Hybrid overload relay

A overload trip unit for selectively applying current from a current source to the coil of an overload relay which is configured to interrupt the current in at least one electric power conductor, the unit comprising:

• a transducer configured to produce a first signal representative of the current in the power conductor;
• at least one pair of contacts which are moved between their opened and closed positions to selectively apply current from the current source to the coil;
• a thermal actuator mechanically coupled to at least one of the contacts and configured to move the contacts relative to each other in response to a predetermined control current applied to the thermal actuator for a predetermined period of time, the control current changing the temperature of the actuator;
• and
• a control circuit electrically coupled to the transducer and the thermal actuator, and configured to produce the predetermined control current for the predetermined period of time in response to the first signal exceeding a predetermined limit.

An overload protection system for monitoring the alternating current in electrical conductors carrying three-phase power to an electrical load, the conductors being coupled to the load by a circuit interrupter and the system changing the flow of a control current applied to the interrupter in response to an overload condition, the system comprising:

• a plurality of current transformers, each transformer being magnetically coupled to one of the phase conductors to produce a phase current signal representative of the current in the respective phase conductor;
• a thermal actuator configured to change shape in response to temperature changes;
• an output terminal;
• a switch which is closed to couple a current source to the output terminal, the thermal actuator moving the switch from the closed position in response to a change in the shape of the thermal actuator;
• and
• a control circuit electrically coupled to the current transformers and the thermal actuator, and configured to produce a predetermined control current for a predetermined period of time in response to the phase current signals exceeding predetermined limits, the control current being applied to the thermal actuator to increase the temperature of the thermal actuator.