page_title: Bayonet mount
section_title: Light bulbs
hierarchical_section_title: Bayonet mount / Light bulbs
caption_attribution_description: English: A blown 40w incandescent bulb with a B15d base.
context_page_description: A bayonet mount or bayonet connector is a fastening mechanism consisting of a cylindrical male side with one or more radial pins, and a female receptor with matching L-shaped slot and with spring to keep the two parts locked together. The slots are shaped like a capital letter L with serif; the pin slides into the vertical arm of the L, rotates across the horizontal arm, then is pushed slightly upwards into the short vertical "serif" by the spring; the connector is no longer free to rotate unless pushed down against the spring until the pin is out of the "serif".
  The bayonet mount is the standard light bulb fitting in the United Kingdom and in many countries that were members of the British Empire including Pakistan, Australia, Hong Kong, Fiji India, Sri Lanka, Ireland, and New Zealand, parts of the Middle East and Africa and, historically, France and Greece.
context_section_description: 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, Hong Kong, 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 or B22. Older installations in some other countries, including France and Greece use this base. First developed by St. George Lane Fox-Pitt in the UK and improved upon by the Brush Electric Company from the late 1870s onward, 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. Older railway carriages in the UK also made use of a 3 pin bulb base to discourage theft. 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 tail light and a brake 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 without complication.  Some special-purpose bulbs, such as infra-red, 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 BA, often with a number after. The number refers to the diameter of the base (e.g., BA22 is a 22 mm diameter bayonet cap lamp). BA15, 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. The entries from the table below pertain to IEC 60061 "Lamp caps and holders together with gauges for the control of interchangeability and safety" and to DIN 49xxx.
  See Replaces list under Amendments section
  On June 1996, all were withdrawn and replaced by DIN EN 60061-1
  
  These are the available sizes in the UK:
  Of these, only the BC (BA22d, often abbreviated as B22) 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.
caption: Incandescent 40 W BA15d bulb
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