Safe Isolation

Safe isolation is the most critical step before any electrical work. It means disconnecting the circuit from the mains supply and verifying that it is dead before touching any conductors.

The Procedure

1Identify MCB2Turn off MCB3Lock off MCB4Test with VTNever rely on an MCB being in the off position alone

Step-by-Step

  1. Identify the circuit — trace which MCB controls the circuit you're working on
  2. Switch off the MCB — flip to the OFF position
  3. Lock off — use an MCB locking clip or masking tape with a "DO NOT SWITCH ON" label
  4. Test with a voltage tester — use a non-contact voltage tester (NCV) or a multi-meter at the conductors to confirm the circuit is dead
  5. Test the tester — test your tester on a known live circuit before and after to confirm it's working

Why Lock Off?

Someone else may not know you're working and flip the MCB back on. A £3 lock-off clip could save your life.

Non-Contact Voltage Testers

An NCV tester beeps or lights up when it detects a live conductor through cable insulation. Always use one before touching any conductor, even if you believe the circuit is isolated.

Proving the Circuit Is Dead

At the final point of work, use a two-pole voltage tester (not just an NCV) to confirm:

  • L to N: 0V
  • L to E: 0V
  • N to E: 0V