The Trouble With Circuit Breakers

by Paul Keleher

The trouble with circuit breakers is that, although they may trip in response to a short-circuit or ground-fault condition, unless they are tested “in-situ” (after installation, under the conditions of their installation), there is no way to know if any given circuit breaker will trip quickly enough in response to a short-circuit or ground-fault to prevent circuit components from overheating  and being damaged, or worse, due to the relatively high current of any parallel fault. This could cause a fire.

There are 3 types of electrical occurrences that can cause excessive flow in an electrical power circuit:

1.    Overload (series fault): a condition where an excessive (greater than the amount of current that the circuit breaker is designed to carry) amount of current is drawn through a branch circuit which is otherwise in serviceable condition. If a branch circuit of a given current-carrying capacity is exposed to an excessive current, the excessive current may cause components of the circuit to fail due to overheating.

2.    Short-circuit (parallel fault): Electricity will only flow through an electrical circuit when the circuit comprises a complete loop which passes through something which converts electrical current to useful energy, and poses some resistance to the flow of current in the process. Power is delivered by an electrical power circuit to the load (any appliance that uses electrical energy to do work) from the source of electrical power and is returned from this load to the source of power. A short-circuit is an unintentional condition that can be created when the source conductor becomes inadvertently connected to the return conductor by-passing the load, causing the amount of current flowing in the circuit to be limited only by the resistance of the circuit conductors themselves. The amount of current that flows through a short-circuit is typically much higher than the amount of current the circuit breaker is designed to handle. If a parallel fault current is not interrupted quickly, the high current will cause the circuit to overheat and this heat will often damage elements of the branch circuit.

3.    Ground Fault (parallel fault): Similar to a short-circuit except unlike a short-circuit, which is an inadvertent connection between conductors of a circuit bypassing any connected load, a ground fault is an inadvertent connection between a circuit conductor and a part of the equipment enclosing the circuit or other electrical equipment that is electrically conductive. The amount of current flowing in a ground-fault is relatively high compared to the amount of current the circuit breaker is designed to carry, and similar to a short-circuit, will cause damage due to excessive heat relatively quickly.

A standard circuit breaker is a thermally controlled, mechanical device with a circuit of building wiring connected to it. This arrangement means that when the bi-metal piece that is built into the standard circuit breaker moves (because an amount of electrical current exceeding a specified value passes through it), that movement causes a pair of spring-loaded mechanical contacts in the circuit breaker to separate. This contact separation causes a break to occur in the circuit mentioned above when the amount of electrical current flowing through the bi-metal becomes excessive, thus interrupting the flow of electrical current not only through the bi-metal element but through the branch circuit that is connected to the circuit breaker as well. Thus interrupts the voltage to the branch circuit. The speed at which this bi-metal moves is proportional to the amplitude of the electrical current flowing through it: the greater the current, the faster it moves, and thus the faster the circuit is interrupted.

In addition to this bi-metal element, which is present in all standard circuit breakers, in order to shorten the response time of circuit breakers operating in low-impedance situations with high available current, Underwriters' Laboratories (UL) introduced an additional requirement of an instantaneous response, enabling a circuit breaker to open the circuit (trip) to interrupt the flow of electrical power as quickly as possible without delay.

Circuit breakers are mechanical switches that are installed in special enclosures called panel boards or load centers. These enclosures containing circuit breakers are installed indoors or outdoors in spaces where the environmental conditions are controlled such as inside of habitable spaces, or in uninhabited spaces such as basements or garages where they are subject to dampness. Moisture and water often penetrate these enclosures and cause the circuit breakers in them to corrode and rust. Furthermore, these mechanical switches are rarely if ever operated causing them to atrophy. Many circuit breakers, especially those in damp or wet environments, become corroded and/or rusty over time and are unlikely to operate as designed should a fault condition arise.

The longstanding requirement of circuit breakers (and fuses) has been to protect all elements of the circuit they are a part of from hazards that might result from an overload, regardless of type. It has not been until recently that a technical definition of how a fuse or circuit breaker should respond under short-circuit or ground-fault conditions has even existed. Fortunately, technology is currently available that conducts a practical, inexpensive and safe test of the short-circuit response of any installed 15 or 20A/120V circuit breaker.