Quick reference · The IET Wiring Regulations, as they affect alarm and CCTV engineers
The UK rules for electrical installations. This guide only covers the parts that alarm, CCTV and access control engineers meet, such as cable resistance and voltage drop.
The UK wiring rules. This page only picks out the parts that matter when you install alarms, cameras and door entry.
BS 7671:2018+A4:2026. Amendment 4 was published on 15 April 2026. Amendment 3 (2024) stays valid until 15 October 2026.
Confirmed two or more sources agree, or a source quotes the standard. Check one source, or a detail that could not be confirmed. Check it against the standard.
BS 7671:2018+A4:2026, published on 15 April 2026. Amendment 3 (2024) stays valid until 15 October 2026.
The current version is the 2018 edition with amendment 4 from April 2026. The earlier amendment 3 can still be used until 15 October 2026.
BS 7671 uses a temperature coefficient of 0.004 per °C for copper, so a conductor at 70 °C has about 1.20 times its 20 °C resistance. The true figure for copper is about 0.00393 per °C.
Copper cable resists current a bit more when it is warm. At 70 °C, allow about 20% more resistance than at 20 °C.
The maximum resistance at 20 °C of plain copper conductors, in ohms per km, from IEC 60228: 0.5 mm² 36.0; 0.75 mm² 24.5; 1.0 mm² 18.1; 1.5 mm² 12.1; 2.5 mm² 7.41; 4 mm² 4.61; 6 mm² 3.08; 10 mm² 1.83; 16 mm² 1.15; 25 mm² 0.727.
A thin cable resists current more than a thick one. For example, 1.5 mm² cable is 12.1 ohms per kilometre and 2.5 mm² is 7.41.
The current goes out and back, so the loop resistance is twice the run length times the resistance per metre. The drop is the current times the loop resistance.
Count the cable both ways. Voltage lost = current × (resistance per metre × 2 × length).
Band I is extra-low voltage, up to 50 V a.c. or 120 V ripple-free d.c. Alarm, control and telecom circuits are usually Band I. Band II runs up to 1,000 V a.c.
Band I is low-voltage circuits such as alarms and phones, up to 50 V a.c. or 120 V d.c. Band II is mains-level circuits.
Band I and Band II circuits must not share a wiring system (regulation 528.1), unless all the conductors are insulated for the highest voltage present, or they run in separate compartments or conduits.
Do not run alarm cables and mains cables together in the same trunking or conduit, unless every cable is insulated for the higher voltage or they are in separate compartments.
At least 100 mm from power cables, according to guidance on regulation 528.2, for cables in ducts or underground.
Keep telecom and power cables 100 mm apart in ducts and underground.
Test at 500 V d.c. and expect at least 1 megohm for circuits up to 500 V. For SELV and PELV circuits, test at 250 V d.c. and expect at least 0.5 megohm (Table 64).
Test ordinary circuits at 500 V and expect at least 1 megohm. Test extra-low voltage circuits at 250 V and expect at least 0.5 megohm.
Yes. Disconnect surge protectors and sensitive electronics before insulation testing. If that is not possible, 250 V d.c. may be used, but the 1 megohm minimum still applies.
Take out or disconnect sensitive equipment before testing. If you cannot, use 250 V, but the result must still be at least 1 megohm.
Guides describe a dedicated circuit with a double-pole fused, keyed spur fused at 3 A, labelled 'Fire alarms - do not switch off', using 3-core cable of 0.75 to 2.5 mm², with no RCD unless BS 7671 requires one and it must not isolate the panel.
Give the panel its own supply through a locked, labelled 3 A fused spur. Avoid an RCD that could switch the panel off.
Nothing in this guide matches. Search all the guides.
Other guides: BS 5839-1, BS 5839-6, BS 5266-1 and BS EN 1838, BS 5306 (extinguishers), BS EN 50131-1 and PD 6662, BS 8418, BS EN 62676-4, BS EN 60839-11-1, BS 7273-4. Not sure what a word means? Explain this text or see the glossary.
For guidance only. Always check against the current edition of the standard and the manufacturer's data.
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