2.schneider-electric.com/documents/contractors/en/shared/training/discover_discrimination.exe