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Wednesday, 19 March 2014

Why doesn't the UK use AFCI's?

Siemens Arc Fault Circuit Breaker
In this post I hope to answer the burning question of, why doesn't the UK include the Arc Fault Circuit Interrupter (AFCI) in their regulations? Actually following the UK naming conventions, I assume they would call them, Arc Fault Circuit Breakers (AFCB) when located within the consumer unit or generically they would be called, Arc Fault Detection Device (AFDD).


AFCI are common in North America but are not required in the UK, why?

AFCI are common in North America but are not required in the UK, why?

Siemens Arc Fault Circuit Breaker

This is a very good question and one that I will have to take an educated guess at. As I am not privy to the discussions that happen in the UK standards bodies I cannot give a definitive answer but I will attempt to guess at the logic behind the decision in this following post. If you're not familiar with what an Arc Fault Circuit Interrupter (AFCI) is, then take a look at the video or read the previous post.
  • Please note in this post, when I refer to an AFCI I am specifically referring to the more advanced "combination" AFCI.
For a longer more in-depth video about AFCI see the video embedded in this post.

Background relating to the UK wiring.

The worlds wiring is generally split between domestic voltage systems of "roughly" 110Vac (split phase) and 230Vac. The higher voltage means a more dangerous wiring system configuration to the residence. The balance of using the more hazardous higher voltage is that, for the same power, the resulting lower current means a more effective power delivery to the load. It can also result in a reduction of the material resource requirements of the physical wiring to the load.

Power is simply calculated as a multiplication of current and voltage. So twice the voltage means half the current, when delivering the same power to an appliance. The higher the voltage the greater the potential of an arc being generated in faulty wiring. Conversely, the higher the current the more energy within the arc itself and consequently the higher the temperature generated in the immediate area of the arc. The higher the temperature generated in the localised fault the greater the risk of secondary effects such as fire.  The energy produced in an arc can be calculated as the square of the current multiplied by the resistance. So twice the current means potentially four times the power in the arc energy of a faulty circuit.

In the USA the standard circuits are supplied with 120V which means that, when compared to UK's 240V systems, they use twice the current to achieve the same power transfer. Most of the world uses a wiring system where their domestic circuits are wired radially from a breaker panel. In North America these are known as branch circuits, while in the UK they would be called radial circuits.
  • Most homes in the USA are supplied with two phases ( AKA split phase), which can be combined to make 240V (note: between the phases and not utilising the neutral conductor!).  Commonly this combined configuration would be used for high power devices such as AC units or washing machines. These devices would normally be protected individually with their own "combined" dual breaker devices.

  • Normally receptacles would be supplied with 120V around the home in the USA. Again it would be common to see the 120V receptacle circuits supplied by a 15A or 20A breaker.

  • It is common to see 20 to 40 breaker units in a USA home, while it would be normal to see only 5 to 15 breakers fitted in a UK consumer unit.
In the UK, we use a form of radial wiring for lighting but we use a "Ring Mains*" for the domestic sockets.  This was originally conceived in the 1940's post World War II, where a shortage of copper coupled with a major reconstruction campaign meant a method needed to be found to effectively wire houses with the minimum use of copper but without compromising on the features or quality. The result was the conception of the wiring system which is still in place today of radial wiring for lighting and ring wiring for appliances.

  • * The term "Ring Main" is really a colloquial expression used by the majority of the British population to "incorrectly" describe their internal domestic wiring. It is also a term regularly used by most electricians when talking to customers about their wiring.  The proper institutional name of the "Ring Mains" is actually a "Final Ring Circuit". Mains is considered to be the distributors cabling to the house and the final circuits are the cabling that comes out of the consumer unit (AKA fuse box) and goes to the various appliances, sockets and lighting around the house. Although it makes institutional engineers and technicians pull faces, I will refer to the "final ring circuit" by its more common incorrect name of "Ring Main" in this post.

In the UK, the radial lighting circuit would normally distribute the mains power along the shortest safe appropriate route between ceiling roses or 4 terminal junction boxes. This minimises cable junctions and length of cable between each light fitting supplied on that particular circuit. The light switch cabling would be run from the appropriate ceiling rose or junction box, utilising the included fourth terminal.  This method keeps the cable runs carrying the greatest current short and consequently lower resistance. It also had the effect of making the wiring system much more versatile as well as reducing the number of cable junctions.

The ring main allowed an unlimited number of sockets to be supplied by a 26A rated cable arranged in a ring configuration, hence the term Ring Main*.  Each socket is powered from two directions allowing a much greater current handling without the need for extensive wiring.  A Ring Main using 26A cable (2.5mm2 copper cores) would normally be protected by a 32A fuse or MCB. Today a Ring Main can have unlimited amount of sockets but is restricted to serve an area no greater than 100m2. The regulations suggest that the current rating of the cable used should be no less than 2/3 the rating of the protective device (see regulations to confirm current regulations).

If just left there, this arrangement would mean that a faulty appliance could potentially draw 32A from the mains socket via a cable that was not rated to carry that level of current.  This potential scenario was neither acceptable nor safe.  The result was an additional safety measure requiring all square pin plugs (BS1363) to be fitted with fuses that limit the current to a "maximum" of 13A.

Selection of plugs showing internal fuse
Selection of BS1362 plugs showing internal fuse
Options for 3A and 5A fuses (BS1362) are also available allowing the appropriate fuse to be
used with the appropriate cable.  Thus, a 200W kitchen hand blender might only be using 1A in normal operation. So the manufacturer would fit it with the lowest rated cable of 3A and an appropriate 3A fuse would be fitted to the plug. Other fuse values are available but 3A, 5A and 13A are the most widely available from local shops.

The British wiring system lends itself to individually fused outlets for appliances, especially installed where the plug and socket would be in a difficult to reach position, behind the kitchen units for instance. Effectively, these circuits are fused spurs from the Ring Main but using an additional fitted fuse of no greater than 13A within the outlet socket's faceplate. A fused spur also allows the fuse to be tailored to the appliances requirements as it uses the same fuses that are fitted in square pin plugs (BS1362) . If the appliance is not wired direct to the fused outlet, irrespective of the fuse fitted in a fused spur's faceplate, the plug will still have its own fuse within, giving the impression that it unnecessarily has two fuses plus the MCB protecting it. (See, post what do fuses protect?)

Guess at the reason why AFCI are not recommended in the UK!

  1. All UK mains standard solid core mains cable must carry a central earth conductor and this runs in the body of the cable between the Live and Neutral conductors in a flat linear type arrangement.  If the cable insulation was damaged to allow arcing and heating, which further compromised the insulation, it is likely the mandated fitted RCCB or RCBO would trip.

  2. As the Ring Main arrangement can handle high current at 240V, it is important to keep resistance of the circuit low.  This means that all connections or extensions are normally large grub screw terminals built inside the junction box, shrouded mechanical crimps or screw operated clamps. Often the hefty recessed grub screw based terminals of the socket are used as a combined terminal junction box connection, allowing easy access to the wires where they are joined or spurred. This facilitates the best mechanical (oxygen free) joints between conductors.  The size of the connection terminals aids in any heat dissipation as well as being in flame resistant housings. The likelihood that these connections would arc is much lower (though far from impossible). 

    • Although wall sockets are usually rated at 13A per socket outlet, the rating of their hefty internal screw terminals is usually 40A plus.  It is common to use these terminals for the dual purpose of socket and junction box thus minimising junctions on the ring.

    • The grub screw terminal allows much more force to be applied to the wires as it is squeezed by the high compression forces. When the wires are partially crushed together by the high forces, the copper is deformed increasing the joint mating surface and expelling oxidising air making the joint low resistance and corrosion resistant in a similar way to crimping. This effect is known as a cold weld.
    • Care must be taken not to sever the wires by enthusiastic over tightening of the grub screw.
    • The UK regulators now insist that any new wiring must be tested for compliance including a resistance test on all conductors and an insulation flash test to confirm the integrity of the insulation. This would pick up any poorly installed wiring in the ring main or lighting circuits.

  3. The fuses within the plugs are rated at or below the rating of the cable, so if parallel arcing occurred it would be more likely to blow the fuse in the plug.

  4. The British system tends to have few push fit connections that are likely places for arcing, notable exceptions being the fuse socket in the plug and the plug fitting itself. Also the plug has excellent integrated strain relief capabilities, which ensure inadvertent force or strain is on the body of the cable and not on the conductors within the plug. The British plug being renowned for its particularly large brass pins being able to handle and dissipate heat much more readily than most other plugs. The net result is that when an appropriate fuse is fitted at the plug, the likelihood that sufficient heat would be generated by arcing to a start a fire is much lower. Sadly it does not stop an individual putting a 13A fuse into a plug meant for a 3A fuse.

    • I have also witnessed some terrible DIY plug installations, retrofitted with straggly conductors and incorrectly fitted (or no) strain relief by people who, "should" know much better! (which is probably the reason why all appliances must be supplied with a factory fitted plug since 1995)

Conclusion and considerations on AFCI/AFDD installation.

All these arguments add to the much lower likelihood of the Arc Fault Current Interrupter (AFCI) being required by the wiring regulation bodies at this time.

Another big consideration is that the regulatory body in the UK are a very conservative bunch and considering that the RCCB technology was around nearly 30 years (late 1960s) before it was adopted as part of the regulations.  The AFCI technology is relatively young at roughly 5 to 10 years (early 1990s).

There are still a number of issues to be ironed out about the AFCI's operation, especially regarding nuisance trips caused by non-compliant appliances as well as compliant ones.  The mains cabling are now being used as a medium for telecommunications within the house and this also can affect AFCIs. Once the AFCI technology matures, I would not be surprised if it does not also become mandatory in the UK, in the same way as the RCCB is today.
  • These telecommunication device nodes usually induce long wave RF onto the domestic mains conductors which are intended to be broadcast locally within the home to other nodes.  Very popular when the Wi-Fi signal is oversubscribed or unreliable.

The modern AFCI is microprocessor controlled and has the capability to replace the RCBO as it can be programmed to perform a host of functions, including overvoltage protection and remote controlled operation.

If you are interested in seeing a long video about that current state of play with AFCIs, then this is an excellent video.  The embedded video below is from a Canadian manufacturer of AFCI called "Eaton" but the principles are the same for all makes. The video is more aimed at the hardcore technophiles who really wants a good understanding of the current state of play with the technology that's available today. Or at least when the video was created.

For further reading and references: please see my resource page

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