Mythbuster #11 - Adapting for change
By James Eade
We all know that it is poor design (and even potentially dangerous) to reduce the current-carrying capacity of a circuit without appropriate overcurrent protection.
For example, connecting a 1.0 mm2 twin and earth cable to a 32 A circuit-breaker is clearly inappropriate, as the current-carrying capacity of the cable is under half the rating of the circuit-breaker.
In a similar manner, during an inspection and test of electrical equipment, many would flag an extension lead consisting of a 32 A BS EN 60309 (‘commando’) plug to a 13 A BS 1363 socket as ‘illegal’, labelling it as ‘dangerous’ and consigning it to the scrap heap or cannibalising it for spares.
The ‘legality’ of such work will be covered later in this article but suffice to say that this is of course the Mythbuster column and you may be surprised to learn that such things may not always be as bad as they first appear. Given the topic, this article has common ground in three areas of electrical work:
- Fixed installations.
- Temporary system connections.
- The in-service inspection and testing of electrical equipment (ISITEE).
General requirements
There is a requirement in BS 7671 that circuits are protected against overload (Regulation 131.4 and expanded upon in Regulation 433), the lack of which could give rise to damage to wiring accessories or detrimental thermal effects, including fire. Regulation 433.1.1 requires (inter-alia) that:
“The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions: (ii) the rated current or current setting of the protective device (In) does not exceed the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit.”
Circuits should also be designed to prevent small overloads of a long duration.
Regulation 433.2.1 requires that a device for protection against overload shall be placed at the point where the reduction in current-carrying capacity occurs. The note following the requirement gives an example of such a reduction in capacity as being “a change in cross-sectional area of a conductor” for a circuit.
This is all quite a straightforward and well-established practice, as anyone who has wired a distribution board will know. However, a good (and routine) example of where this requirement for the current-carrying capacity of a circuit to be greater than (or equal to) the current setting of the protective device is not achieved, is with a ring final circuit; typically, this comprises a 2.5 mm2 cable protected by a 32 A circuit-breaker. With a cable current-carrying capacity of around 25 A, depending on the installation method for the 2.5 mm2 cable, this would be a complete disregard of the requirement in Regulation 433.1.1 (ii) were it not for the fact that it is configured as a ring to allow current sharing around the loop. The national requirement in Regulation 433.1.204 expands on this permitted exception and Appendix 15 provides information on unfused spurs on ring-final circuits.
Is overload protection required?
Having highlighted the need for protection from overload, where does that leave our 1.0 mm2 cable or 13 A adaptor examples above? Cable adaptors are often used and a common example found in most offices and workplaces is the 13 A plug to 10 A IEC connector, such as the ‘kettle’ type or the clover leaf style found on laptop power supplies. With this type of lead, there is clearly a change in current-carrying capacity of the circuit and yet there is no protection fitted, unless of course the fuse in the 13 A plug is changed to 10 A. It is often the subject of much debate amongst those who do routine equipment testing about whether this should ‘pass’ the ISITEE test and inspection, with the result that 13 A fuses are routinely replaced accordingly.
The key to the conundrum is in the title – “Protection against overload”. Not everything electrical is liable to overload; examples include filament lamps, heating elements, power supplies with internal protection and so on. If a circuit is used to power an item of equipment that, owing to its characteristics, can’t overload the cable supplying it, then overload protection is not required.
On that basis, a 1.0 mm2 cable connected to a 32 A circuit-breaker powering a dedicated load of, say, a 20 Watt Wi-Fi access point, could be acceptable in a very controlled environment, as could a 32 A industrial plug to a pair of 16 A sockets (i.e. a ‘Y’ splitter), but there is a notable requirement that must be met if planning such unorthodox work.
Enter the little-known regulation in BS 7671 that covers this: Regulation 433.3.1. It states that:
“433.3.1 General
A device for protection against overload need not be provided:
ii) for a conductor which, because of the characteristics of the load or the supply, is not likely to carry overload current, provided that the conductor is protected against fault current in accordance with the requirements of Section 434.”
Protection against faults here is key and that means disconnection times must be met for faults to earth or between live conductors, assuming automatic disconnection of supply as the protective measure is in use. It would also be appropriate to do an adiabatic check on the conductors to make sure they won’t be damaged in the event of a fault occurring.
Placing of a device for overload protection need not always be at the point where the current-carrying capacity change occurs either. An example of this could be a connection of a distribution board to a busbar, or an external site office with internal distribution connected to a building. If the distribution units in both cases have overload protection on the input, the single supply cable will be protected against overload accordingly. This is allowed for in Regulation 433.2.2, where either the circuit having reduced capacity is protected against fault current, or is limited to 3 m in length and installed in a manner so as to prevent damage and thermal risks. As before though, fault protection should be provided and adiabatic checks carried out.
Is such wiring ‘illegal’?
It is not the place of the IET to provide interpretations of the law, or indeed, standards. That said, a common, and erroneous, belief is that the IET Wiring Regulations (BS 7671 Requirements for Electrical Installations) are ‘law’ and must be obeyed, leading to poor wiring being accused of being ‘illegal’ in common parlance. The use of the word ‘regulations’ within BS 7671, for what in all other British Standards are referred to as ‘clauses’, dates back to the days before it was adopted as a British Standard, its title being Regulations for Electrical Installations in the 15th Edition, for example.
While the standard is widely respected, referenced and used around the world, it is not a legal instrument and not ‘law’ in a legal sense. However, poor design, installation or maintenance of an electrical system may lead to danger or injury, which itself could lead to an infringement of the law, commonly (but not always), the Electricity at Work Regulations for example. The HSE publishes a statement in the front matter of BS 7671, noting that use of BS 7671 is a good way of demonstrating compliance with the relevant aspects of the Electricity at Work Regulations, so while it may not be ‘law’, it does command a certain authority.
A common argument against such work is that the cable adaptor may be used by someone who doesn’t know what they are doing, or someone may modify the circuit feeding the distribution board in the future and so on. While these are reasonable considerations, they both assume the person doing the work or using the installation is electrically unskilled. If that is the case, then availing of Regulation 433.3.1 may not be suitable and overload protection should be fitted accordingly.
While unusual wiring or cable adaptors may appear unorthodox and just ‘wrong’, in skilled hands, such work could be more of a case of deft engineering by a competent person!