Bayonet Mount - Light Bulb Bayonet Mounts

Light Bulb Bayonet Mounts

See also: Automotive lamp types For other types of lightbulb mounts and sockets, see Lightbulb sockets.

The bayonet light bulb mount is the standard fitting in many former members of the British Empire including the United Kingdom, Australia, India, Ireland, and New Zealand, as well as parts of the Middle East and Africa (although not Canada, which primarily uses Edison screw sockets along with the United States and Mexico). The standard size is B22d-2, often referred to in the context of lighting as simply BC. Older installations in some other countries, including France and Greece use this base. Standard bulbs have two pins on opposite sides of the cap; however, some specialized bulbs have three pins (cap designation B22d-3) to prevent use in domestic light fittings. Examples of three-pin bulbs are found in mercury street lamps and fireglow bulbs in some older models of electric radiative heater. Bayonet cap bulbs are also very common worldwide in applications where vibration may loosen screw-mount bulbs, such as automotive lighting and other small indicators, and in many flashlights. In many other countries the Edison screw (E) base is used for lighting.

Some bulbs may have slightly offset lugs to ensure they can be only inserted in one orientation, for example the 1157 automobile tail-light which has two different filaments to act as both a running light and a signal light. In this bulb each filament has a different brightness and is connected to a separate contact on the bottom of the base; the two contacts are symmetrically positioned about the axis of the base, but the pins are offset so that the bulb can only be fitted in the correct orientation. Newer bulbs use a wedge base which can be inserted either way with no issues. Some special-purpose bulbs, such as infrared, have 3 pins 120 degrees apart to prevent them being used in any but the intended socket.

Bayonet bases or caps are often abbreviated to BC, often with a number after. The number refers to the diameter of the base (e.g., BC22 is a 22 mm diameter bayonet cap lamp). BC15, a 15 mm base, can also be referred to as SBC standing for small bayonet cap. The lower-case letter s or d specifies whether the bulb has single or double contacts.

Type	IEC	DIN	ANSI
B15d	IEC 60061-1 (7004-11)	DIN 49721
BA15d	IEC 7004-11 A	DIN 49720
BA15s	IEC 7004-11 A	DIN 49720
BA20d	IEC 7004-12	DIN 49730
B21s-4
B22d	IEC 60061-1 (7004-10)
BY22d
B24s-3
GU10	IEC 60061-1 (7004-121)
GZ10	IEC 60061-1 (7004-120)
GU24			Pending (Mar 2007)

While G actually indicates bi-pin, those listed above have a twist-lock, but with parallel pins from the end instead of opposing pins on the side.

These are the available sizes in the UK:

Designation	Alternative designation	Contacts	Dimension, etc.
BA5s		1	5 mm
BA7s		1	7 mm, elongated pins
BAX9s		1	9 mm, radially offset pins
BA9s	Miniature bayonet cap (MBC)	1	9 mm
BA15d	Small bayonet cap (SBC)	2	15 mm
BAY15d (P21/5W, 1157)		2	15 mm, axially offset pins
BAX15s		1	15 mm, radially offset pins
BA15s (P21W/1156/R5W/R10W)	Single centre contact (SCC)	1	15 mm
BA20s		1	20 mm
BA20d		2	20 mm
BA21d		2	21 mm
B21-4			21 mm 4 pin
BA22d	Bayonet cap (BC)	2	22 mm
BC-3	Bayonet cap (BC)	2	22 mm 3 bayonet pins
B22d-3		2	22 mm double ended (railway)
BX22d		2	22 mm

Of these, only the BC (BA22d) is widely used in homes. The BA20d (sometimes called a Bosch fitting) was once a common automotive (twin filament) headlamp fitting but has largely been superseded by more modern, higher-rated H-series sockets and is only used for some lower-powered applications such as combined automotive tail and stop lamps.