Safe isolation in five steps: prove, confirm live, isolate, test every pair, prove again

Why heating wiring is done dead

The mains supply in a home is 230 volts (230 V). A shock from it can kill, and it can burn.

So the law controls work near live conductors. Regulation 14 of the Electricity at Work Regulations 1989 says nobody may work on or near a live conductor where danger may arise, unless three things are all true: it is unreasonable for it to be dead, it is reasonable to work on it live, and suitable precautions are taken to prevent injury.

Wiring heating controls can always be done with the circuit dead. So it is always done dead. Regulation 12 of the same law asks for a suitable means to cut off the supply and isolate it. Regulation 13 asks for precautions so that equipment made dead cannot become live while you work on it. The Health and Safety Executive’s (HSE’s) guide HSG85, Electricity at work: safe working practices, sets out how.

“Switched off” is not dead

Dead means three things, in this order:

• ISOLATED: disconnected from every supply, and staying disconnected.
• LOCKED OFF: the isolator locked and labelled, so nobody can switch it back on.
• PROVED DEAD: a tester that you know works shows no voltage.

A switch can be faulty or wrongly labelled. A circuit can have a second supply: a frost thermostat on its own feed, controls fed from a different spur than the boiler, a pump fed from somewhere unexpected. A live can even come back the wrong way through a switch: a BACKFEED. And a switched live that is dead only because the programmer is off is not isolated at all: the next timed switch-on can make it live. Only a test, after isolation, proves dead.

The right tester

Prove dead with a TWO-POLE TESTER (a two-pole voltage detector), or a test lamp made for the job. The HSE’s guidance note GS38, Electrical test equipment for use on low voltage electrical systems, asks for:

• a marked rating for the voltage spikes it may meet: measurement category III (CAT III) for a building’s wiring, CAT IV at the origin of the supply, such as the meter;
• finger guards on the probes;
• exposed metal tips of no more than 4 millimetres (4 mm), and where practicable 2 mm or less, or spring-loaded shrouded tips;
• protection against too much current: fused leads on a test lamp, while a two-pole tester made to BS EN 61243-3 limits the current by its design.

A PROVING UNIT is a small box with one job: it gives a known voltage at its sockets, so you can show the tester works without touching a live circuit. GS38 asks for the tester to be proved before and after use, preferably on a proving unit, otherwise on a known live source of similar voltage.

Never prove dead with a neon screwdriver or a non-contact pen. GS38 says single-pole and non-contact detectors are for identifying a live, never for proving dead: they can suggest that a live is there, never that it is not. Do not use a multimeter either. HSG85 does not recommend one for proving dead: set to the wrong function, it can show nothing on a live wire, and wrongly set meters have caused accidents.

The five steps

1. Prove the tester. Put its probes on the proving unit. It lights. Now you know it works today, on these leads.
2. Confirm the supply is live. Test the circuit you are about to isolate, live to neutral at the point of work, with the isolator still on. It shows the supply is there and that your tester can see it. So when the same screws read nothing later, you know your isolation did it.
3. Isolate. Find every supply first. Switch off at the POINT OF ISOLATION, usually the fused spur, normally a double-pole switch that breaks the live and the neutral. Then lock it off.
4. Test every pair, at the point of work. Live to neutral, live to earth, neutral to earth. Then every switched live to neutral and to earth. Why to earth as well? With a broken neutral, live to neutral can read nothing on a conductor that is still live. Why neutral to earth? If live and neutral are swapped somewhere upstream, the “neutral” is the live.
5. Prove the tester again. Back on the proving unit. It still lights, so it worked all through your tests, and every reading of nothing was true. If it stays dark, none of your readings can be trusted: change the tester and start again.

The order never changes. Each step makes the next one worth trusting.

Test the switched lives too

Heating controls are full of SWITCHED LIVES: the programmer’s outputs, the thermostats’ calls, the zone valves’ brown cores, the orange cores, the boiler’s switched live, the pump’s live.

They are fed through the controls, so they can be fed from somewhere else too: another circuit, or a backfeed through a switch. A dead permanent live proves your isolation. A dead switched live proves that nothing reaches the screws you are about to touch.

Where you can, make the switched live live before you isolate, by setting a demand. Then its reading of nothing afterwards means something.

Lock it off

Switching off is not enough. Lock the isolator with a lock-off device and keep the key. If you take the fuse out, keep it with you, and lock the FUSE CARRIER, or fit a lockable blank, so neither it nor another fuse can go back in. Post a caution notice at the point of isolation, so everyone knows work is under way. HSG85 asks for each of these.

The lock and the notice stay until the work is finished and tested. Before the power goes back on: covers closed, earth continuity proved to every part you worked on, and polarity checked while the circuit is still dead. Then remove your lock and notice, and restore the supply.

None of this is for a homeowner to try. It is the start of every job on the wiring, for the person doing that job.