Bayonet Mount - Light Bulb Bayonet Mounts

Light Bulb Bayonet Mounts

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.

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:

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.