Introduction to Electrical Power and Power Electronics

241System Protection

the net current difference in two wires going through the eye of the CT sensor. Under

healthy operation, the lead and return currents are equal, and the sensor output is

zero. A short in one line conductor to ground even via a soft fault (high-impedance

fault) results in higher-than-rated current in the lead wire with no change in the

return current. The net unbalance current is picked up by the sensor, which is usually

very small due to a high CT turn ratio. It is, therefore, amplied and then sent to the

actuator coil of electromagnetic relay to open the circuit breaker.

The NEC requires that all 120-V single-phase outlets ...