pg 20-25 Domestic Electrical Installer – Electrical Inspection, Testing & Documentation (Module 6)

Page 1 – Measurement of External Earth Fault Loop Impedance (Ze)

• Context: Domestic Electrical Installer – Section 1 Part A – Module 6: Electrical Inspection, Testing & Documentation.
• Focus: Measurement of the external earth-fault loop impedance, Z_e, at the origin of the installation (incoming supply) to meet Regulation 643.7.3 of BS 7671.

Why We Measure Z_e

1. BS 7671 compliance: every circuit must have a known earth-fault loop impedance.

2. Protective-device operation: If the total impedance of the loop is too high, fault current will be too low and protective devices may not disconnect within the required time.

3. Ze is an integral part of every circuit’s total earth-fault loop impedance Zs.

   Zs = Ze + (R1 + R2)
   • R1 = phase (line) conductor resistance of the circuit. • R2 = circuit CPC (protective conductor) resistance.

Key Practical Notes

• Direct Ze measurement is a live test. • Unlike continuity tests, neither the earthing conductor nor the bonding conductors need be disconnected while measuring Ze, provided the consumer unit is securely isolated from all live conductors except the earth path under test.
• Typical on-site sequence:

1. Isolate supply; remove consumer-unit cover/barriers; confirm isolation.

2. Temporarily disconnect main earthing conductor at the main earthing terminal (MET) only if required by the instrument manufacturer.

3. Carry out the instrument’s 2-lead or 3-lead external loop test.

4. Reconnect, tighten, and document.

Page 2 – Earth-Fault Loop Impedance of Final Circuits (Zs)

Regulation 643.7.3

• All final circuits (ring, radial, lighting) and distribution circuits must have the earth-fault loop impedance Z_s verified at the farthest point ("end-of-line").
• Socket-outlet circuits supplied through RCDs should be energised and tested with the RCD on (normal operating position).

Measurement Guidance

1. Readings obtained on site may be lower than the calculated value Ze + (R1 + R_2) because of parallel earth paths (e.g.
   steel containment, bonding, structural steel).

2. Record the measured value; do not artificially add resistance to make the measurement "match" the calculation.

3. Ensure all protective devices for the circuit under test are closed/on so the test current returns via the intended loop.

Lighting-Point Tests

• Similar procedure: energise lighting circuit, test at the most remote luminaire or ceiling rose.
• Confirm circuit breaker on; verify CPC continuity before any live measurement.

Page 3 – Functional Testing (BS 7671 643.10)

Equipment Requiring Functional Tests

• Switchgear & controlgear assemblies, drives, controls, interlocks.
• Emergency switching-off / emergency-stopping systems.
• Insulation-monitoring equipment.
Note 1: list not exhaustive.

Protective Devices – Specific Checks

• Devices must be correctly mounted, adjusted, and installed.
• Where fault protection and/or additional protection relies on an RCD, its test facility must be verified.

Residual-Current Device (RCD/RCBO) Testing Workflow

1. Pre-condition: confirm circuit Z_s is low enough for the RCD’s disconnection time.

2. Use an RCD test instrument to simulate fault current.

3. Verify operation with the device’s integral test button (labelled "T" or "Test").

Typical Instrument (e.g. RCDT320)

• Cycles automatically through required tests while allowing re-setting between tests.
• Phase-angle (0° and 180°) selection ensures testing on both half-cycles of AC waveform.

Page 4 – Detailed RCD & AFDD Requirements

Standard RCD Test Parameters

• Additional-protection RCDs must have a rated residual operating current I{ \Delta N} \le 30\,\text{mA}. • Mandatory tests: – 100 % of I{\Delta N} at 0^{\circ}.
– 100 % of I_{\Delta N} at 180^{\circ}.
• Example: for a 30\,\text{mA} device, the instrument injects 30\,\text{mA} at both phase angles.

Ramp Test (Optional)

• Some testers increase current gradually until tripping occurs; provides the actual trip current.
• Only the highest 100 % test time is required on the Electrical Installation Certificate (EIC), but recording all data is considered good practice.

Periodic User Check

• Quarterly pressing of integral test button recommended for end-users but does not replace formal testing.

Arc-Fault Detection Devices (AFDDs)

• Purpose: continuously monitor for persistent arcing caused by damaged insulation or loose connections which conventional OCPDs & RCDs may not detect.
• Installation: recommended at origin of each circuit requiring arc-fault protection.
• Integrated test button ("T") verifies mechanical operation similar to RCDs.

Page 5 – Functional Checks & Test-Instrument Safety

Whole-Installation Functional Checks

• Operate all switches, controls, and systems to confirm intended function.
• Verify correct operation of lamps, luminaires, and any connected equipment.

Appropriate Test Instrumentation

Standards & Guidance:

1. BS EN 61010 – Safety requirements for electrical test equipment.

2. BS EN 61557 (relevant parts) – Performance standards for test instruments used on low-voltage installations.

3. HSE Guidance Note GS38 – Advice on safe selection & use of test equipment ≤ 1000 V.

Key Safety Points for Instruments & Leads

• Instruments and leads must be suitably constructed for the voltage and environment.
• Verify voltage rating before applying to a possibly energised circuit.
• Inspect before each use for damage, cracks, loose connectors, worn insulation.
• Use only in environments for which the instrument is designed (CAT rating, temperature, humidity).
• Test-lead probes:
– Shrouded terminals & finger guards.
– Fuses and current-limiting resistors where appropriate.
– Exposed metal tip ≤ 4 mm (≤ 2 mm strongly recommended).

Key Equations & Numeric References (Consolidated)

• External loop impedance: Ze • Circuit loop impedance: Zs = Ze + (R1 + R2) • Additional-protection RCD trip rating: I{\Delta N} \le 30\,\text{mA}
• RCD mandatory tests: 100 % I{\Delta N} at 0^{\circ} and 180^{\circ} • Example test current: I{\text{test}} = 30\,\text{mA} for a 30\,\text{mA} RCD.

Practical / Ethical / Safety Implications

• Ensuring low earth-fault loop impedance is fundamental to automatic disconnection of supply (ADS), directly affecting personal safety from electric shock & fire.
• Functional testing validates not only the installation but the integrity of protective devices whose hidden failure could have catastrophic consequences.
• Competent persons must adhere to HSE GS38 and relevant BS EN standards to avoid introducing new hazards while testing.