Most circuits will have instantaneous, short term, and long term over load protection. If the circuit includes instantaneous trip devices which react in less than a single AC cycle such as magnetic-only circuit protectors, this could be the cause of the nuisance tripping your plant operators have commented about.
During a motor start-up there are two components that make up the starting current, the instantaneous peak in-rush and the locked rotor current. The instantaneous peak in-rush is the momentary transient that occurs immediately (within half an AC cycle) after the contacts close. Locked rotor current follows the instantaneous current through acceleration to steady state.
The M-Series EMAX performing a six channel 3-phase In-Rush/Start-Up test using the latest MCEGold® software displays both current and voltage transients to assist you in determining the possible cause of these random trips. After completion of an In-Rush test, all three phases of RMS current will be displayed as well as the instantaneous peak current. Comparing these values to the trip settings of the circuit’s over current protection allows you to identify the source of the trip as motor or circuit protection driven. For guidance on increasing the instantaneous trip level to avoid nuisance tripping refer to Section 430 of the latest National Electrical Code.
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