Circuit Protection

Circuit Protection

Circuit protection is the intentional installation of a “weak link” in an electrical circuit. This is usually in the form of a Fuse, Circuit Breaker or Residual Current Device (RCD).  Collectively they are referred to as a circuit protection devices or CPD’s.

There are three general categories of circuit protection devices:

  1. Fuses
  2. Electro-mechanical circuit breakers (including Ground Fault Circuit Interrupter (GFCI) outlets).
  3. RCDs

A very important difference between these categories of CPD’s is that the purpose of fuses and circuit breakers is to protect wiring and appliances, not people; while the purpose of RCDs (and household GCFIs) is to protect people, not wiring and appliances.

 Fuses

There are many different types of fuses for residential and commercial use, but the most common type is made up of a metal wire or filament that is enclosed in a glass or ceramic and metal casing.  In a home, the fuse is typically plugged into a central fuse box where all the building’s wiring passes through.  When the electricity is flowing normally, the fuse permits the power to pass unobstructed across its filament, between circuits. If an overload occurs, the filament melts, opening the circuits and stopping the flow of electricity.

It generally takes very little time for the filament in the type of fuse used in a home to melt, so any power surge is quickly stopped.  Once a fuse is blown, however, it must be discarded and replaced with a new one.  There are many different voltage and amperage rated fuses available that handle different capacities of electricity, and the best fuse for a circuit is typically one that is rated for slightly higher than the normal operating current.

The fuse and circuit breaker both have advantages and disadvantages, each of which can depend on the situation in which they are used.  Fuses are inexpensive and can be purchased from any hardware store.  They also tend to react very quickly to overloading, which means that they can offer more protection to sensitive electronic devices.  This quick reaction can be a disadvantage, however, if the circuit is prone to surges that regularly cause fuses to blow.

Fuses must always be replaced once they are blown, which can be challenging in a darkened room or if the appropriate replacement is not immediately available.  Another issue is that a do-it-yourselfer can mistakenly select a fuse that has a voltage or amperage rating that is too high, which can result in an overheated circuit.  In addition, there may be exposed electrical connections in a fuse box, which can pose a danger to someone who does not follow the proper safety precautions.

Circuit Breakers

A circuit breaker works in one of two ways, with an electromagnet strip (or solenoid), or a bi-metal strip.  In either case, the basic design is the same: when turned on, the breaker allows electrical current to pass from a lower to an upper terminal across the solenoid or strip.  When the current reaches unsafe levels, the magnetic force of the solenoid becomes so strong that a metal lever within the switch mechanism is thrown, and the current is broken.  Alternately, the heat generated by the bi-metal strip becomes sufficiently high to bend the bi-metal strip, throwing the switch and breaking the connection.

To reset the flow of electricity after the problem is resolved, the switch can simply be turned back on, reconnecting the circuit.  Circuit breakers are often found in a cabinet of individual switches, called a switch board.  The simple switch action of a circuit breaker also makes it easy to turn off an individual circuit in a house if it’s necessary to work on the wiring in that location.

Another use of the circuit breaker is a ground fault circuit interrupter (GFCI) outlet, which functions to prevent electric shock instead of overheating.  It works by breaking the circuit in an outlet if the outgoing and return currents becomes unbalanced (similar to RCDs), and can be reset by the push of a button.  This technology is particularly useful in bathrooms or kitchens where electrocution is a risk due to the frequent use of electric appliances near a source of water.

Circuit breakers have many advantages, not the least of which is how quickly they can be reset.  It is usually clear which switch has tripped, and it can be easily reset in most cases.  For the average homeowner, it is also safer because there is no question about choosing the right fuse rating and all of the electrical connections are hidden in a breaker box.  Most modern homes are fitted with circuit breakers rather than fuses.

A drawback to using a circuit breaker is that it is usually more expensive to install and repair.  A circuit breaker also typically does not react as quickly as a fuse to surges in power, meaning that it is possible that electronics connected to the circuit could be damaged by “let-through” energy.  It is also more sensitive to vibration and movement, which can cause a switch to trip for reasons unrelated to an electricity overload.

Residual Current Devices (RCD’s)

A residual-current device (RCD), or residual-current circuit breaker (RCCB), is a type of AC electrical switch that instantly breaks an electric circuit to prevent serious harm from an ongoing electric shock.  An RCD is often referred to as a “Safety Switch” because it is designed to disconnect the power if a person makes contact with a live part of a circuit and earth at the same time.  RCD’s offer a greater level of protection than GFCI outlets (referred to above).

FACT: Currents as low as four thousandths of an ampere (0.004A) can cause pain.

Currents over twenty thousandths of an ampere (0.02A) cause muscles to contract; i.e. you can’t let go of the appliance you are holding.

A standard electrical circuit in Australia is 10amps!!

In Australia residual current devices have been mandatory on power circuits since 1991 and on light circuits since 2000.  A minimum of two RCDs is required per domestic installation. All socket outlets and lighting circuits are to be distributed over circuit RCDs.  A maximum of three subcircuits only, may be connected to a single RCD.

RCD’s operate by using a magnetic field (coil) to detect differential in current between the Active and Neutral cables.  If a differential is observed the RCD will operate and interrupt the circuit within a specified time based on the type of RCD.

RCD’s monitor current through the Active supply wire and the Neutral return wire.  Under normal circumstances the Active and Neutral cables are expected to carry matching currents, and any difference can indicate that a short circuit or other electrical anomaly is present, such as leakage. Leakage can indicate a shock hazard (or shock in progress) which is a potential danger to a person.  This condition can occur if someone comes into contact with the active supply wire while being in contact with earth.  The diagram below shows the basic function of an RCD.

There are two types of RCD used in Australia.  RCDs are classified according to their rated residual current tolerances.

RCD TypeTest CurrentMaximum Trip TimeUsage
Type 110mAMust operate and interrupt circuit within 40 millisecondsVery low trip point. For medical use only (hospitals, etc.)
Type 230mAMust operate and interrupt circuit within 300 millisecondsThis type is most commonly encountered.

ALWAYS REMEMBER there are situations where the RCD will not protect you!

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