SCCR Explanation 1

[TexasPwrEng]

I'm having difficulty understanding the theory behind an SCCR rating being applied only to a specific combination of devices. A current limiting fuse for example interrupts the current in 1/4 cycle. So the maximum current seen by any device in series with that fuse should be whatever the value is at 1/4 cycle.

Why then can we only use UL approved combinations? It seems to me that the current limiting fuse will interrupt the current at 1/4 cycle, regardless of what other devices are in series with it. The SCCR rating should be based upon the fastest protective device.

I understand that I am wrong, but I don't understand why I am wrong. Does anyone have any insight?

 

[rbulsara]

The reasons are both technical and reglulatory (codes), especially in the USA.

1. The current limiting fuses are not current limiting at all currents, only at high currents.

2. UL (or equivalent nationally recognized testing lab)'s listing is a code requirement, whether it is standard SCCR
or series rating. UL will only provide a label if the devices (or combination there of) pass the test.

3. There is no approved calculation method to determine series rating (contrary to what any fuse mfr. might say).

In reality, if you check any published series rating tables by a reputed mfr, you will find that most MCCB are series rated with various types or class of fuses (RK, J, L etc), regardless of the manufacturer of the fuses. While the breakers have to be listed with a specific model number & mfr.

So what you are hoping to have is already in place, except that you need to verify that it is indeed a listed combination.


Rafiq Bulsara

http://www.srengineersct.com