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Journal ArticleDOI

A Better Approach to Motor Circuit Protection

01 May 1973-IEEE Transactions on Industry Applications (IEEE)-Iss: 3, pp 278-284
TL;DR: The first application of instantaneous trip circuit breakers (without time delay) was allowed by the National Electric Code (NEC) as discussed by the authors, provided the pickup can be adjusted above 700 percent to a maximum of 1300 percent of the motor full-load ampere rating.
Abstract: 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.
Citations
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Journal ArticleDOI
TL;DR: It is shown how a solid-state logic approach can sense these circuit variances and result in an economical package providing improved protection, coordinated and easily applied, which can help eliminate or minimize loss due to many of these common problems.
Abstract: Common motor circuit protection has consisted of thermal overload to sense motor overcurrent and a short circuit protective device which acts after a catastrophic condition has occurred. While this combination has provided adequate protection for the most common motor and motor circuit problems, it has not provided good protection against many of the problem conditions that can occur in the motor circuit. Common motor and motor circuit problems, the circuit variances that occur and the use of common devices under these conditions are discussed. It is shown how a solid-state logic approach can sense these circuit variances and result in an economical package providing improved protection, coordinated and easily applied, which can help eliminate or minimize loss due to many of these common problems.

2 citations

Journal ArticleDOI
TL;DR: In this article, the economic factors and compliance with relevant test standards are considered to permit their application in systems having high fault current capability and to retain circuit integrity and reliable usage, various aspects are considered including economic factors, compliance with applicable test standards, and best characteristics of motor control centers are utilized and satisfactory performance is realized by the owner.
Abstract: Motor control centers, as their name implies, provide a convenient means of centralizing the operation, protection, control, and maintenance of motor feeder and branch circuits. To permit their application in systems having high fault current capability and to retain circuit integrity and reliable usage, various aspects are considered including economic factors and compliance with relevant test standards. In this manner, best characteristics of motor control centers are utilized and satisfactory performance is realized by the owner.

1 citations