Reference

Electrical glossary

Plain-English definitions of the UK electrical terms that appear on EICRs, test schedules and quotes.

ADS: Automatic Disconnection of Supply. The primary protective measure against electric shock in BS 7671 — a protective device disconnects a fault within a specified time.
AFDD: Arc Fault Detection Device. Detects the signature of series/parallel arcing faults and disconnects before they cause fire.
BS 7671: The UK national standard for electrical installations — the 'IET Wiring Regulations'. Current edition 18th, Amendment 4 (2026).
CPC: Circuit Protective Conductor. The earth conductor that bonds exposed-conductive-parts of an installation to the main earthing terminal.
DNO: Distribution Network Operator. The regional company that owns the electricity network up to your cut-out — UK Power Networks in the South East.
EICR: Electrical Installation Condition Report. A periodic inspection and test to establish whether an existing installation is safe for continued use.
Ipf / PFC: Prospective Fault Current. The maximum current that would flow if a bolted fault occurred at a given point — the greater of PSCC and PEFC.
MPRN: Meter Point Reference Number — a 21-digit code identifying an electricity meter supply point.
PME: Protective Multiple Earthing. A form of TN-C-S earthing where the DNO combines neutral and earth in the incoming cable.
PEFC: Prospective Earth Fault Current. Fault current that would flow between a live conductor and the CPC/earth.
PSCC: Prospective Short-Circuit Current. Fault current between live conductors (line-to-line or line-to-neutral).
R1 + R2: The sum of the resistance of the line conductor (R1) and the CPC (R2) of a circuit — measured with the supply disconnected, used to verify continuity and calculate Zs.
RCD: Residual Current Device. Disconnects when the sum of currents in the line and neutral conductors differs by more than its rated tripping current.
RCBO: Residual Current Circuit Breaker with Overcurrent protection. Combines an MCB and an RCD in one device — per-circuit RCD protection.
SPD: Surge Protective Device. Protects the installation from transient overvoltages (lightning, switching). See Section 443 and the Surge Protection page.
TN-S: Earthing system where the neutral and protective conductors are separate throughout — DNO provides a dedicated earth (often via cable sheath).
TN-C-S: Earthing system where neutral and earth are combined in the DNO's incoming cable, separated at the intake. See PME.
TT: Earthing system where the DNO does not provide an earth — the installation earth is a local electrode driven into the ground.
Ze: External earth fault loop impedance. Impedance of the fault loop outside the installation, measured at the origin.
Zs: Earth fault loop impedance at a point in the circuit. Zs = Ze + R1 + R2. Must be low enough to cause the protective device to disconnect within the required time.