Arc Fault Detection Devices. More Than Recommended?

November 6th 2018

Simon Rowlinson of Wylex delves into the 18th Edition regulations to explore whether Arc Fault Detection Devices (AFDDs), although defined as a 'recommendation', are essentially 'mandatory' in certain scenarios.

Did you know that the wiring regulations require protection against arcing? Chapter 13 deals with fundamental principles and the protection for safety of people and property (and livestock), while Chapter 42 deals with protection against thermal effects.

Within Chapter 13, regulation 13.1 (vii) says that in electrical installations the risk of injury may result from arcing or burning, and regulation 131.3.1 requires the electrical installation to be arranged so that the risk of ignition of flammable materials due to high temperature or electric arc is minimised.

There’s also a requirement to ensure that electrical equipment doesn't present a fire hazard to adjacent material and to protect people against harmful effects from combustion, ignition or degradation of materials.

‘Danger’ definition

It’s understandable that the above requirements are included when you consider that the definition of danger in the 18th Edition includes the following: “risk of injury to persons (and livestock) from fire, electric shock, burns, arcing and explosion arising from the use of electrical energy”.

Chapter 42 (section 421) includes regulations that deal specifically with protection against fires cause by electrical equipment. Not long ago this section was amended to mandate the use of non-combustible consumer unit enclosures in all domestic household premises, which adequately demonstrates the scope and purpose of this section.

Within section 421 it states that protection is required against the harmful effects of heat or thermal radiation that may be developed by electrical equipment. Persons, livestock and property must be protected against harmful effects of heat or fire which may be generated or propagated in electrical installations.

Such harmful effects may be caused by insulation faults or arcs, sparks and high temperature particles, overcurrent, failure of electrical equipment or heat accumulation, heat radiation, hot components or equipment. This is covered in regulation 421.1.1

Additional Protection

Section 421 also recommends the use of Arc Fault Detection Devices for a particular purpose which is to provide additional protection against fires caused by arc faults.

The subsequent sections of chapter 42 (section 422) deal with specific locations and circumstances that require further considerations. This is in addition to the requirements of section 421 which also apply. Section 422 relates to precautions where a particular risk of fire exists e.g. locations with a risk of fire due to the materials & processes used, buildings with combustible construction materials or fire propagating structures like high rise buildings etc

Regulation 422.1.2, for example, requires electrical equipment to be selected and erected so that its normal temperature rise and foreseeable temperature rise during a fault can't cause a fire. This can be achieved by the construction of the equipment or by additional protective measures during installation.

What causes 'Arc' faults?

Arc faults can occur from loose terminations, damaged insulation, or compressed cables, poorly made connections, frayed conductors, and deterioration of insulation materials that age over time. Faults like these can lead to overheating and ignition of the insulation of the cable and combustion of surrounding flammable materials.

Even though there are requirements within the regulations to ensure that the installation is designed so that the wiring systems are protected against impact, abrasion, tension and compression (Chapter 52) and requirements for every connection to provide durable electrical continuity and adequate mechanical strength, electrically ignited fires continue to occur.

There’s also Chapter 53 which considers the use of devices for the protection against the risk of fire. Regulation 532.2 states that RCDs with a rated residual operating current of not exceeding 300mA shall be used and that RCDs shall be installed at the origin of each circuit to be protected.

But, RCDs are designed to not operate with fault currents below 50% of their rated current. So a 300mA RCD wouldn't disconnect a 150mA earth leakage/arc fault. Also RCDs are unable to detect serial arc faults that occur with loose terminations or damaged (reduced) conductors, while MCBs cannot detect arc faults because the magnitude of the fault current is too low to operate the device.

No surprise then that within the same section of the regulations there is 532.6 which states that Arc Fault Detection Devices (AFDDs complying with BS EN 62606) where specified shall be installed at the origin of the final circuits to be protected (in AC single phase circuits not exceeding 230V).

Clearly there are lots of considerations for designers to take into account to ensure a safe reliable installation including providing adequate protection against arc faults.

The use of AFDDs now offers an additional level of protection against fire caused by arc faults in final AC circuits. Government statistics show that there were more than 13,000 fires in England 2016/17 occurring in electrical distribution and other electrical appliances, which suggests considerable room for improvement.

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