BS 546 is an old British Standard for domestic AC power plugs and sockets. 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).
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.
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.
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.
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 and 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).