Ring of truth

Of late there seems to have been an almost endless stream of misleading and unsubstantiated criticism of the British ring circuit system and BS 1363 plugs and sockets. In most cases, it is difficult to understand why. The UK plug and socket system is probably the safest in Europe, if not the world; the high level of safety is confirmed by statistical evidence from the DTI Consumer Safety Unit.

First, there have been comments that problems such as open or crossed circuits and incorrect extensions can go undetected in a ring circuit until a fault condition occurs (EMC, April 2006, p45-46). This is confusing to installers and consumers since such problems can only go undetected if a system is incorrectly installed, inspected or tested. Furthermore, such problems are equally possible in any radial or tree system.

The testing of a ring circuit is slightly more involved than for a radial or tree circuit, but it is not that complex. It is reasonable to assume that a person who is properly trained and capable of correctly inspecting and testing a radial or tree circuit can do the same to a ring circuit.

Second, protection against overloads, small overcurrents and short-circuit faults in a flexible cord or appliance is provided by the plug fuse. The BS 1363 system cannot be matched in its ability to provide such protection. For example, it is common to find 0·75 mm² flexible cords fitted to appliances, and although such cords have a 6 A rating they are virtually always protected by a 16 A mcb in mainland Europe. This means that the flex can be overloaded or small overcurrents, which can be of a similar magnitude as an overload, can damage flexible cords and equipment.

In the BS 1363 system, with the correct plug fuse fitted, the flexible cord and connected equipment are fully protected. Therefore, the reply to any assertion that the possibility of such overloads or small overcurrents are low is: the chance of any electrical fault is low, but the UK still applies considerable resource to minimising the risk.

The fact that the ring circuit system was invented specifically to allow householders to move 3 kW heaters about their homes seems to have become set in stone. Yet one only has to read Post War Buildings Study No 11 to understand that this is not the case. The major emphasis was to move towards a system that:

• provided a multitude of sockets;
• minimised the number of final circuits;
• maximised diversity of loading;
• provided greater safety against fire and shock – particularly with children;
• would provide correct overload protection of flexible cables and appliances;
• removed the householders’ need to use trailing sockets which become permanent.

Some maintain that the ring-circuit is inflexible, difficult to alter or extend and emphasise the need to break into the ring. Yet it is difficult and intrusive to alter or extend any form of electrical wiring, particularly in the home where cables are behind plaster or architrave, worktops are fitted and carpets are laid. Assertions that tree and radial circuit installation is uncomplicated and alterations are easily arranged are manifestly untrue.

The ring circuit provides the most flexible system available. It allows additions by breaking into the ring or adding a connection unit. It does this while ensuring that overload protection to the additional loads will be maintained through the use of fused plugs or connection units.

With radial or tree circuits, the risk of damage from overloads and overcurrents is always higher as the rating of the protective device is greater than that of the flexible cable and appliance.

A similar emphasis on tree or radial circuits being capable of separate switching is hardly relevant to the ring circuit, since its primary purpose is to provide a high diversity system with a multitude of socket outlets from a single final circuit. Switching control is possible from a ring circuit, but where more complex control is required it has always been normal practice to use radial circuits. After all, no one has advocated using the ring system for lighting circuits.

The testing of a ring circuit is slightly more involved than for a radial or tree circuit, but it is not that complex

Irresponsible comments about the ‘lethal terminal configuration’ of the plugs are sometimes made. The complaint is that where the earth wire is made too short, it will break loose from its terminal before the live or neutral connections when a flex is excessively stressed. Again, these critics have confused misuse with product design.

Where a plug is correctly wired, such danger will not occur since the earth connection will be the last to break. The requirement that all appliances must have a factory-fitted plug will in future virtually eliminate the possibility of such a problem. Furthermore, there are many plug designs outside the UK where the space available and small dimensions make it difficult to achieve the same level of safety as with a BS 1363 plug.

Probably the most misleading and incorrect statements are made about the BS 1363 plug fuse, which must comply with BS 1362. Statements are sometimes made asserting that the fuse overheats, is a nuisance, does not provide shock protection, does not protect the appliance or is unnecessary. One of the main purposes of the fuse is to protect the flexible cord against overloads, small overcurrents or short-circuit faults and to protect appliances that do not have a fuse or circuit-breaker fitted.

Radial circuits with unfused plugs are allowed in the UK and are in common use in other countries. In such circuits, the risk of damage due to overload and small overcurrents, particularly to flexible cables, is greater since the overcurrent protective device (usually 16 or 20 A) has a higher rated current than that of the flex (typically 6 A for a 0·75 mm² flex).

It is worth noting that Wiring Regulations demand that the rated current of fixed cables is equal to or greater than that of the protective device. Why do we allow the opposite and higher risk condition to occur with flexible cables in radial circuits? In practice, are they always attended?

In addition, the plug fuse allows the use of the ring circuit by allowing a high diversity multi-socket system to be supplied from a single, final circuit.

Comments about fuses overheating are incorrect. BS 1363 allows a maximum temperature rise of 52K at rated current on terminals. Products are designed to meet this need, which is well within the capability of moulding materials.

The purpose of the fuse is to protect the flexible cord and connected equipment against fault currents. The plug fuse has never been there to provide protection against direct contact. Statements that suggest the plug fuse is unnecessary because it does not provide such shock protection are incorrect.

It is remarkable how easy it is to justify the British system both on safety and convenience grounds since the safety record and wide use here and overseas speaks for itself. BS 1363 products and the ring circuit system cannot be beaten on safety or convenience grounds.