1.3 The a.c. supply of electricity to the home, wiring, insulation, and plugs

Connections!

The electricity supply to your home is a.c. (alternating current) where the current constant reversing direction e.g. in the UK, an oscillation of 50 Hz (50 cycles/second) is normal (in other countries it may be up to 60 Hz).

Reminder: A d.c. supply only flows in one direction (from + to -) and is often at a much lower potential difference e.g. the p.d. of batteries or cells is usually in the range 1.5 V to 24 V.

The a.c. supply to the ring mains circuits in your house originates from the National Grid system.

Alternating currents are produced from alternating voltages in which the positive and negative terminals of the potential difference keep alternating (+ <=> -).

  

CRO  traces illustrating the difference between AC and DC

The a.c. mains supply in the UK is usually around 230-240 V with a frequency of 50 Hz (50 hertz or 50 cycles/second). It can vary slightly from country to country e.g. some supply systems work on 60 Hz.

Other devices will use a d.c. (direct current) supply from cells or batteries, in which the current only flows in one direction e.g. torch batteries.

A d.c. current is produced by a direct voltage - potential difference (p.d.) and is either positive or negative, but NOT both.

You can convert an ac current into a dc current using a diode.

Many electrical appliances in the home are connected to the ring mains circuit with three core cables fitting into a plug.

The plug (pictures below) is inserted into a socket which is directly connected to the a.c. mains supply.

Most sockets have their own switches connected to the live wire of the ring mains circuit in a house.

A switch must be connected in the live, so the wire is NOT live if the switch is on OFF.

This enables the circuit to be broken and isolate any appliance if there is a risk of electric shock.

The cables consist of a copper wire core and sheathed in an insulating plastic covering, each of which is colour coded to clearly indicate its function (annotated image below).

The colour coding is kept the same for all appliances so that you know exactly which wire is which!

If wired incorrectly you may blow the fuse or have an accident - potentially fatal electrocution (see earth wire), so make sure you know which is which and how to wire a plug safely irrespective of any GCSE physics exam!

The function of each of the three wires in a three core cable.

The live wire - brown colour insulation

The live wire provides the alternating current potential difference with a p.d. of +/- ~230-240 V.

It is the live wire that carries the high potential difference.

The appliance switch must always be in the live wire, otherwise the circuit would always be live!

The live wire carries the p.d. directly from the mains supply and this 'live' wire must never be touched if the circuit is switched on for obvious reasons!

In fact you should never touch or manipulate any wire, especially the live wire, if the circuit is potentially 'live'!

If you touch a live wire, a large potential difference is produced across your body and a surge of current passes through your body. The subsequent electric shock can injure you and can be fatal.

A short circuit of a faulty appliance or anywhere in a circuit, can cause a fire from the energy release - electrical energy to the thermal energy store of the wire and surroundings.

For an appliance, the sequence of wiring in the live wire is:

plug ==> fuse ==> switch ==> heating element

The action of a fuse or circuit breaker protects you from harm and minimises fire risk.

The neutral wire - blue colour insulation

The neutral wire completes the circuit to the appliance and carries away the current.

The neutral wire provides the return path to the local electricity sub-station (transformer).

The neutral wire is earthed, so that it is as close to being an earth potential of 0 V.

This allows the current to flow in through the live wire (maximum p.d. of ~230 to 240 V) and out through the neutral wire (minimum p.d. ~0 V).

The p.d. between the live wire and neutral wire is ~230-240 V for the mains electricity supply.

The earth wire - green + yellow coloured bands insulation

The earth wire has a safety function to protect the wiring and YOU!

It doesn't normally carry a current and its p.d. should be 0 V.

The p.d. between the live wire and the earth wire is ~230-240 V.

There is no p.d. between the neutral wire and earth wire, both are at a p.d. of 0 V.

The earth wire is connected to the metal casing of an appliance and carries the current away safely if a fault develops in the circuit.

If a fault develops and the live wire touches any conducting part of the appliance, the current will run to earth through the earth wire and NOT through you if you touch the appliances.

This also might, and should, blow the fuse because of the surge in current, so the circuit is broken and made safe.

See also Part 5. for

More on the dangers of the live wire, fuses and earthing appliances for extra safety

The danger of the live wire

The danger of electrocution - safety function of earth wire

Under normal conditions your body has a p.d. of 0 V with respect to the ground ('earth').

Unfortunately, if you touch the live wire with the circuit switched on, a potential difference is produced across your body and the current flows through you to the ground - 'to earth'.

In other words, you will experience electrocution - potential injury from an electric shock, and, if the current is large enough, it may kill you!

It doesn't matter whether the appliance is switched on or not, if the plug is in the socket, there is a connection to the live wire which always has a p.d. of ~230-240 V!

If there is any low resistance connection between the live wire and earth wire a sudden huge current can flow to earth, which is dangerous.

This is the cause of many house fires due to a faulty connection where lots of heat is produced.

For more on electrical safety see live wire and fuses notes.