Safe electrical isolation steps

Plan electrical work to be done dead. Being competent does not make dangerous live work acceptable. Live work is allowed only in an exceptional case, where all three of these hold:

• it is unreasonable to make the equipment dead;
• it is reasonable to work live in the circumstances; and
• suitable precautions prevent injury.

A working RCD does not meet those conditions. HSE's HSG85 is plain that all three must be met: if even one cannot be, live working is not allowed and the work is done dead.

• Identify the equipment and every relevant supply. Check for independent feeds, backfeeds and stored energy: not every cable belongs to one circuit.
• Switch off and disconnect the identified sources using devices suitable for isolation. Deal with stored energy by the equipment’s safe procedure.
• Lock off and label. Secure the isolation with a lock and a warning notice so nobody can switch it back on while you work.
• Check the detector’s response. Use a compatible proving unit or a known live source of suitable voltage immediately before the dead test.
• Prove dead. Test the conductors to confirm there is no voltage present, then test your indicator again to confirm it still works.
• Only then work. Treat the circuit as live until you have personally proved it dead.

Keep the isolation locked off while you check the detector, test the conductors and check the detector again. The first check shows the detector works; the last one catches a detector that failed during testing, which would otherwise look like a dead circuit. In England, Part P of the Building Regulations sets electrical-safety requirements for dwellings; Approved Document P gives guidance, and where you are not competent to carry out the electrical work it must be done by someone who is. Part P makes three kinds of work in a dwelling notifiable: installing a new circuit, replacing a consumer unit, and adding to or altering an existing circuit in a special location, such as the space around a bath or shower. Work in kitchens and outdoors used to be notifiable but no longer is: other additions and alterations, and maintenance, replacement and repair, are not notifiable. A registered competent person gives the BS 7671 electrical installation certificate to the person ordering the work, and within 30 days of completion the installer or their scheme gives the occupier a Building Regulations compliance certificate and gives building control the certificate or its details. An installer who is not registered can appoint a registered third-party certifier before work starts, and must notify the certifier within 5 days of finishing. Check the full procedure against current guidance.

Why a screwdriver tester is not good enough

A neon screwdriver lights by passing a small current through your body to earth, so what it shows depends on your footwear and the floor as much as on the circuit. It can glow on a dead conductor running beside a live one, and stay dark on one that is live. It is not built or tested to any recognised standard for proving dead, and neither is a socket tester or a non-contact pen. A non-contact pen reacts to an electric field without touching the conductor, so it can miss a live conductor behind metal or inside screened cable. Treat either as a first warning at most, never as proof.

Choose a contact instrument intended for absence-of-voltage checks and suitable for the system voltage, measurement category and duty. A two-pole detector, normally to BS EN 61243-3, checks between its probes without using your body as the return path. Follow its instructions, including any loading function: models differ. A compliant detector has built-in protection against excessive current and energy, so it does not always need fused leads as well.

Inspect the detector, leads and probes before use. Finger guards and limited exposed tips reduce accidental contact and bridging: GS38 gives a maximum exposed tip of 4 mm and recommends 2 mm or less where practicable, or retractable screened probes. Remove damaged equipment from service; tape does not restore damaged insulation, shrouds or the original rating.

GS38 also sorts measurements into categories, and probes, clips and leads are marked with theirs:

• CAT IV at the source of the installation, such as the meter and the main overcurrent protection;
• CAT III in the building's installation, such as distribution boards, socket outlets and permanently connected equipment like a fused spur;
• CAT II on circuits connected directly to the installation, such as appliances and portable tools.

Using equipment rated for a lower category than the point being tested, such as CAT II kit on a CAT III installation, is one of the causes of accidents GS38 lists. When a multimeter is used, its probes carry high-breaking-capacity fuses, usually of no more than 500 mA, or a current-limiting resistor and a fuse; a loop impedance or RCD tester's leads typically use 10 A. Replace a fuse only with the correct type and rating: glass fuses are not suitable.

For simply checking whether a voltage is present, GS38 prefers a test lamp or two-pole voltage detector to a multimeter, because incorrectly set multimeters have often caused accidents. When a multimeter is used, connect the test leads to the meter first, before connecting to the circuit, and set its function and range before any connection is made. Where the voltage is in doubt, select the highest range first, provided the highest voltage possible is within the meter's range. A multimeter left on a current or resistance range while it is used to measure voltage draws excessive current through its probes and leads, and the arcing that follows can burn.

Why a switched-off circuit can still be live

A single-pole switch or isolator breaks the line conductor only and leaves the neutral connected — which matters because if the circuit's polarity has been reversed somewhere, the conductor left connected is the live one. Choose an isolation arrangement that suits the supply, the equipment and every possible feed; a double-pole device disconnects line and neutral together where that is needed. Ordinary isolation leaves the protective conductor connected. A switch that looks off does not mean the circuit is safely isolated.

A neutral can also be shared, or “borrowed”, between two circuits. Switch off and lock one of them, and its neutral can still be energised from the other circuit, which is still on. So a conductor you are about to touch can be live even though its own breaker is off. Several cables, permanent lives and switched lives may belong to one supply, or an enclosure may contain independent feeds. Find the actual arrangement and isolate every source: counting cables does not tell you. A display still lit after isolation may mean another supply or stored energy, so investigate. An unpowered display can also keep showing an image, and a blank display does not prove the circuit dead: test the conductors at the point of work.

Opening circuit breakers or the consumer unit’s own main switch does not isolate its incoming tails and supply-side terminals. There may be a separate isolator upstream, but you still have to establish whether it is suitable and exactly what it isolates; never assume the incoming terminals are dead. The cover over that is protection, not an access panel — removing it to reach pipework behind can expose live parts, and that is work for someone competent and authorised to do.

None of this shows up by looking. It is exactly why every conductor is identified and proved dead at the point of work, on every combination, rather than assumed dead because a switch is off or a display has gone blank.

Don't trust a label, trust the test

A circuit label records what somebody believed when they wrote it, and boards get altered, circuits get extended and ways get reused afterwards without the label following. A label, or a neat row of similar switches with no label at all, only tells you where to start looking. You still trace the circuit and prove it dead at the point of work, which is where being wrong would matter. Colours can mislead in the same way. Single-phase fixed wiring installed before the 2004 change to harmonised colours uses red for line and black for neutral, where today's cable is brown and blue, and an altered installation can contain both. Identify each conductor rather than assuming its job from its colour.

Keep the lock-off key on your own person while you work, rather than leaving it in the device or hanging nearby. A lock is only as good as its key: left within reach, it lets someone else restore the supply in good faith, with no way of knowing you have your hands in an enclosure elsewhere in the building.

The lock matters as much as the key. Electrical Safety First's guide to safe isolation says insulating tape over a circuit-breaker is not an acceptable way of holding it off. Lock the breaker off with a locking-off clip and a padlock that only a unique key opens, and attach a prohibition sign at the point of isolation. It does not recommend combination padlocks, because other people may know the combination. Before proving dead, make sure you have the right point of isolation: where it is possible and safe, test the circuit with the isolating device on and then off, which shows that the circuit is under its control.

An RCD does not replace isolation

On a single-phase circuit, an RCD compares the currents passing through its line and neutral paths. An earth fault may cause it to operate before anyone makes contact. If a person becomes the leakage path, it can cut the shock short, but that is not guaranteed, and a shock from line to neutral, which stays balanced, may not be detected at all. The device can fail too. Securely isolate every supply, deal with stored energy and prove dead at the point of work. The RCD protects the installation while it is live; it is not permission to touch live conductors.

A plain RCCB detects residual current but does not provide overload or short-circuit protection; an RCBO combines those functions. An RCD cannot see an overload, because the current stays balanced. Provide the overcurrent and residual-current protection required for the actual circuit, with each device selected for its duty.

Connecting an immersion heater or fan convector

Follow the appliance's own electrical requirements. Heatrae Sadia's immersion heater instructions ask for the heater to be earthed, wired in heat-resistant flexible cord of at least 1.5 mm², and supplied through a double-pole isolating switch with at least 3 mm contact separation in both poles. The supply must be off before any connection is made, and the cylinder full before the heater is switched on. Myson's Kickspace fan convector needs a fused (3 A) spur whose switch has 3 mm separation on all poles, in an easily reached position.

From the street to the circuits

Know which parts an isolation point actually covers. The network operator's service cable ends at the cut-out, a sealed unit holding the service fuse; it belongs to the network operator, and only someone they authorise may withdraw that fuse. The supply then passes through the meter, which is the energy supplier's equipment, and along the meter tails to the consumer unit. Inside the consumer unit the main switch feeds a busbar, the busbar feeds the RCDs and circuit-breakers, and each of those feeds its final circuits. Switching the main switch off disconnects everything after it, while the tails and the switch's own incoming terminals stay live.

Many socket circuits in UK homes are ring final circuits. The line, neutral and protective conductors each leave the board, loop through every socket on the circuit and return to the same terminals, so each socket is fed from both directions. Rings are commonly wired in 2.5 mm² twin and earth cable with a 1.5 mm² protective conductor. A radial circuit runs out from the board and ends at its last point. A large load has a circuit of its own. Mira's guide for its Sport electric showers asks for a separate supply from the consumer unit to the shower, with nothing else, such as an extractor fan or a pump, supplied through it. It recommends a 32 A to 45 A circuit-breaker, depending on the model's power.