BS 546

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

BS 546, "Two-pole and earthing-pin plugs, socket-outlets and socket-outlet adaptors for AC (50-60 Hz) circuits up to 250V" is a British Standard for three pin AC power plugs and sockets. Originally published in April 1934, it was updated by a 1950 edition which is still current, with eight amendments up to 1999. BS 546 is also the precursor of current Indian and South African plug standards. This plug is also used in Singapore and Hong Kong for some applications. Although BS 546 plugs and sockets are still permitted in the U.K. and are used for special purposes, new installations since 1947 generally have used BS 1363 sockets and fused plugs.

History

The 1934 edition of the standard, titled "Two-Pole and Earthing-Pin Plugs and Socket Outlets" superseded BS 372 "Side-Entry Wall Plugs and Sockets for Domestic Purposes" Part II, which was published in 1930. BS 546:1934 clause 2 specifies interchangeability with BS 372 Part II which includes the same four plug and socket sizes. BS 372 Part II had itself superseded BS 317 "Hand-Shield and Side Entry Pattern Three-Pin Wall Plugs and Sockets (Two Pin and Earth Type) published in 1928. BS 372 Part II states that there are only minor alterations from BS 317. (BS 372 Part I was a standard for two pin non-earthed plugs which were never included in BS 546 and which were incompatible due to different pin spacings.)

Also in 1934 the 10th Edition of the IEE’s “Regulations for the Electrical Equipment of Buildings” introduced the requirement for all sockets to have an earth contact.

Prior to BS 546, British Standards for domestic plugs and sockets included dimensional specifications for the socket contact tubes. In BS 546 there are no dimensions for socket contacts, instead they are required to make good contact with the specified plug pins.

The 1911 GEC Catalogue included several earthed sockets intended for industrial use, but one of the earliest references to an earthed plug and socket for domestic use is found in Maud Lucas Lancaster's 1914 book Electric cooking, heating, cleaning, etc where she mentions an earthed iron-clad plug and socket by the English firm of Reyrolle and Co. In 1926 GH Scholes Ltd (Wylex) introduced a three-pin plug. in three sizes, 5A, 10A and 15A with a round earth pin and rectangular live pins, a fused 13A version of this continued to be available after the introduction of BS 1363, illustrating that BS 546 was not used exclusively at any time.

Although still permitted by the UK wiring regulations, BS 546 sockets are no longer used for general purposes. Some of the varieties remain in use in other countries and in specialist applications such as stage lighting.

The sockets were not shuttered when BS 546 was in common use domestically in the UK. The current revision of the standard allows for shutters similar to those of BS 1363. Current UK wiring regulations require socket outlets installed in homes to be shuttered.

Applications

There are four ratings of plug and socket in BS 546, (2 amperes, 5A, 15A and 30A). Each has the same general appearance but they are different physical sizes to prevent interchangeability. Plugs fitted with BS 646 fuses have been optional since the original BS 546:1934 with maximum fuse ratings of 2A in the 2A plug, and 5A in the 5A, 15A and 30A plugs. In practice most BS546 plugs are unfused with fused versions being unusual and expensive.

The 15 Amp(A) sockets were generally given a dedicated 15 A circuit. Mulitiple 5 A sockets might be on a 15 A circuit, or on a dedicated 5 A circuit. Lighting circuits fused at 5 A were generally used to feed the 2 A sockets. Adaptors were available from 15 A down to 5 A and from 5 A down to 2 A so in practice it was 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 the protection offered by the BS 1363 fused plug.

In theory, 15 A plugs were to be used for large appliances such as electric heaters, while small appliances such as radios or table lamps were supposed to use 5 A. In practice this was 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 larger top pin is the earth connection, the left hand pin is neutral and the right hand pin as line when looking at a socket or at the rear of a plug.

2 Ampere

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 Ampere

5 Amp switched socket-outlet to BS 546

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). 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 Ampere

15 Amp switched socket-outlet to BS 546

This is the largest plug in domestic use. This type is commonly used in the UK for indoor dimmable theatre and architectural lighting installations. 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. 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 Ampere

The 30 A plug is the largest of the family. 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 three-pin plugs

current rating	30A	15A	5A	2A
Diameter, Line and Neutral pins	0.312 inches (7.9 mm)	0.278 inches (7.1 mm)	0.200 inches (5.1 mm)	0.140 inches (3.6 mm)
Length, Line and Neutral pins	1.125 inches (28.6 mm)	0.733 inches (18.6 mm)	0.585 inches (14.9 mm)	0.500 inches (12.7 mm)
Diameter, Earth pin	0.375 inches (9.5 mm)	0.343 inches (8.7 mm)	0.278 inches (7.1 mm)	0.200 inches (5.1 mm)
Length, Earth pin	1.437 inches (36.5 mm)	1.125 inches (28.6 mm)	0.812 inches (20.6 mm)	0.625 inches (15.9 mm)
Distance between L and N pin centres	1.437 inches (36.5 mm)	1.000 inch (25.4 mm)	0.750 inches (19.1 mm)	0.570 inches (14.5 mm)
Distance between E centre and centre line of L and N pins	1.562 inches (39.7 mm)	1.125 inches (28.6 mm)	0.875 inches (22.2 mm)	0.570 inches (14.5 mm)

Standards derived from BS 546

South African SANS 164

The South African standard SANS 164 Plug and socket-outlet systems for household and similar purposes for use in South Africa defines a number of derivatives of BS 546. The main household plug and socket is defined in SANS 164-1, and is essentially a modernised version of the 15 A BS 546 (the essential differences are that pins can be hollowed to reduce the amount of metal used, the dimensions are metricated, and it is rated 16A). SANS 164-3 defines a 6 A plug and socket based on the 5 A type BS 546.

SANS 164-4 defines three variants of the 16 A plug and socket intended for specialist (known as "dedicated") applications. The variants use a flattened earth pin, each at a different specified rotational position. This arrangement ensures that the dedicated plugs can all plug into an ordinary ("non-dedicated") socket, but that the various dedicated plug and socket combinations are not interchangeable (nor can a non-dedicated plug be inserted into a dedicated socket).

The dedicated versions have specific colours assigned to them, depending on the rotational position of the flattened portion. These are black (-53°), red (0°), and blue (+53°). The red (0°) version is by far the most common, and is widely used on computer and telecommunication equipment (although this is not required in the standard). In this application the "dedicated" socket refers to one that is not connected to a residual current circuit breaker, which is otherwise mandated for all normal power sockets.

Indian IS 1293

Indian standard IS 1293:2005 Plugs and Socket-Outlets of Rated Voltage up to and including 250 Volts and Rated Current up to and including 16 Amperes includes versions of the 5 A and 15 A BS 546 connectors, but they are rated at 6 A and 16 A respectively. These are also used in Pakistan, Sri Lanka and Nepal. Some 5 A 3 pin sockets also have two extra holes above the line and neutral holes to allow a 5 A 2 pin plug to be connected.

Russian GOST 7396

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

References

• BS 546 Supplement No. 1: "Specification for plugs made of resilient material" (1960)
• BS 546 amendements: PD 1752, PD 4389, AMD 251, AMD 2307, AMD 4045, AMD 5809, AMD 6144 and AMD 8914. AMD 5809 also includes Supplement No. 2 (1987) to BS 546:1950.