Load-Side Voltages on Open Low-Voltage Circuit Breakers Due to Parasitic Capacitive Coupling

The unavoidable parasitic capacitance that exists between circuit breaker contacts can cause an ac voltage to exist on the load side of an open circuit breaker. While this phenomenon is well-known for high-voltage circuit breakers, it also exists for low-voltage circuit breakers but is not as well documented. With the proliferation of trip units capable of measuring voltage, if the load-side voltage is monitored the operator may be surprised to see such a voltage on the load side of an open circuit breaker and this may raise safety concerns. This paper explores this phenomenon in detail, describing the system conditions that affect the voltage level encountered, the means to lower such voltages, and the safety impact.