BS 546

BS 546 3-pin plugs. Left to right: 15 A, 5 A and 2 A.

BS 546 is an old British Standard for domestic AC power plugs and sockets now most commonly used in India and Pakistan. These are also known as type D (for the 5-A 3-pin) and type M (for the 15-A 3-pin) in US documentation. When viewed in photographs with no indication of scale the different ratings look similar, but where a photograph shows more than one type the difference in size is obvious. There are 6 different versions that are not interconnectable. Whilst these sockets are no longer used in Britain for general appliances (although still permitted by the UK wiring regulations), some of the varieties remain in use in other countries and in more specialist applications (a notable example being stage lighting).

Prior to the development of the BS 546 standards there were around two dozen different types of power socket in use throughout Britain with different types in use in different cities.

15 A sockets were generally given a dedicated 15 A circuit. 5 A sockets might be on a 15 A circuit with multiple sockets or on a dedicated 5 A circuit. 2 A sockets were generally connected to the lighting circuit which was fused at 5 A. Adaptors were available from 15 A down to 5 A and from 5 A down to 2 A so in practice it was quite possible for an appliance with the smallest size of flex to be protected only by a 15 A fuse. This is a similar level of protection to that seen for portable appliances in other countries but less than that offered by the BS 1363 fused plug that replaced them. Fused BS 546 plugs using a BS 646 fuse are available but are rarely seen in practice.

In theory, 15 A plugs were proposed to be used for larger appliances (e.g. electric heaters) while smaller appliances (e.g. radios, table lamps) were supposed to use 5 A. In practice this was found to be inconvenient and in many households the 15 amp plug was used for everything. Adaptors were available which allowed 2 or 5 amp plugs to be used in 15 amp sockets.

The 3-pin sockets were not shuttered when BS 546 was in common use domestically in the UK, but the current revision of the standard does specify shutters and BS 546 sockets sold in the UK today are shuttered in the same basic manner as BS 1363 ones. Current UK wiring regulations require socket outlets installed in homes to be shuttered. The three pin variety had the larger top pin as the earth connection, the left hand pin as Neutral and the Right hand pin as Live [1] (when looking at a socket or at the rear of a plug)

2 A 2 pin

BS 546 un-shuttered 5 A and 2 A 2 pin sockets

This plug did not find general use and is now rarely seen. It was rendered incompatible with the 2 A 3 pin variant by variation of the pin gauge and this may have contributed to its lack of adoption.

5 A 2 pin

This plug is what became the UK standard shaver plug (BS 4573) and is similar but slightly larger than the Europlug. British shaver sockets and adaptors tend to be sized to accept this, Europlugs and two pin American and Australian plugs.

2 A 3 pin

This plug was used to connect low power appliances to the lighting circuit (and to adaptors from the larger socket types). Sometimes still used to connect lamps (and other lights which aren't attached, or built-in to the fabric of the building - e.g. lights which are mounted on kitchen cabinets, bookshelves etc.) to the lighting circuit.

5 A 3 pin

D

This plug was used for moderate sized appliances either being on its own 5 A circuit or a multi socket 15 A circuit and also on many adaptors (both multi socket 5 A adaptors and adaptors that also had 15 A pins). Some 5 A 3 pin sockets also have 2 extra holes above the live and neutral holes to allow a 5 A 2 pin plug to be connected.

India has standardised on this plug as Indian Standard IA6A3, rated at 6A / 250V, and it is now almost exclusively used in India, Sri Lanka, Nepal and Namibia. However, this 5 A plug, along with its 2 A cousin, is sometimes used in the UK for centrally switched domestic lighting circuits, in order to distinguish them from normal power circuits. This is quite common in hotel rooms. This plug was also once used in theatrical installations for the same reasons as the 15 A model below, but is now only used for this purpose in schools and other rarely used installations (presumably because those operating multiple installations wanted to standardise on one plug type for all their lights and the 5 A plug was unsuitable for larger lights).

15 A 3 pin

M

This is the largest plug in domestic use. Live and neutral are spaced 1 inch apart, and earth is 1⅛ inches away from each of them. Although the preceding type is standard in Pakistan, India (Indian Standard IA16A3), Sri Lanka, Nepal, and Namibia, the 15 A version is also used in these countries for larger appliances. Some countries like South Africa use it as the main domestic plug and socket type as South African standard SABS 164. Sockets almost always have an on–off switch built into them. The Type M is almost universally used in the UK for indoor dimmable theatre and architectural lighting installations. It is also often used for non-dimmed but centrally controlled sockets within such installations. The main reason for doing this is that fused plugs, while convenient for domestic wiring (as they allow 32 A socket circuits to be used safely), are not convenient if the plugs and sockets are in hard to access locations (like lighting bars) or if using chains of extension leads (since it is hard to figure out which fuse has blown). Both of these situations are common in theatre wiring. This plug is also widely used in Israel, Malaysia and Singapore for air conditioners and washing machines.

30 A 3 pin

The 30 A plug is the largest of the family, being slightly larger than the 15 A plug. This was used for high power industrial equipment up to 7.2kW, such as industrial kitchen appliances, dimmer racks for stage lighting, etc. Plugs and sockets were usually of an industrial waterproof design with a screw locking ring on the plug to hold it in the socket against waterproof seals, and sockets often had a screw cap chained to them to be used when no plug was inserted to keep them waterproof. Use of the BS 546 30 A plugs and sockets diminished through the 1970s as they were replaced with BS 4343 (which later became IEC 60309) industrial combo plugs and sockets.