Electrical isolation – protecting yourself and those around you from shocks

05 Feb 2018

The Gas Safe Register’s Technical Bulletin (TB) 118 provides guidance on the necessary safety precautions to take when preparing appliances for work – specifically their electrical isolation.  The document sets out the procedures involved in isolating the electrical connections of equipment and stresses that it is important that the person carrying out the work is competent.

The dangers of electric shock

Fundamentally, the steps laid out in TB 118 are to prevent electric shock, which as we all know can cause serious injury or even death. Even if the current is not of sufficient strength to kill, consequential hazards from the shock, such as falling from a ladder, can exacerbate harm. It’s essential to avoid risky situations by isolating any equipment with an electrical current before work is carried out.


Electric isolation of the equipment must be the responsibility of the person carrying out the work and the following precautions should be carried out before work begins on the appliance itself:

  • Warning notices should be posted at the point of isolation.
  • Locking devices should be used on plugs or switched fused connection units.
  • If the isolation of a circuit breaker is required, then a proprietary locking device with a padlock should be used, not insulating tape.
  • Where fuses are used, removal of the fuse/fuse carrier is a suitable means of isolation. Where the carrier cannot be withdrawn, a lockable fuse insert should be used.

Once the appliance has been isolated, it’s very important to check that it is electrically dead before any further work is undertaken on it. A GS38 approved voltage indicator such as a proprietary LED test lamp or a two-pole voltage detector should be used to prove that the equipment is dead. The HSE doesn’t recommend the use of a multi-meter, non-contact voltage indicators or neon screw drivers for this task.

Here is a summary of the steps they set out for isolating an appliance, such as a gas boiler:

  1. Remove the load from the circuit by turning off the equipment if possible, by operation any on/off switches or removing the plug.
  2. Test the voltage indicator on a known source/supply before use, for example on a proving unit.
  3. Using the proven voltage indicator, check that there are no dangerous voltages present between the following connections:
    • Earth and line
    • Neutral and line
    • Earth and neutral

It is also important to check all other terminals, such as a pump over-run, switched live and any other external connection to the appliance.

  1. Reprove the voltage indicator on the known source to ensure that the voltage indicator is working correctly.

TB 118 gives detailed guidance on electrical isolation and can be found here

LCL offers a qualification in Essential Electrics and Safe Isolation of Electrical Equipment to help Heating and Refrigeration Engineers protect themselves and fellow workers from harm. For more information, please click here.