Please read and accept the following before using these tools:
Safe Isolation
Safe to touch · isolate · prove dead (single-phase)
Step 1 of 12
Work dead.The law does not allow work on live electrical equipment unless working dead would be unreasonable, working live is reasonable, and precautions against injury are in place. This procedure makes sure the appliance is dead before you start.
Job details▾
Supervising engineer details are entered at sign-off.
Gas Safe TB118A procedure
Twelve steps to a dead, locked-off appliance.
Work down the page. Each step unlocks when the one before it is complete. If a check fails, the tool tells you what to do next.
01
Step 1
Permission to isolate
Isolating can affect other equipment and the people in the property, so agree it first. Some premises, particularly non-domestic, may need a permit to work.
Non-contact voltage detector being proved on a known live source
02
Step 2
Non-contact voltage detectorVoltage detector checked
These detectors can show that something is live, but cannot prove it dead. Use it to the manufacturer's instructions.
If the detector is unsatisfactory, do not go on until a working detector is available.
Complete Step 1 to continue.
Safe-to-touch sweep of the appliance casing with a non-contact detector
03
Step 3
Safe-to-touch sweep
A fault such as a broken neutral on a PME (TN-C-S) supply can leave pipework carrying current without a fuse operating.
Complete Step 2 to continue.
STOP: voltage indicated
Do not touch the appliance or pipework. Tell the responsible person the installation needs urgent attention from an electrically competent person, and contact an electrically competent person. What happens next depends on the situation, so a risk assessment and/or advice from an electrically competent person is needed.
The Isolation Record will show “Procedure stopped at Step 3: voltage indicated”. The bonding question and sign-off below are still available.
Examples of isolation points: SFCU, plug and socket-outlet, circuit-breaker
04
Step 4
Identify the point of isolation
Complete Step 3 to continue.
Not carried out: procedure stopped at Step 3.
Two-pole voltage indicator being proved on a proving unit
05
Step 5
Two-pole voltage indicator
Proving dead needs a device that makes contact with the conductors. A multimeter is not recommended because it can be set to the wrong function.
If the indicator is unsatisfactory, replace it with a suitable indicator before continuing.
Complete Step 4 to continue.
Not carried out: procedure stopped at Step 3.
06
Step 6
Appliance functional and single-source
The circuit may need checking. Go back to Step 4.
Complete Step 5 to continue.
Not carried out: procedure stopped at Step 3.
07
Step 7
Identify a suitable means of isolation
Having confirmed the appliance has only one supply (Step 6), choose a means of isolation that will disconnect it. Step 10 proves whether it has.
An SFCU does not always break both line and neutral. On a TT earthing system the incoming neutral may not be at earth potential, so the means of isolation must disconnect line and neutral. Action: on a TT system, if you cannot confirm the chosen point breaks both, choose one that does.
Complete Step 6 to continue.
Not carried out: procedure stopped at Step 3.
08
Step 8
Isolate
Turn off the circuit at the switched fused connection unit.
Remove the plug from the socket-outlet.
Remove the fuse.
Switch off the circuit-breaker (MCB) at the consumer unit.
Remove the fuse (and carrier where applicable) for the circuit supplying the appliance.
Switch off the isolator.
Lock-off details follow at Step 9.
Complete Step 7 to continue.
Not carried out: procedure stopped at Step 3.
MCB locked off with padlock and warning tag
09
Step 9
Lock off and label
Plug/socket: switch off if switched and remove the plug. Remove the plug fuse if practicable. If the socket is not next to the work, secure the plug against someone reconnecting it.
SFCU: switch off if switched and remove the fuse. Lock the fuse carrier open. Keep the fuse on your person, not on show near the unit.
MCB: switch off and lock off with a proprietary device. Insulation tape is not an acceptable way to hold an MCB off.
Fused consumer unit: remove the fuse (and carrier where applicable). If the carrier cannot be withdrawn, use a lockable fuse insert. Keep the fuse on your person.
Complete Step 8 to continue.
Not carried out: procedure stopped at Step 3.
Proving dead at the appliance terminal block: earth to line (E–L), no voltage indicated
10
Step 10
Prove dead (at the appliance)
Test at the appliance terminals with the two-pole indicator and record each result.
A small residual reading between neutral and line (up to 15 V) still counts as dead.
Earth probe on first, off last.When testing between earth and line or neutral, connect the earth probe first and remove it last. Why: if the terminal is still live, connecting to it first can put a dangerous voltage on the tip of the other, unconnected probe. With the earth probe connected first, the free probe is never the live one.
A test is not dead. Go back to Step 4 and re-identify the point of isolation.
Complete Step 9 to continue.
Not carried out: procedure stopped at Step 3.
11
Step 11
Re-prove the indicator
The Step 10 results cannot be relied on. Go back to Step 5.
Complete Step 10 to continue.
Not carried out: procedure stopped at Step 3.
12
Step 12
Safe to work
Stay alert, and re-prove with the indicator and proving unit whenever you leave the appliance and come back to it.
Complete Step 11 to continue.
Not carried out: procedure stopped at Step 3.
After the work
Re-energising
Available once Step 12 is complete.
Not carried out: procedure stopped at Step 3.
Main protective bond on the gas pipework at the meter, with its warning label
Separate question
Main protective bonding
Bonding keeps the metalwork in the dwelling, including gas pipework, within safe voltage differences, so an earth fault operates the protective device rather than leaving pipework live.
Expected position: on the consumer's rigid metal pipework, as close as practicable to where the pipe enters the premises, before any branch; where practicable within approximately 600 mm downstream of the meter's outlet union; for an external meter, where the pipe enters the building. It should be accessible for inspection and carry a “safety electrical connection” warning label.
A plastic (MDPE) gas service pipe generally does not need bonding, but a competent electrician should confirm that.
Deciding whether bonding is needed, especially on PME supplies, may be beyond a gas engineer's competence. If so, tell the responsible person and leave further action to them.
Photos▾
Sign-off▾
Sign above
Sign above
Electrical Safety Advice Notice▾
Details are copied from the job details and sign-off. Edit them here if needed; changes here do not alter the job details.
Sign above
Signatures from the sign-off section are used on the notice.
Bonding Advice Notice▾
Details are copied from the job details, bonding question and sign-off. Edit them here if needed; changes here do not alter the job details.
Sign above
Signatures from the sign-off section are used on the notice.
Isolation Record▾
Create any notices first, then the Isolation Record, so the record shows them as issued.
No notices are needed for the answers recorded so far.
Disclaimer: This tool is provided as an aid to help competent persons follow and record a safe isolation procedure based on Gas Safe Register Technical Bulletin 118a. It does not constitute electrical certification, make the user electrically competent, or replace professional judgement. Users must be competent for the work being undertaken. Users are solely responsible for the accuracy of all entries, for the selection and proving of test equipment, and for ensuring that all work is carried out in accordance with the Electricity at Work Regulations 1989, the Gas Safety (Installation and Use) Regulations 1998 (as amended) and all other applicable legislation and standards. To the fullest extent permitted by law, the author accepts no liability for any error, omission, or any direct or indirect loss or damage arising from use of this tool or reliance on its output.
This tool guides you through the Gas Safe best-practice procedure for checking an appliance is safe to touch, isolating it, and proving it electrically dead on a low-voltage single-phase supply before gas work. It always produces an Isolation Record PDF. Where needed, it also produces an Electrical Safety Advice Notice (safe-to-touch fail) and/or a Bonding Advice Notice (bonding not present or not confirmed).
It does not make anyone electrically competent, and it does not cover three-phase supplies or isolating a whole installation or board.
Guided shows every step with photos and explanations. Quick checklist shows the same steps, in the same order and with the same stop rules, in a compact list. Both record the same information and produce the same PDFs.
Safe isolation sequence and fail branches
The legal position
The Electricity at Work Regulations 1989 do not allow work on live electrical equipment unless all three conditions are met: working dead would be unreasonable in the circumstances; working live is reasonable in the circumstances; and precautions against injury are in place. If any one of these cannot be met, the work must be done dead.
Electrical work should be done with the system dead whenever possible.
Testing or fault-finding on live equipment is itself live working.
The voltage tests in this procedure are themselves live testing. GS38 treats testing for voltage as an essential part of proving a system dead before work starts. That is why they must be done with GS38-compliant equipment that has been proved before and after use.
Gas engineers must not remove covers from electrical components unless they are competent for the work. No one should do work they are not competent to do.
Why it matters
Electric shock is a harmful physiological effect caused by current passing through the body. It can range from discomfort to death.
A current as small as about 50 mA can be fatal. At that level, muscles may contract so the person cannot let go.
Body resistance can drop to about 1000 Ω, so contact with 230 V could drive around 230 mA through a person, several times the dangerous level. A fuse will not protect a person at currents this small.
A residual current device (RCD) is additional protection only. It is no substitute for basic protection or safe isolation.
Most electrical accidents happen on equipment that was thought to be dead. Treat all exposed electrical parts as live until proven otherwise.
Test equipment
GS38: the two-pole voltage indicator used to prove dead should meet HSE Guidance Note GS38.
Non-contact detectors (“voltage sticks”) are for finding live equipment only, never for proving dead.
Proving dead needs a device that contacts the conductors: a two-pole voltage indicator or a proprietary test lamp. A multimeter is not recommended for this, and illuminated “neon” screwdrivers are not suitable.
Standards and construction: probes, clips and leads should be made to BS EN 61010-031, or BS EN 61243-3 for two-pole detectors. They should be marked with their CAT rating and maker. Finger barriers are needed, with no more than 4 mm of exposed tip (2 mm or less, or spring-loaded retractable covers, strongly recommended).
Measurement category (CAT) rating: the device must be rated for the measurement category and voltage at the point of test. CAT IV is the source of the installation (e.g. meters, main overcurrent protection); CAT III is the building installation (e.g. distribution boards, socket-outlets, permanently connected equipment); CAT II is circuits connected directly to the installation (e.g. appliances, portable tools).
Prove it works before and after use, ideally with a proving unit; failing that, on a live supply of a similar voltage known to be working.
Proving units produce lethal voltages, so follow the maker's instructions.
Look after it: check for cracked cases, damaged insulation, loose terminals and damaged tip covers. Discard damaged leads.
Safe to touch
Before touching an appliance casing or metal pipework, sweep it with a non-contact detector. A fault can make metalwork live, so the sweep comes before any contact. If the detector shows voltage, stop and contact an electrically competent person. If it shows nothing, prove it again on a known live source to confirm it was working.
On protective multiple earthing (PME, also known as TN-C-S) supplies, a broken neutral can make the pipework a current path without blowing a fuse. Separating pipework can then draw an arc or put the operative in the circuit.
Isolation and lock-off
Speak to the responsible person first, and check whether a permit to work is needed.
Confirm the appliance is working and fed from one source, identify the means of isolation, isolate, lock off, label, and keep the key and any fuse on your person.
A switched fused connection unit (SFCU) does not always break both line and neutral. On a TT system the incoming neutral may not be at earth potential, so the means of isolation must disconnect line and neutral. Check other terminals (pump overrun, switched live, external connections) too.
Method-by-method details: see Step 9 of the checklist. Never use insulation tape to hold a miniature circuit-breaker (MCB) off.
Proving dead
Test earth–line (E–L), earth–neutral (E–N) and neutral–line (N–L) at the appliance with the proved two-pole indicator. Earth probe on first and off last: if a terminal is still live, touching it first could leave a dangerous voltage on the other probe tip. A residual of up to 15 V N–L still counts as dead. Then re-prove the indicator on the same proving unit. If anything reads live, recheck what has actually been isolated.
While working and re-energising
Re-prove dead whenever you come back to the appliance.
Capacitors can hold a charge even after isolation. Be aware of them.
When cutting into pipework or removing a meter, fit a temporary continuity bond of at least 10 mm² across the break and leave it until permanent continuity is restored.
After the work: inspect for damage, remove locks and labels, restore the supply, check the appliance for safe operation, and complete the paperwork.
Protective equipotential bonding
What it is: main equipotential bonding connects the consumer's earth point to the gas installation pipe. It creates a zone in which voltage differences stay at acceptable levels, reducing shock risk.
Where it should be: on the consumer's rigid metal pipework, close to where it enters the premises and ahead of any branch, and where practicable within approximately 600 mm downstream of the meter's outlet union (for an external meter, where the pipe enters the building). It should be accessible, visible, labelled, and mechanically and electrically sound. It must not be clamped directly onto corrugated stainless steel or flexible meter-outlet connectors.
External meter boxes: the bond may be made inside the box on the meter outlet before any tee, provided the box's integrity and any sleeve seal are not compromised. Medium-pressure meter boxes have extra restrictions on how cables leave the box.
When bonding is commonly required: for new-build gas installations, and the first time gas pipework is put into an existing property.
Plastic service pipes: an MDPE (non-metallic) service generally does not need bonding, but a competent electrician should confirm.
PME supplies: bonding requirements are stricter, and judging them may be beyond the gas engineer's competence. If so, tell the responsible person and leave further action to them.
Supplementary bonding may be needed in higher-risk locations such as bathrooms; consult a competent electrician.
The engineer's duty: tell the responsible person, in writing, that bonding may be needed and should be carried out or checked by a competent person. Do not fit or alter it unless you are competent to.
Legal basis for the Bonding Advice Notice
Anyone connecting installation pipework to a primary meter must, where bonding may be needed, tell the responsible person that it should be done by a competent person.
L56 guidance says the advice should be put in writing. It also says the same action is expected when an engineer notices an apparent bonding defect at other times, such as during maintenance, for both main and supplementary bonding.
“In writing” includes email as well as paper.
Gas Safe describes a matching duty: where bonding may be needed, the engineer should advise the responsible person to get a competent person to check the electrical installation.
References
Gas Safe Register, Technical Bulletin 118a (Feb 2023). Controlled copy: Gas Safe Register Technical Information area (login).
HSE, GS38 Electrical test equipment for use on low voltage electrical systems, 4th edn (2015): hse.gov.uk/pubns/gs38.htm
HSE, HSG85 Electricity at work: safe working practices (cited by TB118A and GS38)
Gas Safety (Installation and Use) Regulations 1998 (as amended), Regulation 18(2)
BS 7671:2018+A2:2022 Requirements for Electrical Installations (IET Wiring Regulations, 18th edition): Part 2 definitions, Regulation 312.2 and Abbreviations (used for the appendix)
HSE, L56 Safety in the installation and use of gas systems and appliances, 5th edn (2018), paras 200–206: hse.gov.uk/pubns/books/l56.htm
Gas Safe Register, Technical Bulletin 102 (Oct 2019). Controlled copy: Gas Safe Register Technical Information area (login).
The three proving-dead tests: earth to line, earth to neutral, neutral to line
SFCU
Switched fused connection unit
MCB
Miniature circuit-breaker: a type of circuit-breaker, found in consumer units. It carries normal current and switches off automatically when abnormal current flows, such as a short-circuit current
RCD
Residual current device (an RCCB or an RCBO)
PME
Protective multiple earthing: a form of TN-C-S system (BS 7671 Figure 3.9)
PEN
Combined protective and neutral conductor
TN-C-S
An earthing system in which, for part of the system, one conductor (the PEN conductor) does the job of both neutral and protective conductor. T: the supply source is directly connected to Earth. N: the installation’s earthing is connected to the earthed point of the supply, normally its neutral point. C-S: combined in part of the system, separate in the rest.
TT
An earthing system in which the supply source is earthed at one point, and the installation’s earthing is provided by its own earth electrode(s), electrically independent of the supply’s earth electrodes. First T: the source is directly connected to Earth. Second T: the installation is connected directly to Earth, independently of the source’s earthing. On a TT system the incoming neutral may not be at earth potential, so isolation must disconnect both line and neutral.
CAT II / III / IV
Measurement category of test equipment: II circuits connected directly to the installation (e.g. appliances, portable tools); III the building installation (e.g. distribution boards, socket-outlets, permanently connected equipment); IV the source of the installation (e.g. meters, main overcurrent protection)
MDPE
A plastic (non-metallic) material used for gas service pipes
GS38
HSE Guidance Note GS38, Electrical test equipment for use on low voltage electrical systems
GS(I&U)R
Gas Safety (Installation and Use) Regulations 1998 (as amended)
HSE
Health and Safety Executive
TB
Gas Safe Register Technical Bulletin (e.g. TB118A, TB 102)
V, mA, Ω
Volts (electrical potential), milliamps (thousandths of an amp of current), ohms (resistance)
mm²
Square millimetres (cross-sectional size of a conductor)
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