MCB short circuit capacity

jamesBoater · Feb 5, 2013

6 amp BS EN 60898 .....6ka being the maximum fault current the device can handle, although the device may no longer work after that. The 3 underneath this being the fault current (ka) the device can handle without damaging it and then being able to be reset.

I have been told 2 different reasons that the 3 appears under the 6ka so would just like to see what anyone else has on this.

Thanks James.

kingeri · Feb 5, 2013

What were the reasons?

jamesBoater · Feb 5, 2013

The 3 being what fault current the device will safely interrupt without internal damage to it. (Was taught this today in college by a teacher of 35 years in the trade)

The 3 having no relevance to RSCC with the device.

Paul.M · Feb 5, 2013

I'm interested in what your tutor would reply to this:-

If you have a prospective fault current of 7.5kA once established by testing, why would you not change the MCB's to a 10kA ratting?

How can he test the MCB after a fault occurred and what fault current flowed at the time?

jamesBoater · Feb 5, 2013

Another MCB i looked at (10ka) also has a 3 under it!

Darkwood · Feb 5, 2013

If you have an upstream device that has higher than fault current rating then mcbs etc downstream dont need to meet the kA rating of the fault current but if you have the option to its good practice to do so and fit a higher kA rating.

You need to seek manufacturers advice to the actual tolerances against tabulated but any device that has had a fault current exceeding it kA rating should be replaced but has anyone ever bothered ... especially domestic when numerous faults may have occured on any given breaker that may have exceeded it kA rating.

Andyroid5 · Feb 5, 2013

jamesBoater said:
Another MCB i looked at (10ka) also has a 3 under it!

As I understand it, Its the class rating of the device,, it can be 1,2 or 3 ,3 giving the least let through energy or I2t on a fault (the best)
If you look at a manufacturers data sheet it gives you the values which can then be used to design your installation.

Here you go .. have a look on page 12 of this
http://energy.te.com/getDocument.aspx?PRD_ID=3628