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

Vocabulary flashcards covering key terms and definitions related to earth fault loop impedance, functional testing, RCD and AFDD testing, and safety standards for electrical test instrumentation.

External Earth Fault Loop Impedance (Ze)

The impedance of the earth fault loop measured at the origin of an installation, forming part of every circuit’s earth fault path.

Purpose of Measuring Ze

To verify compliance with BS 7671 and ensure the earth fault current will be high enough to operate protective devices within required disconnection times.

Earth Fault Loop Impedance of a Circuit (Zs)

Total earth fault loop impedance for a specific circuit, comprising Ze plus the circuit conductors’ resistance (R1 + R2).

Zs Formula

Zs = Ze + (R1 + R2).

Live Measurement of Ze

Ze can be measured only on live installations and does not require disconnection of earthing or bonding conductors.

End-of-Line Testing

Earth fault loop impedance tests must be carried out at the farthest point of each circuit (the ‘end of line’) with all loads connected.

Functional Testing (BS 7671 643.10)

Verification that equipment is properly mounted, adjusted, installed and operates correctly according to the standard.

Examples of Equipment Requiring Functional Tests

Switchgear, controlgear assemblies, drives, interlocks, emergency switching/stopping systems, insulation monitoring devices.

Residual Current Device (RCD)

A protective device that disconnects a circuit when it detects an imbalance between live conductors, indicating leakage to earth.

Standard RCD Trip Rating for Additional Protection

30 mA maximum residual operating current.

RCD Test Conditions

Device must be tested at 100 % of rated current (e.g., 30 mA) at both 0° and 180° of the AC waveform.

Ramp Test (RCD)

Instrument gradually increases test current until the RCD trips, determining the actual tripping current at 0° and 180°.

Integral Test Button (RCD)

Built-in ‘T’ or ‘Test’ button for routine user checks; verifies mechanical operation but does not replace formal testing.

Ze Requirement Before RCD Testing

Earth fault loop impedance must be confirmed sufficiently low before using an RCD test instrument.

Arc Fault Detection Device (AFDD)

Device that continuously monitors circuits for dangerous arcing conditions not detected by other protective devices, reducing fire risk.

Types of Arcs Detected by AFDD

Arcs caused by damaged or broken insulation that create hazardous flash-over between conductors.

AFDD Test Facility

Integrated ‘T’ or ‘Test’ button used to check the mechanical operation of the AFDD.

BS EN 61010

Standard specifying safety requirements for electrical test equipment and instrumentation.

BS EN 61557

Series of standards defining performance requirements for insulation, continuity, loop, and RCD test instruments.

HSE GS38 Guidance

UK Health & Safety Executive note giving advice on selection and use of electrical test equipment up to 1000 V for electricians.

Test Lead Safety Features

Leads must have shrouded terminals, finger guards, fuses, and current-limiting resistors where appropriate.

Maximum Exposed Metal Tip Length

Probes should expose no more than 4 mm of metal; ≤ 2 mm is strongly recommended.