pg 15-20 Electrical Inspection, Testing & Documentation – Module 6

Page 1 – Overview & Preparatory Guidance

• Module context: Domestic Electrical Installer – Section 1, Part A, Module 6: Electrical Inspection, Testing and Documentation.
• Central theme: how to conduct insulation-resistance tests safely, accurately, and in compliance with BS 7671.

Removing / Isolating Loads Before Testing

1. ALL electrical loads (lamps, motors, control gear, appliances) must be physically removed or unplugged prior to insulation-resistance (IR) testing.

2. Any circuit or piece of equipment that is vulnerable to the standard IR test voltages must be clearly indicated on installation drawings, schedules, and handed over as part of the final electrical documentation.

When Complete Disconnection Is Impractical

• Where loads cannot be fully extracted:
– Bridge line (L) and neutral (N) together, then test only between the joined conductors and the earthing arrangement.
– Ensures sensitive electronic devices do not experience the full test voltage.

Supply Neutral Precaution

• Incoming neutral conductor must be disconnected so that no inadvertent connection with earth remains from the supply.

Instrument Requirements

• Test set must deliver the correct DC insulation-test voltage at 1\,\text{mA} short-circuit current (per BS 7671).
• Consult BS 7671 for mandatory test voltages (e.g. 250 V DC for SELV/PELV, 500 V DC for final circuits ≤ 500 V).

Compliance Check Reference

• Regulation 643.3.1: mandates measuring insulation resistance between live conductors and protective conductor connected to the earthing arrangement.

Page 2 – Step-by-Step Procedure for IR Tests

Test 1 – General Isolation & Preparation

1. Securely isolate the circuit (open line and neutral).

2. Disconnect ALL current-using equipment:
   – Fluorescent / discharge luminaires.
   – Remove filament lamps.

3. Verify every switch is closed; if lamps/equipment cannot be removed, local switches must be open.

Test 2 – Carrying Out the Measurement

1. Remove/isolated vulnerable equipment (electronics, surge devices, RCDs, dimmers, etc.).

2. Confirm test instrument & leads are intact; check battery condition.

3. Select the correct IR test voltage & range.

4. Measure and record L → N insulation.

Additional Measurements (only if equipment is not vulnerable)

• L → E (line to earth).
• N → E (neutral to earth).

If Vulnerable Loads Cannot Be Removed

• Link L + N together and test (L + N) → E at the distribution board.

Acceptable Results & Interpretation

• For a standard 230\,\text{V} system, IR > 1\,\text{M}\Omega is normally acceptable.
• Readings < 2\,\text{M}\Omega demand caution:
– Retest each conductor separately after fully disconnecting equipment.
– Investigate persistent low values (possible insulation breakdown, moisture ingress, damaged cable).

Why We Test IR

• Verifies insulation is adequate for:
– Protection against direct contact.
– Prevention of short-circuits and earth faults.
• Detects unintended leakage currents that threaten:
– Electric shock to persons/livestock.
– Progressive insulation deterioration → potential fire.
• Must be performed:
– On new work before energising.
– Periodically on existing installations during condition reports.

Page 3 – Other Specialist Tests & Polarity Requirements

Specialist / Advanced Tests (detailed in later modules)

• Protection by SELV (Separated Extra-Low Voltage) or PELV (Protected Extra-Low Voltage).
• FELV (Functional Extra-Low Voltage) verification.
• Basic protection via barriers or enclosures present during erection.
• Insulation resistance / impedance of floors and walls (important for medical & wet locations).
• Earth-electrode resistance (TT systems, lightning protection).
• Prospective fault current (PFC) measurement.
• Phase sequence confirmation on 3-phase supplies.
• Voltage-drop verification in long circuits.

Polarity Testing – Regulation 643.6 (BS 7671)

Polarity must be checked to verify:

1. **Every fuse, single-pole control or protective device is inserted in the *line* conductor only.**

2. For circuits with a neutral conductor (a) all bayonet centres, and (b) Edison-screw lamp holders must have their outer/screwed contact on neutral, except E14 & E27 holders (BS EN 60238) which allow reverse.

3. All wiring is correctly connected throughout the installation (no cross-polarity).

Page 4 – Practical Polarity & Continuity (R₁ + R₂ Method)

Method 1 Overview

• During continuity tests you can simultaneously prove polarity by measuring R1 + R2 (line conductor resistance plus CPC resistance) at each outlet.
• If every point returns a low, consistent reading, polarity is by implication correct—no extra dead-polarity test is required.

Dead vs. Live Tests

• All continuity, insulation, and most specialist checks are dead tests (supply isolated).
• Polarity must be proved twice:

1. Dead, to confirm new work is wired correctly.

2. Live, at the origin, to confirm the distributor’s service polarity is correct before energising the installation.

Live Polarity Options

• Often combined with the earth-fault loop-impedance measurement.
• Alternatively use an approved voltage indicator/test lamp between conductors.

Page 5 – Earth-Fault Loop Impedance (Zₑ, Zₛ)

Purpose & Legal Reference

• BS 7671 Regulation 643.7.3: where protective measures rely on earth-fault loop impedance, it shall be measured (or determined by calculation/enquiry).

Definitions

• External loop impedance (Ze): impedance of the supply network from the source, through line conductor, protective-earth path and back.
• Total loop impedance (Zs): Z_e plus the resistance of the installation’s live and CPC conductors.

Ze + (R1 + R2) = Zs

Measurement Procedure for Z_e

1. Isolate installation; disconnect the earthing conductor from the main earthing terminal (to eliminate parallel paths via bonding, CPCs, etc.).

2. Use a loop-tester to measure at the origin.

3. Record value on:
   – Electrical Installation Certificate (EIC).
   – Minor Works Certificate (if applicable).
   – Electrical Installation Condition Report (EICR).

Supply System Example – TN-C-S (PME)

• Source employs a PEN (combined neutral/earth) up to the service head; installation uses separate N & PE conductors.
• Additional source earth electrodes may be present to lower overall impedances.

Alternative Determination Methods

• Calculation using known conductor resistances & lengths.
• Enquiry (ask DNO / utility provider for maximum Z_e figures).

Cross-Connections & Practical Implications

• All tests build a chain of verification ensuring protective devices will operate within disconnection times.
• Low insulation or high loop impedance can compromise ADS (Automatic Disconnection of Supply) and must be rectified before energising.
• Ethical responsibility: failure to test/record accurately can endanger life and property; proper documentation is a legal requirement under the Electricity at Work Regulations 1989 (UK).

Key Numerical & Formula Summary

• Test instrument short-circuit current: 1\,\text{mA}.
• Standard test voltages: 250 V, 500 V (per BS 7671 tables).
• Acceptable IR for 230 V circuits: > 1\,\text{M}\Omega (investigate if < 2\,\text{M}\Omega).
• Continuity/polarity assessment: R1 + R2 method.
• Loop-impedance formula: Zs = Ze + (R1 + R2).