SPD
Surge Protective Device
A device that diverts brief, high- spikes — from lightning activity or heavy switching — safely to earth before they reach sensitive equipment. Usually fitted at or near the , SPDs have moved from a luxury to an expected part of new installations as homes fill with vulnerable electronics.
Where SPD fits in the learning path
The Installation section covers these units:
- Wiring Systems & Enclosures
- Cable Selection & Circuit Design
- Earthing, Bonding & Consumer Units
Curriculum mapping: C&G 2365 Units 203/204/305.
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This section is on the Plus path. Lesson availability depends on content review and your learning path.
Read, practise, return
1. Read it in context
- BS 7671 Amendment 4The Regs and Practice Papers
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- 18th Edition Exam — quiz previewPlus · The Regs and Practice Papers
3. Return to the meaning
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Recall a related term
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The distribution board of a domestic installation. A main switch, residual-current protection and a row of circuit devices on a DIN rail split the incoming supply into individually protected final circuits. Modern UK consumer units sit in non-combustible enclosures and increasingly give every circuit its own RCBO instead of sharing RCDs across banks.
Consumer unit
Read the Consumer unit entryCompare related terms
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Consumer unit
The distribution board of a domestic installation. A main switch, residual-current protection and a row of circuit devices on a DIN rail split the incoming supply into individually protected final circuits. Modern UK consumer units sit in non-combustible enclosures and increasingly give every circuit its own RCBO instead of sharing RCDs across banks.
Read the Consumer unit entryAFDD — Arc Fault Detection Device
A protective device that recognises the electrical signature of dangerous arcing — the sputtering current of a damaged cable or a loose terminal — and disconnects the circuit before the arc starts a fire. Ordinary MCBs and RCDs cannot see these faults, because a series arc neither leaks current to earth nor draws obvious overcurrent.
Read the AFDD entryFunctional earthing
An earth connection made because equipment needs it to work — a clean signal reference for ICT and electronic systems — rather than for shock protection. It matters now because Amendment 4:2026 of BS 7671 updated the requirements around functional earthing for information and communication technology installations.
Read the Functional earthing entryA definition is a start. Using it in a real question is where it sticks.