Panel Short Circuit Rating

Hi everybody

you select the breaking capacity for the incomer to be 50 KA for 1 sec which supposed to protect MDB and as soon as we know that a discremination time difference between the incoming and outgoing must take place as a preventive protection action your selection is good but in some industrial application it is required as your client required to be the same breaking capacities due to various types of highly inductive loads

Best Regards

Just because you have a breaker rated for 50 ka in the MDB does not prevent the fault current of 50 ka from occuring at the breaker rated at 36 ka. I assume the two breakers are in the same panel and the distance between them is small. Does the main breaker have some current limiting feature?

Friend,

I believe you are talking about "series ratings", but there is not enough information to be sure. Various manufacturers do have test results to show that a specific model of MCCB can be used to provide fault current protection to lesser-rated branch breakers. The NSX series is pretty intelligent, so it wouldn't surprise me if there is this sort of rating. You will have get a rating in writing from Schneider, with certain limits to settings or components.

--JMM

Please confirm which MCCB's are used, we would need to know the incomer plus the largest outgoing and smallest outgoing.

Schneider do produce tables to confirm if cascading is achieved.

There is nothing in IEC 61439-2 that restricts this practice, however it may be your client who restricts it, since if you have a short circuit in one circuit you will loose the entire board of 35 circuits which may not be acceptable. This is normally the reason for pushing to get full discrimination.

Out of interest have you carried out full design verification to IEC 61439-2 on this assembly design?

there is a difference between discrimination where a small breaker trips before a larger one and cascading. The upshot is if the smallest device sees the full fault level for a very small time it must be able to contain it all be it unusable afterwards. Merlin Gerin / Schneider produce cascade tables where they show which device will cascade with another. Obviously if you have mixed manufacturers you will be goosed for getting the test information. This information might sway your client.

Cascade tables are produced by most reputable manufacturers but each device is not shown and its generally hard work.

No question! Your outgoing MCCBs must also be rated for 50kA only.

With the exception the mdb is for a peculiar application, i'll say your client is a contractor who knows little or too well of what he is requesting. And Schneider will say no to keep on the safe side of everything but economic feasibility of your panel.