A Better Approach to Motor Circuit Protection

Over the years, various approaches including both thermal magnetic breakers and several types of fusible devices have been used in motor circuit protection schemes with acceptable results. Evidence reported in the trade magazines indicated that the level of protection from these types of devices was not totally effective. For the first time, with the printing of the 1968 National Electric Code, the application of instantaneous trip circuit breakers (without time delay) was allowed provided the pickup could be adjusted above 700 percent to a maximum of 1300 percent of the motor full-load ampere rating. After extensive testing the first product complying with these requirements was introduced to the market in 1969. Experience in motor failures indicates that most faults occur at relatively low levels of fault current just above lock rotor values rather than at higher levels. With rapid clearing of faults in the low-level range such as can be accomplished with the use of instantaneous trip circuit breakers, extensive damage to motors as well as to control equipment can be greatly reduced. By combining low-level protection obtained with the high interrupting capabilities of specially designed current limiters, a full range of coordinated protection is available for both high-and low-level faults.