Disclaimer
IANAE. (I am not an electrician). However, I've done quite a lot of electrical wiring work around my house and haven't caused any damage, started a fire or killed anybody yet. If you do any work based on what I say below, I won't be held responsible for any problems, injuries, damage to property or death that may result. Always consult a qualified electrician if you're not sure what you're doing.
Specifications
Mains electricity in the UK runs on overhead power lines at anything up to 400 kilovolts in the Supergrid. However, it is transformed to a nominal 230 volts between Live and Neutral (which isn't necessarily 0v with respect to Earth!) before arriving in the home. The allowed variation on this is -6% and +10% (ie 216 to 253 volts). Traditionally, Britain ran at 240 volts, and this is well within the allowed range. The power is AC (Alternating Current), at a frequency of 50 Hertz. This frequency is guaranteed to be incredibly accurate over a period of a few days (and this is essential as many mains-powered devices with clocks use it as a timing pulse).
Electricity is supplied to residential property on a single phase.
Supplier owned property on site
There are a number of pieces of equipment in the home which are owned by the electricity board.
The main fuse
There is always a main fuse which is owned by the supplier. This is the first thing that the incoming power comes across, and is a "last resort" to blow if there is a major electrical fault in the property. Depending on the size of the property, this is usually rated at 50 or 100 amperes (amps). It can be either a cartridge fuse or a piece of fuse wire. I have never seen this as a circuit breaker, and it probably wouldn't be sensible to have it as something that is so easy to reset. If this fuse goes, there's usually a VERY good reason for it!
It is usually mounted in a protective case with a tamper-proof tag on it. Tampering with it is against the law. Furthermore, if your electricity supply is being cut off, this is the item that the supplier will send someone to remove.
The Meter
Every site has a meter on the property. Traditionally these would be mounted in the garage, although more often nowdays they are in a box on the outside of the property. This means that when someone is sent round to read the meter, they can do it from the outside. Again, tampering with the meter is illegal.
The meter measures electricity use in KiloWattHours - 1 KiloWattHour means that there has been a drain of 1 Kilowatt for the duration of one hour. So, for example, if you have 20 x 60 watt lightbulbs, running for 2 hours, you have drained a total of 20 x 60 x 2 = 2400 watthours or 2.4 Kilowatthours.
Some properties have a dual meter and a timeswitch, that can change between two separate meters. You pay a slightly higher standing charge for this service, but if you have a large overnight electricity usage (eg if you have storage heaters), this can end up cheaper as you pay a lower amount for units used overnight.
The Consumer Unit (or "Fuse Box")
The Consumer Unit (which used to be known as the "Fuse Box") is the first piece of equipment that is owned by the property owner. As expected, this contains a row of fuses - or nowdays, a row of circuit breakers, one for each major circuit around the house. Most modern houses will have the following circuits.
- Lighting circuits. 1 or more, usually fused at about 6 amps. Small houses may only have 1 lighting circuit, larger houses will have more. But each one will supply power to a number of rooms.
- Ring main. 1 or more, usually fused at about 30 or 32 amps. Again, the number of circuits will depend on the size of the house.
- Cooker circuit. 1, usually fused at about 30 or 32 amps.
- Immersion circuit. 1, for an immersion heater or electric boiler. Usually fused at about 15 amps.
There will be a "master cut out" switch on the fuse box. Many modern units also include one of the following.
- An "Earth Leakage Circuit Breaker" (ELCB). This monitors any current flowing down the Earth lines around the property. If this is happening, it means that an appliance has a fault. If this trips, all power is cut out. However, it won't detect current that is flowing to earth by another route (eg via a person).
- A "Residual Current Circuit Breaker" (RCCB). These are better than ELCBs. Rather than monitoring the earth line, they instead monitor the current flowing in the "Live" (in) and "Neutral" (out) lines. If there is a difference, the power is cut out. This means they will detect if any current is flowing out by any other method - via the earth, or via a person.
Wiring - Lighting Circuits
The lighting circuit is essentially a ring main that runs to every ceiling rose in the property (or in the case of a property with multiple lighting circuits, to a section of the ceiling roses). There are no individual fuses on each rose, so the only fuse for the circuit is the one at the consumer unit.
Many modern houses don't use traditional ceiling roses, and instead simply have a pair of wires hanging down for each light fitting. If this is the case, it means that all you need to do is connect the light bulb across them, and everything will work.
If you have traditional ceiling roses, things are more complicated, and when you unscrew the rose, you will find 8 or 9 separate wires! These are as follows.
- Live Ring In (Red)
- Live Ring Out (Red)
- Neutral Ring In (Black)
- Neutral Ring Out (Black)
- To Switch (Red)
- From Switch (Black - but note when the switch is on, this is LIVE)
- To Bulb (Brown)
- From Bulb (Blue)
- Earth (optional) (Green and Yellow, or may be uncased)
The ceiling rose itself has 9 connectors in it, and these are simply blocks that connect multiple terminals together. There are two blocks of 3 terminals, 1 block of 2 terminals and a single terminal. Connect them up as follows.
- In the first block of 3: Live Ring In, Live Ring Out, To Switch.
- In the block of 2: From Switch, To Bulb.
- In the second block of 3: From Bulb, Neutral Ring In, Neutral Ring Out.
- To the single terminal: Earth (if present). If the earth lines are uncased, slide some green and yellow insulation over them.
Essentially, what we're doing here is:
- Maintaining the two ring connections.
- Sending Live to the Switch.
- Sending the return from the Switch to the Bulb.
- Sending the return from the Bulb back to the Neutral.
Some people have a timeswitch added at the consumer unit to the entire lighting circuit. This allows all the lights in the house to be turned on and off at specified times. Separate timeswitches need to be added for each circuit if there are more than one. Beware, multiple circuits are not necessarily "upstairs" and "downstairs" - they can be "front" and "back", or sometimes something that seems totally arbitrary!
Always turn off a circuit that you are doing any work on before starting work. Check that you are definitely turning off the correct circuit by turning on the light and making sure it goes off.
Ring Mains and Sockets
The ring main is the main circuit that provides power to power sockets around the house. Most sockets are single or double, but triple ones are also available. Power is distributed around the house in one or more ring mains, in which the live and neutral lines run in and out of every socket in the property. This means that even if there is a fault somewhere, power can still get to the sockets.
Individual sockets are not fused, however, as noted below, every standard British plug is fused. This is why there only needs to be a single fuse or breaker for each ring main at the consumer unit.
All sockets are rated at at least 13 amperes. With a nominal voltage of 230v, this means that any device plugged into a normal British socket can have a rating of up to 3 kW (Kilowatts). This means that unlike in the USA, where special sockets have to be provided for units such as air conditioners, everything in the UK can run into a standard socket, except for cookers (and even some small cookers can).
Sockets can either be surface mounted or flush mounted. The same socket can be used for both - the difference is the box that goes behind the socket. In the case of a surface mount socket, a white plastic box is usually used. For flush mounting, the box is a thin metal one which is placed and then screwed into a hole in the wall.
Once the box has been fitted to or in the wall, simply pull the live and neutral wires in. As mentioned above, there should be 2 of each for the "Ring In" and "Ring Out". Twist each pair together, and push into the correct hole on the back of the socket. An earth cable should also be provided. Connect this into the earth hole on the back of the socket. If the earth lines are uncased, slide some green and yellow insulation over them. If using a metal box, this should also be connected to the earth.
All British sockets are shuttered - there are small plastic covers inside the socket over both the Live and Neutral holes. This prevents babies sticking things into the sockets and killing themselves! On older sockets, the shutters are released when the earth pin goes in (which is longer than the other pins). On newer sockets, they are released when both depressed simultaneously. All modern sockets also have separate switches for each socket.
If you are adding a single socket, this can be done as a spur from the ring. Run a cable on or in the wall from an existing socket, to the new socket, and simply twist it into the existing Live, Neutral and Earth cables. Never add more than one spur off a socket directly on the ring - if you take the socket off the wall and see three sets of wires coming into it, there already is a spur. Never have more spurs on the ring than direct sockets. Additionally, if you are hard-wiring an appliance directly into the mains, remember to add a fuse.
Always turn off a circuit that you are doing any work on before starting work. Check that you are definitely turning off the correct circuit by plugging something into the socket and making sure it goes off.
Cooker and Immersion circuits
These are essentially the same idea - a single circuit from the consumer unit to a specific device which needs particularly high power. There should never be any need to play with these!
Plugs
The standard British 13 Amp plug is bigger than most other plugs. From the front, it looks like this. Some are slightly squarer but this is the basic shape when viewed from the front, with the pins pointing at you.
_____________
/ +-+ \
/ E | | \
/ +-+ \
/ \
| (FUSE) |
| L N |
| |
| +--+ +--+ |
| +--+ +--+ |
+--------+ +--------+
| |
The pins all have a rectangular cross section, with the earth pin being longer and larger than the other pins. The Live and Neutral pins are shrouded for their first half coming out of the plug. This means that even if the plug isn't inserted fully, or if fingers creep a bit too far round the plug while it's been inserted or removed, it's not possible to touch live metal.
All plugs are fused. Most are supplied with 13 amp fuses, and 1 amp, 2 amp and 5 amp fuses are commonly available. Always change the fuse to one which is appropriate for the unit. rootbeer277 says that the fuse should be the one appropriate for the power cord, not the appliance. To a certain extent, these are one and the same, in as much as it's important that an appliance which will drain 13 amps has a power cord that can cope with it. On the other hand, it's safe to have a 13 amp power cord on a device that will only use, say, 1 amp. But in that case, I'd rather have a 1 amp fuse.
The earth pin is usually connected to the metal frame of the appliance, so that if there is a loose wire inside, current will divert to earth rather than just leaving the case live. As mentioned above, this will often trip a major circuit breaker at the consumer unit. Additionally, even if the appliance isn't earthed, there will always be an earth pin on the plug (even if it's made out of plastic!) This is in case the plug is being inserted into a socket that uses the earth pin to release the shutters over the live and neutral pins. It also guarantees that British plugs are always polarised - live and neutral are always connected the correct way round.
All appliances are now supplied with a moulded plug. In these cases, the fuse holder is usually exposed on the face of the plug where I have marked "FUSE" above. This can be popped out with a screwdriver and replaced if required.
If you have purchased a plug and are fitting it to an appliance, follow these steps.
- Cut off the end of the wire to get a clean end.
- Strip back about 3 inches of the outer insulation from the wire.
- Open up the plug, and release the cable grip.
- Lay the cable on the inside of the plug, so that the outer insulation will be gripped by the cable grip.
- Note how much of each of the 2 or 3 internal wires you need to just reach the terminals inside the plug, without being stretched, but without having lots of excess. Note the one on the right (when looking from the inside) is the Live, with the fuse.
- Cut the inner wires appropriately.
- Remove the fuse, and then pull all three terminals out of the plug.
- Strip about 0.5 cm of the inner insulation from each wire. Push the wire into its terminal and then screw tight.
- Replace the pins back in the plug. The wire should fit comfortably if you measured correctly!
- Replace the fuse and cable grip. Screw the plug back together.
Cable colours
Cable buried in the walls most often uses the "old" colours. Modern flex usually uses the "new" colours.
+---------------------------+---------------------------+
| Old | New |
+----------+---------------------------+---------------------------+
| Live | Red | Brown |
+----------+---------------------------+---------------------------+
| Neutral | Black | Blue |
+----------+---------------------------+---------------------------+
| Earth | Green or uncased | Green and Yellow striped |
+----------+---------------------------+---------------------------+
Special cases
Bathrooms
Bathrooms, for obvious reasons (water + electricity = death) have special restrictions on them. As far as I know, the following rules apply.
- No conventional wall-mounted light switches. All lights must be turned on by either a ceiling pull switch, or a wall mounted switch outside the bathroom. One could argue that if you're not going to dry your hands, they'll still be wet if you turn the light off outside - but I think part of the difference is that there'll be less steam condensed onto a switch from outside. rootbeer277 also says that there's less water on the floor, so less chance of power being conducted through your body to the earth by that route.
- No "normal" mains sockets. At all.
- Mains can be supplied by shaver sockets. However, these are always current-limited and have built-in isolating transformers.
Power Showers (high power electric showers that heat up their own water locally - useful if you have a small water tank or too low a hot water pressure head) are connected into the main electricity supply, but this will always be done in such a way that any electricity is well shielded from water.
Gardens
I've never done any work on outdoor sockets, so this is speculation. As I say above, always speak to someone who does know what they're talking about! Outdoor sockets must be in special waterproof enclosures, and must have a separate fuse attached to them.
Never use an electric device outside in the rain. And if you're using anything outside, use a RCCB (residual current circuit breaker) that plugs into the socket (many custom outside sockets have one built in - if you're using, say, a lawnmower plugged into a socket, plug it in via a RCCB). This will cut the power out if you cut through the cable.
Reference
BS7671 is the current standard for British wiring. (Thanks Ponder).
Thanks also to rootbeer277 for making some editorial and technical comments on this, and for the inspiration to write it (after reading some of his excellent WUs).