long time delay primary injection testing

[KLLU26]

i've been testing for about 4 months now and one question i seem to keep asking (and haven't gotten a definitive answer) pertains to the recorded times of the long time delay during primary injection testing of a molded case square d circuit breaker.

while testing A phase, at X current we would record roughly 60 seconds.

B phase would drop to around 35-40 seconds,

C phase would be similar.

now i am familiar with the fact that as the breaker heats up the trip time will decrease, so my question is about the value that we record. in my mind under a normal load the breaker will be warmer, obviously, so during an overload the breaker would most likely trip around the 35-40 second mark. so why would we record and expect our values to be around 60 seconds from a cold test? i think it would be more accurate to record and expect the 35-40 second reading in a real world situation.

is this just part of the manufacturers specification or NETA standards?

[johnierw]

This SQ D IB gives you some reference specifically page 14.
335
i've been testing for about 4 months now and one question i seem to keep asking (and haven't gotten a definitive answer) pertains to the recorded times of the long time delay during primary injection testing of a molded case square d circuit breaker.

while testing A phase, at X current we would record roughly 60 seconds.

B phase would drop to around 35-40 seconds,

C phase would be similar.

now i am familiar with the fact that as the breaker heats up the trip time will decrease, so my question is about the value that we record. in my mind under a normal load the breaker will be warmer, obviously, so during an overload the breaker would most likely trip around the 35-40 second mark. so why would we record and expect our values to be around 60 seconds from a cold test? i think it would be more accurate to record and expect the 35-40 second reading in a real world situation.

is this just part of the manufacturers specification or NETA standards?

[gchoquette]

This SQ D IB gives you some reference specifically page 14.
335

From the reference above:
"Trip times are measured from a “cold start.” A cold start, as defined by Underwriters Laboratories Inc. Standard 489 occurs at 25°C ± 3° (77°F ± 5°). Therefore, before beginning overcurrent testing, the circuit breaker must be in 25°C (77°F) ambient temperature long enough for all parts to reach that temperature. Circuit breakers that have been in higher ambient temperatures may take two to four hours to reach the steady state temperatures mentioned above."

The problem with testing a circuit breaker warm of even hot, is that the trip time values cannot be referenced to a standard value because we don't know how "warm" is warm. That's why we test at 25C or 77F.

Ideally, there could be a table of trip times as a function of CB temperature, but that would be difficult to measure accurately and it would drop during the test.

[CEBrighton]

What your experiencing is thermal memory. (Thermal imaging effect) Typically it explains this in the manual and lets you know what time that specific breaker needs to reset. The ones we have been testing lately have been 20 min.