The circuit breaker has saved a lot of lives. David Herres outlines the role and characteristics of a simple but effective device.

 Circuit breakers play a vital role in all electrical systems – residential, commercial and industrial.

Breakers are rated in amps, and the amount is plainly printed on the end of the handle.

The purpose is to automatically open a circuit in the event of over-current, by which is meant electrical current in excess of the rated ampacity of the conductors.

If the ampacity is exceeded, the temperature of the wire will rise. This is usually due to an increase in supply voltage, short circuit caused by a line-to-line or line-to-ground fault, or an increase in the connected load.

Prolonged or repeated temperature rise will damage the wire insulation. In a severe case, the copper can get red hot inside the wall, igniting nearby combustible material and causing a widespread fire.

Breakers perform the additional function of acting as a manual switch. After a fault has been corrected, the tripped breaker can easily be reset.

Fuses are also effective over-current devices. A moulded-case switch looks just like a breaker and snaps into a load centre in the same way, but it has no over-current interrupting capability.

Codes require that if you brace the handle of a breaker in the on position the device will still trip without moving the handle in the event of overload.

Circuit breakers have been around for a long time and they are extremely reliable. They were used in early telegraph networks long before the first power distribution system was built.

If a breaker becomes faulty, it is preferable for it to trip when there is no overload rather than fail to trip when needed. In fact, that is how they are designed. Breakers do not stick or lock in the on position. The price we pay for that kind of performance is that they occasionally nuisance trip.

To prevent damage to a computer’s hard drive, an inexpensive uninterruptible power supply (UPS) will provide continuity of power when a circuit cuts out.

Life support equipment, either in a health-care facility or in the home, generally has elaborate automatic back-up power. Increasingly, this set-up is seen in family homes.

When an over-current device powers down a branch circuit, the first move is to determine whether it is performing its protective function or just nuisance tripping. For the most part, circuit breakers are reliable and rarely fail. It is a simple matter to swing the wires over to a good breaker.

An individual circuit outage generally falls into one of three categories, although there are grey areas.

The breaker may fail to reset, thereby signifying a dead short, or it may hold for a short interval before tripping again. Otherwise, it may hold for quite a long time, sometimes hours, before cutting out.

The latter is because breakers in common use are inverse time devices, with the ability to tolerate a small amount of over-current for a long time and a greater amount of current for a short time.

An accurate picture can be gained from current measurement using a clamp-on ammeter (trade name Amprobe). If the breaker holds long enough, you can check the current at each outlet until the fault is located.

If there are loads plugged in, one of them is probably faulting. Electrical equipment, especially motors, tend to draw more current as they age, due to internal insulation gradually breaking down. This can also happen if a transformer is connected.

A hair dryer or laptop computer, when the internal battery is being charged, may be the culprit. Unplugging connected loads one at a time, with a clamp-on ammeter at the entrance panel, will quickly locate the fault. The Amprobe ‘hold’ function is useful in this procedure.

Nuisance tripping can also be a problem with residual current devices (RCDs). In the United States and Canada these are known as ground-fault circuit interrupters (GFCIs). The purpose and inner workings of both are similar, but the circuit deployment varies due to different phasing arrangements.

NEC and CEC, as well as the Australian and New Zealand Wiring Rules, require such devices in sensitive locations – wherever the proximity of electrical power and water may lead to a shock.

This residual current device interrupts both conductors.

Since their introduction in the 1960s, and subsequent widespread use, RCDs have greatly reduced the number of non-utility electrocutions, especially in homes.

The equipment-grounding conductor usually completes the circuit to the entrance panel, then the over-current device trips. However, in some instances that conductor breaks or has not been properly hooked up. The result can be an electric shock – sometimes fatal.

The RCD takes the form of a breaker in the entrance panel or load centre, a receptacle at the wall outlet, or a moulded-case device built into a power cord and located adjacent to the plug.

The hot wire and neutral return conductor of a 240V circuit both pass through the device. Normally, the current in the supply and return conductors is equal (conforming to Kirchhoff’s Current Law that electrical current is everywhere the same in a non-branching circuit).

In a two-fault situation (involving connectivity of the hot wire and loss of connectivity of the equipment-grounding conductor), when someone touches the energised metal, some of the current goes to earth.

The RCD monitors both current paths, performing a continuous differential measurement. If the difference exceeds a specified limit, typically 6mA, the device interrupts the circuit. The amount and duration of the brief electric shock is not enough to constitute a hazard.

Many of these devices combine the function of an over-current device, and they can also be used as a switch by pressing the ‘test’ button.

The newer RCDs have an LED that indicates a tripped condition, so it is possible to know at a glance where the problem lies. Moreover, if the RCD is not getting power, it will not reset, providing immediate information on the status of the upstream portion of the branch circuit.

Unfortunately, RCDs are prone to nuisance tripping, which is often caused by a defective load. The first step is to unplug all downstream loads, one at a time.

(Remember: nothing that happens upstream of the RCD, including conductor imbalance, can cause tripping. The device monitors only downstream wiring, devices and loads.)

Some loads are incompatible with RCDs, and refrigeration equipment is one example.

In a hermetically sealed compressor, motor windings are submerged in the grounded refrigerant, and any imperfection in the light coating of insulation will allow current leakage to ground.

Moreover, an undetected nuisance-tripping incident can lead to the loss of freezer contents.

A dedicated non-RCD circuit should be run for a refrigerator, even in a kitchen.

RCDs should not be used in circuits for fire alarms, life support and other essential services.

If the cause of nuisance tripping is not apparent, and substitution has ruled out the device, take a good look at the wiring. Cable rubbing on grounded metal pipe or an errant nail can create a problem, or there may be moisture in the wall.

Does the nuisance tripping occur after heavy rain? Perhaps an outdoor receptacle has been bugged off a kitchen or bathroom receptacle to obtain the required RCD protection.

If cable connectors have been over-tightened in older concrete block or masonry constructions with metal wall boxes, there may be enough current leakage to cause RCD tripping. If so, it may be possible to loosen the connectors and slide in a slit piece of cable jacket.

If wiring runs are suspected, the diagnostic procedure is to progressively isolate segments of the branch circuit by temporarily unhooking the hot wires, one at a time, until the fault has been located.

However, in residential construction most of the wiring is behind finished walls, so some hard work may be in order.