Whether you're a contractor or a DIY enthusiast, there are some basic principles and rules you need to be aware of if you're doing electrical work. All electricians are familiar with certain issues that come up time and time again; follow these guidelines and you'll avoid some of the major pitfalls and mistakes we see all the time, which are often both dangerous and costly to rectify.
Note that all domestic electrical work done in the UK must be carried out according to the current amendment of BS7671. The easiest way to achieve this is by hiring an electrician to do the work for you. This page is in no way intended to help non-experts understand the complex and detailed regulations. The information here should be taken as helpful advice and will not apply to every situation or circumstance. If in any doubt at all, consult a specialist.
In the UK it's common to use Twin & Earth cable in domestic use. This has an insulated line (live) and neutral cable, along with an earth wire (usually thinner), all contained within sheathing. The cable is rated by the size of the conductor. For example, 2.5mm² Twin & Earth (T&E) cable has line and neutral wires with a cross-sectional area (CSA) of 2.5mm², and an earth (CPC) wire CSA of 1.5mm².
In the event of a fault condition, it's important that all wiring is protected by a device which will disconnect the supply before the wiring is damaged or catches fire. Normally this will be a fuse or a Type B MCB, both of which protect against short circuit faults.
If the wiring is too small for the fuse/MCB, or the fuse/MCB is too big for the wiring, then the safe time to disconnect the supply will not be achieved, which may lead to fire and other damage. Consequently, every piece of wiring must be protected by an appropriate upstream fuse or MCB.
In general, the following fuse/MCB values should be used as a baseline. In many situations, LOWER MCB values should be used; installation methods and derating are outside the scope of this advice.
In the UK it's common to install 32A ring mains: that is to say, the cable forms a ring from the MCB to socket to socket to socket and ultimately back to the same MCB. This allows 2.5mm² cable to be used, as the current has two alternative routes around the ring.
If you are extending a 32A ring, you have the following choices:
You should not use 2.5mm² cable to spur off a 32A ring unless a fused connection unit is between the 2.5mm² cable and the ring.
Finally, be aware that accessories such as switches, sockets and light fittings are designed to be protected in the same way as the wiring. Check the manufacturer's specification and ensure the correct fuse or MCB is protecting the device. For example, most light fittings should be protected by a 6A MCB or 5A fuse; if you installed such a fitting on a 32A ring main, you would have to do so through a fused connection unit fitted with a 5A fuse.
In a nutshell, you must ensure that every cable and device attached to it is protected by the correct value fuse or MCB between it and the point of supply.
Note that use of an RCD in no way changes the fuse/MCB requirements to protect cables and devices. An RCD protects people in the event of current leakage, whereas a fuse or MCB protects cables and devices in the event of short circuits. Read more on the differences here.
After writing the start of this page, literally the very next day we did a job which highlights the danger of many of these pitfalls.
The client had reported that they frequently got substantial electric shocks from a light switch in a barn conversion.
When we attended, the first thing we found was that the earth wire in the light switch was not sheathed. Unnecessary exposed sections of live wire in the switch (see the black wire in the photo) were coming into contact with the earth wire, making the metallic box behind the switch live. This in turn was making the screws at either side of the switch live.
(Note that the black wire in the photo is actually a line wire, not a neutral as the colour suggests - tape should have been used to denote this is a switched line (live) wire.)
This in itself was a problem as it was giving potentially fatal 230V shocks to the householder. However, we knew the whole house was RCD protected, which means the RCD should have tripped immediately in the event of a large earth leakage like this; that's what they're there for. Further investigation showed that the earth wire (CPC) on this circuit wasn't actually earthed at all...
The lights this switch was connected to were metal (Class 1), and were earthed to the CPC, but the CPC wasn't connected to earth, and in fact was live. So not only were the screws in the switch live, but the metallic light fittings were also live, and within easy reach of anybody using the bed directly below them.
The final nail in the coffin of this installation was that the junction box for the lighting had been buried behind a stud wall, meaning it was not only inaccessible, but would be extremely difficult even to locate in order to drill a hole in the wall to access.
Apart from sheathing the earth wire, testing the CPC and not burying the junction box, the installer should have done R1+R2 testing to confirm the CPC was connected to earth, and Zs testing to confirm that the disconnection time would be met. Clearly neither of these tests had been carried out, otherwise these dangerous and potentially fatal problems would have been identified immediately!
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