Electrical fires are a significant problem across a wide range of domestic, commercial and industrial installations. For example, according to the Department for Communities and Local Government, in England alone between 2016 and 2017 there were close to 8,000 fires attended by fire and rescue services that had been ignited by electrical distribution equipment. A further 5,000 fires came from electrical appliances over the same period.
Although conventional circuit breakers provide protection against some faults, such as short circuits and overloads, they do not afford protection from dangerous arc faults.
In a bid to address this issue, new guidance has been issued by the Institution of Engineering and Technology (IET) alongside the British Standards Institute which now recommends the installation of Arc Fault Detection Devices (AFDDs) in final AC circuits like those supplying the sockets in almost all buildings. All new and amended electrical installations in the UK must comply with the IET regulations and the latest edition of the IET Wiring Regulations, the 18th edition, comes into effect from January 2019.
Among the significant changes within the latest edition are recommendations, though not requirements, for AFDDs. Other measures introduced in the latest version include requirements for devices for protection against overvoltage, and a change to the requirement for the methods of supporting wiring systems within buildings against their premature collapse in the event of a fire.
Causes of arc faults
Arc faults in AC circuits can originate from numerous sources. For example, series and parallel arc faults can occur as a result of damaged cables that have been crushed, broken or drilled through, or where insulation has been damaged by rodents or as a result of loose connections and terminals. Parallel arc faults can occur between live and neutral conductors or live and earth conductors, again due to damaged insulation. Here a fault current may be too low to trip other protection devices such as fuses, or where short-duration, high-peak-current events take place.
Mark Coles, head of technical regulations at the IET, explains: “There are different faults that you see on circuits. There are series faults that could be related to a loose connection in a joint box or in the back of a socket, for example if the screws haven’t been tightened up sufficiently. You might get an arc, but it’s not jumping across to another conductor or to earth; rather, it is going on the correct route through the appliance. That can obviously lead to overheating which can then lead to fire. It is a serious fault, but an RCD [residual-current device: circuit breaker] element wouldn’t detect or address that, as it’s not a discharge to earth or a short circuit.” Due to their nature, arc fault conditions may also develop over time and occur out of sight and unnoticed.
In any event, arcing is a potential source of ignition and therefore AFDDs are recommended in locations where the risks associated with fires are considered higher. Coles continues: “With an installation up to BS7671 standards using the correct material, if it’s maintained in good condition and inspected regularly, you’re not likely to get arcing going on. But if the installation isn’t looked after, there’s a hole in the roof and water gets in, for example, AFDDs are there to protect against those situations.”
As the technical regulations head notes: “Arc fault detection is there to either protect you against a deteriorating connection through poor installation practice or poor maintenance, or against an occurrence like mechanical damage to a cable by an action in the building.”
AFDDs could also be appropriate for (well-maintained) premises with sleeping accommodation, or locations with a risk of fire is greater due to the nature of on-site processes or stored materials. The latter might include, for example, flour mills or woodworking shops.
Unlike conventional circuit breakers, AFDDs do not have an electromechanical trigger. Instead, within the AFDD are detection coils and microelectronics. As the current passes through the device, the waveform ‘signature’ of the current and voltage are analysed in comparison with a library of characteristic waveforms.
Different types of equipment produce different waveforms. Arcs can occur in equipment that is working normally, for example. For instance, switches may produce arcs as the contacts open, and bushes produce arcs as a motor spins.
“It’s not looking for any particular level of current or any imbalance of current like an RCD. It’s actually looking to high frequency currents and voltage signatures,” explains Paul Collins, project & technical support manager at electrical equipment manufacturer Hager Group.
The arc fault detection device looks for signature sine waves and any distortion because of harmonics. The dangerous types of arc have very definite signature waveforms that are differentiated from normal kinds of arcs to prevent excessive trips. Activated by both series and parallel faults, AFDDs are extremely sensitive, but are designed to sense and respond only to potentially dangerous arcs.
As this ‘library’ of characteristics expands over time, the microprocessor theoretically will become better, because it will be able to recognise a greater number of risky signals.
In terms of products, equipment manufacturers are developing AFDDs as a combined modular unit with a circuit breaker. Collins says: “Applied to single-phase final circuits and located within the distribution board or consumer unit, these devices have been incorporated into a miniature circuit breaker (MCB)-type module by many manufacturers. One half is the MCB and the other is the arc fault detection device.”
However, installation usually requires some modification to a distribution board. At the moment, distribution consumer units tend to have just single-pole devices attached to a bus bar. For AFDDs to function, the neutral has to go through the AFDD as well, so they are double pole units (Doepke example pictured at right).
What is driving the adoption of AFDDs are changes in technical standards. The international IEC standard 60364 is being introduced at national level across Europe and elsewhere, as Coles explains: “These devices have been absorbed into the world installation standard at International Electrotechnical Commission (IEC) in the 60364 series of electrical installation standards going into regions like Europe, North America, Australasia, and then from there into individual countries.”
He adds: “Our route is from a global international standard IEC, into Europe, which is Cenelec, and then into the UK, which is BS 7671.”
Health and safety legislation, safe practices at work, insurance and other considerations also have to be taken into account. Although there might be a low risk of fire, that risk is appreciable.
Although the new recommendations are not yet in force, there has been early engagement, points out the project & technical support manager: “Since the edition was published we’re seeing lots of interest. There’s a lot of awareness.”
Pricing a challenge
However, pricing is likely to pose a challenge initially. “They are considerably more expensive than MCBs or RCBOs to manufacture. Market pricing is being established as manufacturers develop their offer,” according to Collins. RCBOs are MCBs plus RCDs; the abbreviation stands for residual-current circuit breaker with overcurrent protection.
Nonetheless, as the use of AFDDs ramps up, manufacturing costs are expected to fall. “I would certainly think that within five years it would be quite common, and the more common they are the cheaper they become,” states Collins.
He adds: “Going back maybe 30 years, when RCBs were just coming in, they were quite expensive, but now they are incredibly common and low-cost devices. RCBOs have followed a similar trajectory.”
As Collins notes: “Although currently it is only a recommendation, electrical designers or the responsible person cannot dismiss this lightly. Ignoring that recommendation means the first time there is a fire there will be questions asked.”
Also, what starts as a recommendation may not stay that way, Coles observes: “Recommendations often turn into requirements over time as the value of such devices becomes more proven. We’ve seen it with other devices, such as surge protection devices.”
He concludes: “They’re not that expensive to install and the benefits are that if something does occur within your installation then these devices obviously would stop a catastrophic fire.”