A circuit breaker handle however would allow the machinist to switch the circuit breaker into an existing short-circuit. A RESET button on the panel door allows the machine operator to reset an overload trip only in order to restart the machine again. The cost savings for cables are almost in every instance higher than the purchase price of an overload relay.Ĭ) An additional overload relay behind the line contactor facilitates the discrimination between a short-circuit trip and an overload trip. If an overcurrent relay is installed after the line contactor all 6 conductors running to the motor may be rated for the set current of this overload relay (Ie motor x 0,58). An overcurrent relay installed behind the line contactor will protect the motor in star- AND delta mode.ī) All 6 conductors to the motor have to be rated for 100 % motor full load current. During start-up in star-mode this winding is protected by a motor protection circuit breaker that can allow 173 % `overcurrent` for the winding without tripping! This is not sufficient and can lead to a burnt out motor if the event of a YD-time relay failure. If only one motor protection circuit breaker without an additional overload relay is used in a star delta starter, the following disadvantages must be taken into consideration:Ī) The actual rated current of a motor winding is only 58 % of the rated motor full load current.
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