BS 546

BS 546 is a pre-Second World War^{[citation needed]} British Standard for domestic AC power plugs and sockets that is also the precursor of current Indian and South African plug standards. This plug is also used in Singapore to a limited extent. 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). The current edition of the standard was issued in 1950, with supplements since issue.

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 to be used for larger appliances such as electric heaters, while smaller appliances such as radios or 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. The current revision of the standard does specify shutters similar to those of BS 1363. 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).

The three pin 2 A, 5 A, and 15 A, connectors of BS 546 are duplicated by Group B1 of the GOST 7396 standard.

2 A 2 pin

This plug did not find general use and is now rarely seen. It was 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 became the UK standard shaver plug (BS 4573) and is similar but slightly larger than the Europlug (and the pins are only 16mm apart). 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 (and to adaptors from the larger socket types). It is sometimes still used to connect lamps to a lighting circuit.

5 A 3 pin

This plug was used for moderate sized appliances, either on its own 5 A circuit or on 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 two 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. 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.

15 A 3 pin

M

This is the largest plug in domestic use. Live and neutral are spaced 1 inch (25 mm) apart, and earth is 1+1⁄8 inches (29 mm) away from each of them. The 15 A version is also used in Pakistan, India, Sri Lanka, Nepal, and Namibia for larger appliances. In India it is rated at 16 A under Indian Standard IA16A3. 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. This type is almost universally used in the UK for indoor dimmable theatre and architectural lighting installations, and for centrally controlled sockets. Fused plugs 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 and is slightly larger than the 15 A plug. This was used for high power industrial equipment up to 7.2 kW, such as industrial kitchen appliances, or dimmer racks for stage lighting. 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.

Characteristics of BS 546 plugs

	type ? 30A	type M 15A	type D 5A	type ? 2A
Diameter L and N pins	7.92 mm	7.06 mm	5.08 mm	3.56 mm
Diameter earth pin	9.53 mm	8.71 mm	7.06 mm	5.08 mm
Centre distance between L and N pin	36.50 mm	25.40 mm	19.05 mm	14.48 mm

References

BS 546: "Specification. Two-pole and earthing-pin plugs, socket-outlets and socket-outlet adaptors" (1950)
BS 546:Supplement No. 1: "Specification for plugs made of resilient material " (1960)