Breaking current >Making current ??????

I am looking for an anwser to the great debate on which is the greater value of current, the making current or the breaking current with reguads to circuit breakers, and why?
We know that a circuit breaker must be able to make on to a fault ( close) just in case the fault is not cleared, Hence the making current must be at lest equal to the breaking fault current. Please can someone explain the answer to the question.
Paul.

 

[gordonl]

Making current is higher than breaking current. When a fault occurs it decreases over the first several cycles. So when a breaker closes onto a cycle it is at the begining of the fault when current is highest. But when it comes to breaking a fault, usually breakers have a small delay of several cycles, so the fault has started to decay, so it needs to interupt less current than it makes. So the required making current is higher than the required breaking current.

 

[JBD]

First you need to define what type of circuit breaker you are discussing.

For example for molded case breakers (taken form NEMA AB 1-1986,
"For AC, the short circuit making capacity of a circuit breaker shall not be less than its rated short-circuit breaking capacity, multiplied by the factor 'n' of Table 2-1... A rated short-circuit breaking capacity requires that the circuit breaker shall be able to break any value of current up to and including the value corresponding to that rated value..."

Abbreviated Table 2-1
Rated Short Minimum required
circuit breaking making capacity
Icu (amperes) Power Factor n x Icu
-----------------------------------------------------------
0-10,000 .50 1.7 x Icu
10,001 - 20,000 .30 2.0 x Icu
20,001 - and up .20 2.2 x Icu

 

[EKTORAS]

Make current is greater than the break current.
Note the difference:

Make current is considered as the peak assymetrical value of the fault current and is measured at 10ms (first half cycle) after the fault onset.

Break current is considered as the rms assymetrical value and is measured in the region of 5 cycles (100ms). This is the region the circuit breakers operate. Some level, but low, of dc component is still present.

General practical rule for the calculation:

We calculate the rms value of fault current (If) at the onset of it (t=0sec), using subtransient values of reactances for the sources and taking no account of the dc component.

Then,

the make current is : 2.55*If
and break current is : 1.255*If

with dc component included in both cases.

 

[dpc]

If I understand the question to be related to the capability of the breaker, then I agree with everyone else - the breaker is capable of closing into higher currents than it is capable of interrupting. When "breaking", the breaker must be capable of extinquishing the arc and withstanding the transient recovery voltage. The worst case is for highly reactive loads when the current and voltage are badly out of phase.

ANSI standards for medium and high voltage breakers require a higher "close and latch" rating than interrupting rating.

dpc