Posted by buickanddeere on April 06, 2018 at 05:18:23 from (184.151.36.218):
In Reply to: Electrical Question posted by pburchett on April 05, 2018 at 10:54:22:
Depends on the breaker specs and what they are set to. There is instantaneous and short term which are short circuit protection. Then there is long term trip settings which are thermal protection. Also there is the breaker’s interrupting capacity which maybe 4KA, 10KA, 50KA or what ever you chose to pay for . Sometimes breakers blowup because they had more short circuit current through them than what they were rated for .
Then the trip curves matter . I tried to tell the company engineers that they were screwing up the set points on the new 600 and 1200 amp breakers in the new buss gear . I pointed to the factory instructions on for the” how many times “ the rated current of the downstream MCC panels for instantaneous, short term and ground protection . The engineers decided to be “safer” and set the breakers to just over the normal operating current of the combined loads .
Some of you know where this is going .
Months later a crew was installing a new 2Kw 600V three phase heater in the control room . They did not megger the circuit prior to energization and failed to find the shorted heating element . They closed the 15amp branch circuit breaker which exceeded the instantaneous protective circuit set point on a 1200amp supply breaker . Which took down half of the facility. The 15amp breaker never tripped as the current was interrupted upstream before enough cycles occurred to trip the 15amp breaker .
Needless to say I spent a lot of time rubbing it in and saying “I told you so”.
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