The ground trip/ look-ahead relay is designed for use with 120 VAC under-voltage or shunt trip breakers on circuits limited to 25 amps or less ground fault current. It is factory adjusted to trip at less than 6 amps ground fault through a current transformer. This relay uses a breaker for tripping and indication. When a ground fault current occurs the breaker trips and must be manually reset. A resistor in series with the breaker under-voltage to limit current on a ground fault is not required.
The relay now features a lookhead circuit to verify no fault is present before closing the contactor/circuit breaker. Typical molded case circuit breakers require at least 30 seconds prior to reclose to dissipate any ionized air. The relay has a minimum 30-second to allow this to take place. The time delay is scalable to 180 seconds for applications where inertial loads require more time to dissipate back emf from the motor circuit.
The ground fault with relay is designed to perform in two modes of operation as well as a CT test function.
Look Ahead mode- the purpose of this mode is to test the high voltage circuit for a cable or motor ground fault. The Look Ahead circuit performs this function by connecting to the motor circuit through the High Tension Coupler after the 30-180 second time delay. Internal circuitry will first test for any AC voltage on the motor circuit. When no AC voltage is detected a constant current DC low voltage will applied through the High Tension Coupler. A voltage will develop across any leakage resistance. This voltage is read, converted to a resistance and compared to a pre-selected resistance threshold. If the measured resistance is greater than the pre-selected threshold an internal register is set and the indicating LED will illuminate GREEN. If the measured resistance is less than the pre-selected threshold a ground fault is detected, indicated by a RED LED, tripping the circuit.
The second mode is the Ground Fault mode - This mode will monitor the signal from the current transformer and convert it to an ampere reading. When this reading is greater than the programmed limit the embedded microcontroller will trip the circuit breaker and the CURRENT FAULT LED will be illuminated flashing RED, tripping the circuit.
The circuit breaker, in conjunction with the internal circuitry, will allow the output relay to close if no fault is present. If the circuit breaker is not closed or there was a detected fault from earlier testing the output relay will not close.
The operation mode is determined by the contactor/circuit breaker status. This signal is connected to the ‘Contactor Input’ terminals at the rear of the Look Ahead Relay. If a 120 Volt AC signal is present on the Contactor Input, the contactor/circuit breaker is closed and the Look Ahead Relay is set to operate in the Ground Fault monitor mode. When this voltage is not present, the contactor/circuit breaker is open and the Look Ahead Relay enters a selectable time delay, indicated by illuminating the DELAY LED flashing YELLOW. After the delay, the Look Ahead Relay will test the CT function, and finally the Look Ahead function. This state is duplicated upon power up to the Look Ahead Relay, provided the Contactor/circuit breaker is open. If a voltage is detected on the Contactor Input terminals of the relay before the delay is completed, the delay is sustained, the DELAY LED is extinguished, and the Look Ahead Relay is set to Ground Fault Monitor mode. Upon the removal of the Contactor Input voltage, the delay mode will resume and the DELAY LED will flash YELLOW.
CT Test Function - In addition to the Look Ahead and Ground Fault modes, the circuit will test the current transformer used for current monitoring to verify operation. The Look Ahead Relay will send a small AC current to the test coil on the current transformer and measure the return transformer signal. If this signal verifies the CT is operating properly, the CT FAULT LED will turn off. If the CT is not operating properly the relay will set and the CT FAULT LED will illuminate flashing RED.