MCB Question .

NoseyMike2010 wrote: »

Something I'd like to know. I am wondering if you had two idential systems and 1 had a 6 amp breaker Type B and the other type K which would trip 1st if they had exactly the same surge.

I have looked at the Characteristics graph and have no idea whats happening in them! Sorry. A virgin to all this if I am honest!


NoseyMike2010

Simple answer type B would trip first.

Type K is designed for use in application where temporary surges (motor starts etc) are common and hence would take time to trip.

However in the event of a direct short circuit they should both trip simultaneously.

Side note I was not aware that Type K were available in singles, Always assumed they were a 3 phase breaker only..

NoseyMike2010 wrote: »

So is Type B a slow tripping breaker?

If you can explain with a mathematical solution it would help alot.

Not exactly sure where you got that from.

The Type K is designed to allow short momentary surges such as those associated with loads such as AC motors where you have a short intense current draw during start up.

Type B are designed for a totally different application where surges are not expected such a domestic situation.

They both have specific applications and as such are not directly comparable.

They most certainly could not be described as slow and quick tripping. As I said in my original post in a short circuit situation they should both trip at the same speed.

There are 4 types of MCB type B, C, D, K, Z and there performance characteristics or trip curve are defined by BS EN 60898 and BS EN 60947-2

In simple terms the letter indicates the instantaneous tripping current, that is the minimum value of current that causes the circuit-breaker to trip without intentional time delay (i.e., in less than 100 ms), expressed in terms of In:

for a Type B N is defined as greater than 3 up to or equal to 5

So for a 6 amp type B MCB the instantaneous tripping current would be between 18amp to 30amp (ie. between 3 and 5 times the rated current)

For a type C its 5 to 10 times
Type D its 10 to 20 times
Type K its 8 to 12 times
and for Type Z its 2 to 3 times

But that a crude description of an MCB

All types will trip for any load over their rated load but the trip time will not be instantaneous (ie. longer than 100ms).

I would recommend you do more reading.

Start here http://en.wikipedia.org/wiki/Circuit_breaker and here http://www.moeller.co.uk/ta_mcb_basics.htm and then look up the relevant standards.