Protection against motor over - load usually adopt thermal relay, but protection against line over - load usually adopt long delay over- current release of circuit- breaker.
电动机过载保护通常采用继电器，而配电线路过载保护多采用断路器的长延时过电流脱扣器来完成。
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The article analyzes the operation character and the invalidation mode of thermal over-loadrelay and puts forward the reliability index and the calculation method of reliability characteristic.