Page 675 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 11 Electrical principles and processes for building services engineering

You will be involved with isolating the circuit to complete works for two
main reasons. You may be asked to remove an appliance from a plug,
which is the easiest method of isolation. You might be asked to isolate an
immersion heater from a fused connection unit in order to repair, replace or
decommission an electrical shower. Failure to isolate supplies correctly can
result in electric shock.

The following procedures will enable you to complete these works correctly and
stay safe. The sequence below will enable safe isolation from the consumer unit
(if the wires need disconnecting for a complete decommission, remember that
you are not allowed to work inside the consumer unit).

Safe isolation procedure

Step 1 Step 2 Step 3 Step 4
Check with the occupier/user Identify the type of supply system: Locate and identify the circuit/equipment to be isolated. Select an approved voltage-
that it is acceptable to isolate • TN–S: Double-pole main switch indicating device; this must
the circuit/equipment. • TN–C–S: Double-pole main switch be verified:
• TT: DP isolation – all circuits and • on a known supply or
equipment. • on a proving unit.

no
Step 8 Step 7 Step 6 Step 5

Fit appropriate lock-off device Isolate circuit/equipment by Identify suitable Verify the circuit/equipment is
and locks. Person carrying out switching off: means of isolation. functional. Use the approved
works to retain key. • double-pole/three-phase isolator satisfactory voltage-indicating device to
• circuit-breakers (or by with- verify circuit/
drawing the fuse). equipment. If the
circuit is non-
operational, dead
testing may be
required to verify
the circuit.
no

Step 9 Step 10 Step 11 Step 12
Fit warning label for isolation Verify the circuit/equipment is Re-check the approved voltage- Circuit/equipment should be safe
and identified work. isolated. Use the approved voltage- indicating device is still functional: no to carry out the work. Always
indicating device to verify circuit is • on the same known supply as remain vigilant and recheck with
dead. Test between all conductors: Step 4 voltage-indicating device when
• Line–Line • on the same proving unit as moving away and returning to
• Line–Neutral Step 4. the circuit/equipment.
• Line–Earth satisfactory satisfactory
• Neutral–Earth.

yes yes

p Figure 11.35 Safe isolation procedure

Once you have done all the above, you should have confirmed that circuit is
dead. It is important to remember to explain to the customer what you are
about to do to ensure that they are happy, but also so that if they notice any

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