Chapter 42: Protection against thermal effects

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420.1 — Scope

To prevent people, livestock and property from the effects of arcing, sparking, overcurrents and many other risks of fire.

The consequence of arcing can be serious

Detrimental thermal effects result from badly designed or incorrectly installed electrical equipment including:

  • Harmful effects of heat or thermal radiation developed by electrical equipment.
  • Ignition, combustion or degradation of materials.
  • Flames and smoke (where a fire hazard could be propagated from an electrical installation to other nearby fire compartments.
  • Safety services being cut off by the failure of electrical equipment.

421.1.2   Fixed electrical equipment shall be selected and erected such that its temperature in normal operation will not cause a fire.

421.1.3   Where arcs, sparks or particles at high temperature may be emitted by fixed equipment in normal service, the equipment shall meet one or more of the following requirements.

Regulation 421.1.7 expands and clarifies requirements relating to the install of arc fault detection devices (AFDD) in higher risk residential buildings (and at the same time removing reference to stored material, construction in combustible materials etc). The revised regulation is:

421.1.7 Arc fault detection devices (AFDD) conforming to BS EN 62606 shall be provided for single-phase AC final circuits supplying socket-outlets with a rated current not exceeding 32 A in:

  • Higher Risk Residential Buildings (HRRB)
  • Houses in Multiple Occupation (HMO)
  • Purpose-built student accommodation
  • Care homes.

NOTE 1: Higher Risk Residential Buildings are assumed to be residential buildings over 18 m in height or in excess of six storeys, whichever is met first. It is anticipated that in many areas higher risk residential buildings will be defined in legislation which can be subject to change over time, as well as in risk management procedures adopted by fire and rescue services. Current legislation should be applied.

For all other premises, the use of AFDDs conforming to BS EN 62606 is recommended for single-phase AC final circuits supplying socket-outlets not exceeding 32 A.

Where used, AFDDs shall be placed at the origin of the circuit to be protected.

The use of AFDDs does not obviate the need to apply one or more measures provided in other clauses in BS 7671.

NOTE 2: For busbar systems conforming to BS EN 61439-6 and Powertrack systems to BS EN 61534, the AFDD may be placed at a location other than the origin of the circuit.

422 Precautions wher particular risks of fire exist

The requirements of this regulation shall be applied in addition to those of Section 421 for installations in locations where any of the conditions of external influence described in Regulations 422.2 to 422.6 exist.

422.2 – Protected escape routes

Cables or other electrical equipment shall not be installed in a protected escape route unless part of:

  1. an essential fire safety or related safety system
  2. general needs lighting
  3. socket-outlets provided for cleaning or maintenance.

422.2.1 – Only cables that meet the following requirements shall be installed in protected escape routes:

  1. cables meeting the requirements of 560.8.1 or cables with resistance to flame propagation according to the recommended requirements of the relevant part of BS EN 60332-3 series or, where cable management systems according to (iii) or (iv) below are used, to the recommended requirements of BS EN 60332-1-2 and
  2. a minimum of 60 % light transmittance when tested in accordance with BS EN 61034-2.Cables in protected escape routes shall be as short as practicable.Where used in protected escape routes, cable management systems shall be one or more of the following types and shall be of limited smoke production so as not to inhibit escape:
  3. conduit systems classified as non-flame propagating according to BS EN 61386
  4. cable trunking systems and cable ducting systems classified as non-flame propagating according to BS EN 50085
  5. cable tray and cable ladder systems classified as non-flame propagating according to BS EN 61537
  6. powertrack systems meeting the requirements of BS EN 61534

422.3.4  –  cable shall as a minimum meet the requirements of BS EN 60332-1-2 unless it is fully embedded in non-combustible material such as plaster or concrete, or be installed in one or more of the following

  1. A conduit system satisfying the test under fire conditions specified in BS EN 61386-1.
  2. A cable trunking system or cable ducting system satisfying the test under fire conditions specified in the appropriate part of the BS EN 50085 series.

422.3.1   Except for equipment for which an appropriate product standard specifies requirements, a luminaire shall be kept at an adequate distance from combustible materials. Unless otherwise recommended by the manufacturer, a small spotlight or projector shall be installed at the following minimum distance from combustible materials:

(i) Rating up to 100 W 0.5 m
(ii) Over 100 and up to 300 W 0.8 m
(iii) Over 300 and up to 500 W 1.0 m.


Accessible part Material of accessible surfaces Maximum temperature
A hand-held part Metallic / Non-metallic 55°C / 65°C
A part intended to be touched but not hand-held Metallic / Non-metallic 70°C / 80°C
A part which need not be touched for normal operation Metallic / Non-metallic 80°C / 90°C