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Various common situations are discussed in which overload relays do not always adequately protect today's motors. NEMA class designations will be explained along with their relevance to motor damage curves. Single phasing and voltage unbalance will also be shown to cause motor heating beyond that which the conventional overload relay is designed to protect. Viable solutions exist for these problems, and insight will be given as to how best to solve particular problems. The important thing is to clearly understand why motor burnouts occur even though the heaters have been sized correctly.
Brighton et al. (Mon,) studied this question.