Control.com - Nerds in Control

What is Medium voltage circuit breaker making capacity? Why it is more than the CB breaking capacity?

Krishna, the requirement that a circuit breaker have a larger “make” capacity than a “break” capacity is true regardless of voltage rating. In some countries “make” capacity is referred to as its “momentary-duty” while its “break” capacity is called “interrupting-duty.”

The breaker must withstand the maximum current (called the 1st-cycle peak) it is subjected to upon closing “into” a faulted circuit. This peak is not only larger than the corresponding rms ac-current wave by sqrt(2, but an asymmetrical offset caused by the circuit’s L/R ratio, furher increases the current. Thus the “make” current is substantially larger than the “break” current. The resultant mechanical forces (proportional to I^2) could cause mechanical deformity. In addition, the breaker must be able to "close" into a fault without its contacts welding.

Regards, Phil Corso, PE (cepsicon@aol.com)

Making capacity is the max fault current it can carry during closing of breaker, e.g. You are closing a breaker on fault. The making capacity is always higher because it is expressed in peak value not rms, and also because of the DC offset, voltage and PF while closing.

Hi,

Could you please tell why the making is expressed in peak and not rms??

Responding to Samuel’s 06-Sep (21:59) query:

The “make” (or “close”) current occurs at the time the first phase reaches peak value! (An aside, at the time the first phase peaks, the other two phases reach lower values.) The peak-current is comprised of two components, one a.c., the other d.c.! Of course, an rms value can be calculated, but it is meaningless because the electrical stress and mechanical force the breaker must withstand are produced by the maximum current, i.e., the first-cycle peak!

Regards, Phil Corso (cepsicon@aol.com)

Hi Phil,

I read your valued replies to the questions posted.

I felt that this much should be said, especially to them who are interested in fault protection!

The questions of making and breaking capacity really has its introduction due to a breaker's application in a system.

Circuit breakers for a non-protection expert would mean the rated current, i.e. trip current with the amps listed on the handle, voltage rating, and at most frequency of cycles per second.

What the non-protection expert does not know is that there is a far greater current value in the kA, and it comes thru situations when there is a fault and at that very instance of the fault time:

1. The circuit breaker contacts are opened, this is exactly what a circuit breaker does.

2. The circuit breaker is closed and as it is it closes into a fault, just so happens as said there is a fault and the beaker is closed in at a fault.

The above situations are defined as 1. breaking current, and 2. making current.

The other condition is that it's between 1 and 2 above. What could that be if it is between the 2 conditions?

It is that the circuit breaker is not the guilty party in the fault, but that it is connected to the busbar that has a fault and the fault is detected by one of the other circuit breakers connected to the busbar.

Thus, condition 3 is:

3. The circuit breaker has to withstand the fault current for a short duration while the other circuit breaker is clearing the fault.

The above situation 3. is short time current.

All three in capacity terms are:
1. breaking capacity
2. making capacity
3. short time capacity.

I hope this helped describe the overall capacity myth of just how these faults came about, as there would be others who need the answers to the same question.

Regards,
karl