Electric motor protection depends on the accurate selection of overloads, fuses and/or circuit breakers. Over the years the protective devices have been selected according to the applicable code requirements with only minimal nuisance tripping.
However, in recent years, the problem of nuisance tripping due to the high motor inrush currents that occur during motor starting has gained increased attention.
In order to avoid the problem of nuisance tripping, application engineers have been forced to either set the HMCP magnetic circuit breaker above code requirements or take a step backward and exchange the HMCP circuit breaker for an inverse time circuit breaker. Both scenarios have the disadvantage of sacrificing the close coordination protection for which HMCPs were initially designed.
The National Electric Code (NEC) was changed slightly in 1996 to address this problem.
The problem stems from the fact that the NEC allows certain settings for
HMCPs (currently 800% of full load current, 1100% for design E motors) based on the motor’s locked rotor current (LRC), which is generally 600% to 700% of full load current (FLC). However, with high efficiency motors the inrush current may exceed the 800% of FLC. Also, the application voltage may be over the nominal by 3 to 5%.
These factors will cause the initial inrush current to be much higher than usual. Additionally, one other phenomenon that will exacerbate the situation is that the initial peak inrush current will not be symmetrical.
However, in recent years, the problem of nuisance tripping due to the high motor inrush currents that occur during motor starting has gained increased attention.
In order to avoid the problem of nuisance tripping, application engineers have been forced to either set the HMCP magnetic circuit breaker above code requirements or take a step backward and exchange the HMCP circuit breaker for an inverse time circuit breaker. Both scenarios have the disadvantage of sacrificing the close coordination protection for which HMCPs were initially designed.
The National Electric Code (NEC) was changed slightly in 1996 to address this problem.
The problem stems from the fact that the NEC allows certain settings for
HMCPs (currently 800% of full load current, 1100% for design E motors) based on the motor’s locked rotor current (LRC), which is generally 600% to 700% of full load current (FLC). However, with high efficiency motors the inrush current may exceed the 800% of FLC. Also, the application voltage may be over the nominal by 3 to 5%.
These factors will cause the initial inrush current to be much higher than usual. Additionally, one other phenomenon that will exacerbate the situation is that the initial peak inrush current will not be symmetrical.