Safe isolation and basic electrical checks

Most gas appliances have a mains electrical supply, and gas engineers have been killed by electric shock. Before you work on an appliance, you need to know its supply is off, that it can't be switched back on while you work, and that it is genuinely dead. Gas Safe Register's Technical Bulletin 118 sets out how, and health and safety enforcing authorities expect registered engineers to follow it as best practice.

Why it matters

An electric shock is current passing through the body. Its effects range from slight discomfort to death, and even a mild shock can cause a fall, for example from a stepladder. Around 50 mA is generally accepted as a lethal level. Above it, muscles can lock so you can't let go, and the heart's rhythm can be disrupted or stop. A fuse or circuit breaker won't normally protect you, because the body's resistance keeps the current too low to trip it. A residual current device (RCD) may detect it, if one is fitted.

The law

• The Health and Safety at Work etc. Act 1974 requires employers to ensure, so far as reasonably practicable, the health and safety of their employees.
• The Electricity at Work Regulations 1989 require precautions against death or injury from electricity at work. Working dead should be the normal method. Live work is only allowed where it's unreasonable to work dead (for example some fault finding and testing), it's reasonable to work live, and suitable precautions are taken (reg. 14).

Before touching pipework or appliances

Before working on pipework, carry out a risk assessment that includes stray electrical currents, and check for stray voltage. A non-contact voltage detector (a "volt stick") able to indicate 50 V or more, used on all the exposed metalwork in the work area, helps detect dangerous voltage. But it only shows that something is live. It can't prove that anything is dead. If you have any doubt about the electrical installation, get advice from a competent electrician.

Isolating the supply

The means of isolation must be under your control while you work.

• Plug, or switched fused spur, next to the appliance and in view: isolating there is enough, as long as nobody else can switch it back on.
• Out of sight: lock it off (a locking device on the plug or spur), and post a warning notice at the point of isolation.
• Circuit breaker: use a proprietary lock-off with a padlock (not insulating tape), and keep the key yourself.
• Fuse: remove it (and its carrier where possible) and keep it with you. If the carrier can't be removed, use a lockable fuse insert. Post a warning notice.
• TT earthing system: the incoming neutral may not be at earth potential, so isolate both line and neutral.

The safe isolation procedure

1. Isolate. With the responsible person's permission, and once you've checked it's safe to do so, switch the appliance off, then isolate the supply as above.
2. Prove your tester. Check your approved voltage indicator (a proprietary test lamp or two-pole voltage detector) on a known supply or proving unit. Don't use a multimeter to prove dead.
3. Test dead. Check there's no dangerous voltage between line and earth, line and neutral, and neutral and earth. Check other terminals too, such as pump overrun, switched live and any external connections. When testing to earth, connect to earth first and remove it last.
4. Prove your tester again on the known source, to confirm it was working all along.

Checking the earth

When you work on Class I (earthed) equipment such as a gas appliance, the earthing of its supply must be adequate, and in a customer's home you won't know whether it is. Measure the earth fault loop impedance with an instrument made for the purpose, and make sure you know how to read the result. Plug-in testers generally only check polarity and show that an earth may be present. If the earth is inadequate, tell the customer that the work can't continue until it's put right.

A safe place to work

When testing live in a customer's home, set up a safe test area: consider a temporary barrier, give yourself room to move away if you get a shock, keep children and pets away, make sure everyone knows the dangers, and never leave an appliance unattended with live parts exposed. Test equipment should be made to BS EN 61010, BS EN 61557 or BS EN 61243-3.

Sources: Gas Safe Register TB 118 (2018)isolation, testing BS 6891:2015 8.3.1–8.3.2risk assessment, voltage detector Electricity at Work Regulations 1989 reg. 14 (via TB 118)dead working CCN1 Issue 16 §5assessed

▸Tutor notes

• Have each learner carry out the four-stage procedure on a training boiler, naming each test pair aloud.
• Ask why the tester is proved again after testing, and why the earth lead goes on first and comes off last.
• CCN1 §5 item 28 assesses the purpose and suitability of a non-contact voltage tester: stress what it can and can't tell you.