Electrical principles - Components

Circuit protection

All circuits require protection so the power can be switched off if a fault occurs or to allow maintenance. The rating of the circuit protection is based on the rating of the cable, not the appliance.

Two main forms of circuit protection are:

• fuses
• circuit breakers.
  

Fuses

A fuse is an enclosed piece of wire. When the current is too great, the wire will heat up and melt so the circuit will open. Fuses are rated according to the size of the wire; the larger the wire, the greater amount of current required to melt it.

Semi-rewirable fuses are now illegal but can still be found in old houses. These fuses are illegal because the fuse wire is not enclosed and is therefore open to outside elements. If it is a cold and windy day it will take longer for the fuse wire to melt.

Circuit breakers

A circuit breaker is a switch that opens when a predetermined current rating is reached.

There are three main types of circuit breakers:

• magnetic
• thermal
• thermal/magnetic.

Magnetic
This circuit breaker uses magnetism produced by a coil, to cause the circuit breaker to trip (open). As current travels through the coil, a magnetic field is created. If the current flow exceeds the rating of the circuit breaker, the magnetism is so great that it causes a metal switch contact to be attracted. This attraction and movement of the switch contact opens the circuit, preventing further current flow.

 

Thermal
A thermal circuit breaker uses two dissimilar metals (bi-metallic) to open a switch. When the current is high (above circuit breaker rating), the heat generated cause the two different metals to expand at different rates. This causes the bi-metallic arm to bend, releasing a latching mechanism and opening the circuit.

 

Thermal/magnetic

This circuit breaker combines the effects of the thermal and magnetic circuit breaker. This results in the the fast operation of high current faults (magnetic) plus the slower operation of low fault currents (thermal).

 

Residual Current Device (RCD)

An RCD is commonly called a safety switch and is legally required in all houses. It runs the active and the neutral cable through a coil. If the current is the same in both the neutral and the active, the coil has no magnetic field in it. However, if there is a difference of 30 mA or more the current will be induced into the coil, causing a magnetic field that pulls the switch towards it and opens the circuit.