MCCB

As the terms imply, Making Capacity is what the CB can make on to, without damage, and Breaking Capacity is what it can break safely. You should choose a MCB or MCCB whose capacities are higher than the maximum that the system can deliver. For this purpose, you need to be able to calculate the system fault level.

Thanks for information. Also, plz let me know how to calculate the fault level in a given system eg control panel. I need to know is there any specific mathematical formula.

There are many papers downloadable from the web which guide you in this calculation. A good paper is ECT 158 by Schneider Electric.

The making capacity is usually more than the breaking capacity.

Log on to www.electrotechnik.net/2010/11/making and breaking capacity of circuit breaker

Also refer www.electrical engineering portal.com. Breaking capacity is estimated SC current the PD can break while Making capacity is the highest value the PD can reach under specified(PF)conditions. A circuit can be broken in fraction of a cycle by modern CBs but making current can reach high values depending on circuit parameters like C,L,PF,motor starting,pt in the wave form where it is closed etc. An induction motor takes 6 to 8 times rated current on DOL but its inrush current can reach 20times rated current.

i am unable to reach the two websites that you have mentioned. You know, you can create directly clickable links by using the right click of the mouse after pasting any web address, maybe that will make it easier for others to access the webpages.

Anyway, i can't quite make out...are you disagreeing with the IEC definition of Making and Breaking capacities?

I agree with KVS! Making capacity is the maximum value of the "peak" current experienced when the circuit breaker is closing on to a three phase short circuit fault. As explained by KVS, this includes the peak value of the RMS value of the fault current (Root 2 x RMS) and the Aymmetry Factor or the "KHI" Factor, which is due to the DC contribution to the fault from the stored energy in the system reactances (which depends upon the system "X/R" Ratio). This peak value is helpful in claculating the electrodynamic forces (could be attractive force or repulsive force, depeneding upon the contact arrangedment in the breaker) developed during the breaker's closing during a short circuit. Whereas the RMS value of the fault current is useful in calculating the thermal effects of the short circuit current. Peak current does not play a role in breaking capacity, because after a fault, by the time a protective release senses, sends a trip commnad to the breaker and by the time the breaker actually breaks the current, the fault current would definitely have subsided to the symmetrical RMS value from the Asymmetrical peak value (even with the most modern numerical relays and most modern fast acting circuit breakers).

Then why the two websites didn't mention it?.

i can't say, since i can't get to those websites as i said before.... please check again whether there is any mistake in typing them. Better still, give the weblinks as i mentioned before, and maybe we can solve the mystery.

British Standard 7671?