pg 6-10 Domestic Electrical Installer – Module 6: Electrical Inspection, Testing & Documentation

Page 1: Pre-Work Survey & Safety Considerations

Key preliminary questions to answer before any test work begins

• Which earthing arrangement (e.g. TN-S, TN-C-S, TT) is in place?

• What is the maximum required disconnection time demanded by BS 7671 for that circuit?

• What isolation and switching provisions exist and are they adequate?

• Are all mandatory labels (e.g. main switch, RCD testing, identification of protective device ratings) present and legible?

• Does the measured earth-fault loop impedance Zs fall below the relevant tabulated maximum Z{max} values in BS 7671?

• Is an RCD or RCBO required and, if fitted, is it appropriate for the circuit type and location?

• Is the installation situated in a special location (e.g. bathrooms, swimming pools, agricultural premises) invoking extra regulations?

• What specific risks or hazards (electric shock, arc flash, inadvertent energisation) need to be mitigated?

Inspector’s safety obligations

• Electrical testing always contains inherent danger; it is the inspector’s duty to protect self and others from:
  • contact with live parts,
  • dangers arising during the test procedure itself.

• A thorough pre-work survey is compulsory. It verifies that:
  • all test equipment/leads meet GS38 requirements,
  • the installation is safe to work on,
  • the surrounding environment does not introduce extra hazards (e.g. wet floor, poor lighting).

Essential safe-working techniques

• Safe isolation (prove dead, lock-off, label) before touching conductors.

• Dual working / buddy system where appropriate.

• Verification that test leads are correctly fused, shrouded and rated.

Page 2: Continuity of Circuit Protective Conductors (CPCs)

Context & rationale

• BS 7671’s principal protective measure is Automatic Disconnection of Supply (ADS):
  • Protective earthing (PE)
  • Protective equipotential bonding (PEB)
  • Automatic disconnection upon fault.

• For ADS to work, all CPCs, main bonding and supplementary bonding must be electrically continuous.

What is continuity?

• A CPC is typically a copper conductor integral to a multi-core cable linking every exposed-conductive-part to the Main Earth Terminal (MET).

• A main protective bonding conductor:
  • green/yellow insulation,
  • single-core copper,
  • cross-section \ge 10\,\text{mm}^2,
  • links MET to extraneous-conductive-parts (e.g. metallic water/gas services).

• Continuity = no open circuit, no loose/corroded joints, no excessive resistance caused by conductor length or CSA mismatch.

Test equipment

• Low-resistance ohmmeter capable of milli-ohm resolution.

• Conductor designations:
  • R1 = resistance of line conductor • R2 = resistance of CPC
  • R_n = resistance of neutral

• Deduct test-lead resistance (null function or manual subtraction).

Two approved test methods

1. Method 1: R1 + R2 (short-link method) – also confirms polarity.

2. Method 2: Long-lead or “wander-lead” method – ideal for large circuits or when one end of conductor is remote.

Page 3: Detailed Test Methods

Method 1 – R1 + R2 / Short-Link Procedure

1. Securely isolate the circuit.

2. Insert a temporary link between phase (line) and CPC at the origin of the circuit (e.g. at the distribution board).

3. At each accessory (socket, luminaire, etc.):
   • Apply test probes between the earthing terminal and the centre contact (line) of an Edison screw lampholder or line terminal of a socket.
   • Switch on local switches to include downstream wiring.

4. The meter displays R1 + R2 for that point.

5. Remove the temporary link after testing.

Regulation references: BS 7671 643.2 (continuity) and 643.6 (polarity).

Method 2 – Long-Lead / Wander-Lead Technique

1. Connect an extended lead from the MET (or CPC at the board) to one probe of the ohmmeter.

2. Walk to each point on the circuit, probing the CPC
   • Confirms continuity of CPC over long runs or where disconnection at origin is impractical.

3. Also usable for:
   • Main or supplementary bonding verification,
   • Positive identification of circuits,
   • Live conductor continuity (with circuit dead).

Illustrated examples in slides: consumer-unit origin, lighting switch-drop, Edison lampholder.

Page 4: Significance of CPC & Bonding Continuity

Why it matters

• A break in CPC or bonding leaves exposed metalwork without fault protection; under an earth fault the metal may become live.

Using R1 + R2 data

• Values contribute to calculation of earth-fault loop impedance Zs at the accessory: Zs = R1 + R2 + R_{external}

• Ensures measured Zs complies with tabulated Z{max} to achieve required disconnection time.

Long-lead method for bonding conductors

• Main bonding: resistance between service pipe clamp and MET should be \le 0.05\,\Omega.

• Supplementary bonding: same \le 0.05\,\Omega criterion.

Metallic enclosures as CPCs

• When conduit, trunking or SWA armour is used as the CPC, inspector must:
  • Visually inspect for mechanical soundness along entire length.
  • Perform ohmmeter test to verify low resistance continuity.

Continuity of live conductors

• As vital as CPC continuity; broken line or neutral can create hazardous conditions or mis-operation of protective devices.

Page 5: Continuity of Ring Final Circuit Conductors

Regulatory requirement

• BS 7671 643.2.1 mandates continuity verification of all conductors in a ring final circuit (line, neutral, CPC).

Construction recap

• Ring circuit leaves distribution board, loops through every point, and returns to the same terminals. Each conductor therefore forms a complete loop with no branch interconnections.

Test instrument

• Low-resistance ohmmeter with resolution to distinguish differences of 0.05\,\Omega or better.

Three-step test sequence
Step 1 – End-to-End Resistances

1. Isolate the installation.

2. Identify and disconnect line (L), neutral (N) and CPC at the consumer-unit terminals.

3. Measure:
   • R1 = line end-to-end resistance • Rn = neutral end-to-end resistance
   • R_2 = CPC end-to-end resistance

4. Expected observations:
   • R1 \approx Rn (identical CSA)
   • If CPC CSA is reduced (common case: 1.5\,\text{mm}^2), then
   R2 \approx 1.67 R1
   for a ring with 2.5\,\text{mm}^2 line/neutral conductors.

Steps 2 & 3 (not detailed in the slide but standard practice)

• Cross-connect L of one leg to N of the other, measure at every point (checks polarity and interconnections).

• Repeat with L to CPC cross-connect to obtain R1 + R2 for each socket.

Purpose of results

• Detects:
  • Broken conductors masked by interconnections,
  • Mis-wired rings configured as radials,
  • High-resistance joints.

Documentation

• Record individual R1, Rn, R_2 values and subsequent figure-of-eight readings on the Electrical Installation Certificate (EIC) or Minor Works Certificate as evidence of compliance.