Breaking current >Making current ??????
Breaking current >Making current ??????
(OP)
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.
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.






RE: Breaking current >Making current ??????
RE: Breaking current >Making current ??????
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
RE: Breaking current >Making 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.
RE: Breaking current >Making current ??????
ANSI standards for medium and high voltage breakers require a higher "close and latch" rating than interrupting rating.
dpc