Electricity comes in many forms. Your residential house supply is 230 volts alternating current (AC from here on). The AC is supplied on two pins, and an earth connection on another. The AC pins are live and neutral. You shouldn’t go around touching any bare electrical wires, but if you did touch the earth or the neutral nothing would happen to you.
The neutral line is held at zero volts. The live line is the one that alternates. It alternates (you might be surprised to know) between +325 volts and –325 volts fifty times a second in a sinusoidal shape. This gives it the equivalent power of a 230 volt DC line.
The earth (sometimes called ground) line’s job is to act as an electrical safety valve. It is a low-impedance path for current. Low-impedance means that if a live voltage is connected to it, the current from that line will flow unrestricted at any current. This unrestricted flow will be so high that it will blow any fuse in the line. Thus, earth is used to make electrical equipment safe.
Let’s say you have a metal toaster with no earth connection, the electrical heating elements are (when its on) connected from live to neutral. Let’s say over the years, the element gets broken and the live wire falls off and touches the metal casing. That metal casing is now at 230 volts AC. If anyone were to touch it, the electricity would have a path, through their body, to ground, which it would take, and in the process would electrocute them. Now imagine that the case was connected to earth, as soon as the live touched the case unlimited current would flow: much higher than the 13A fuse in the plug would allow, and so that fuse would blow, cutting the power and making the whole thing safe. We expect then that there will never be any current flowing into earth, if there is, then there is a fault in the equipment.
You should be able to see then that your equipment will work without an earth connection (some low-voltage, non-metallic cased, equipment doesn’t need an earth), it is dangerous not to connect it. The live connection supplied the power, and the neutral connection supplies the return path for the electrical current to flow.
Now you know what the three wires are, let’s look at how they are connected in a plug. In the electrical equipment (i.e. the wiring that uses the power, rather than the wiring that supplies the power) the three wires are brown (live); blue (neutral); and green and yellow (earth).
UK plug
The UK plug is the best design in the world (I think). It’s solid, robust, safe and easy to wire. The rectangular pins, arranged in a triangle give it a mechanically sound connection when it’s in the socket.
Wiring it, at first sight, is simply a matter of connecting the right colour to the right pin. See the ultimate handyman for a quick photo-guide to connecting. Here is some additional useful practical advice that’s worth knowing:
- The cable is made up of an outer sheath, which contains three wires with different coloured insulation. The wires themselves are stranded copper — that is to say they are made of hundreds of tiny little copper wires, braided together. Be careful with the little wires, it’s not hard to stab yourself with one, leaving a nasty metal splinter in your finger.
- Notice that the wire paths for the neutral; earth; and live are all different lengths. While cutting them to the same length will work, you’ll find it easier to work with if you cut them to more appropriate lengths. Do this by laying them in the plug and measuring — you don’t have to be exact, and you should leave a little slack. Leave more slack on the earth than on either of the other two. This ensures that should there ever be enough force on the cable to pull the wires out, the earth will be the last to be disconnected.
- The ends should be stripped, but not by a huge amount. Look how deep the brass screw terminals are — between five and eight millimetres of exposed wire will be sufficient. Some people strip more and then fold the wire in half for a better grip. If you do do this, then the folded wire should be inserted into the terminal so that it folds in a clockwise direction — this means that when you tighten the screw, you are tightening the fold.
- When you’ve stripped the insulation, the stranded wires exposed should be twisted to keep them together. This helps to make good electrical contact and prevents a stray strand from poking outside the screw terminal, or stabbing you in the finger.
- When tightening the screw terminals ensure that no insulation is under the screw, you are making an electrical connection, not a mechanical connection. If you trap insulation under the screw, you are making the electrical connection weaker, and potentially creating a fire risk if the electricity passes through the insulator.
- Connect the earth wire first, then live, and neutral in whatever order you find easiest.
- The cable grip is important. A plug without a cable grip means that any pulling on the external cable is actually pulling on the brass screw terminals. Eventually they will break (the copper stranded wires that carry the current have very little mechanical strength). If it happened to be the earth that broke inside the plug, it could work its way across to the live terminal and touch it. At which point, the equipment would be unearthed, and the normally safe to touch parts of it would be connected to live. The cable grip should be loosened before you start, and tightened to leave the wire inside the plug slack. Don’t be shy about tightening the cable grip, make sure it’s solid. You should be tightening the cable grip over the full outer cable sheath, not the three individual wires. If you find that the individual wires are too long to let you push the full cable under the grip, cut them shorter and begin again.
- Don’t forget the fuse. It’s not dangerous if you do forget, but your equipment won’t work without it.
- Don’t use an inappropriate fuse. Whatever the equipment is rated at, that’s the fuse you need. Putting a 13 amp fuse on a night lamp that uses only 500 milliamps, just means that 12.5 additional Amps have to flow through your quivering body before that fuse blows (it’s not quite as bad as that, but you get the idea).
- If you have a piece of equipment that is blowing its fuse regularly, the solution is not to put a bigger fuse in. Equipment that is blowing its rated fuse is faulty. Even if it’s intermittently faulty, it’s still faulty. Putting a bigger fuse in simply hides that fault and puts you in danger.
Update:
This guide to electrical outlets the world over offers good visual evidence of why my choice that the UK outlet is the best is correct. (Incidentally; if anyone thinks it’s a patriotism issue, the British telephone socket is not good).
Key features:
- Three pins
- Non-equilateral triangular pin arrangement
- Rectangular pins
- Mechanically sturdy pins
- Mechanically sturdy socket
- Low profile (i.e. the cable comes out perpendicular to the plug, not parallel to the plug)