Line units


A Line Unit is the term given to define devices used for functions other than fire detection, which are connected to a fire detection and alarm system transmission path or within other components of the fire detection and alarm system e.g. inside Control and Indicating Equipment. For example, the following devices may be regarded as Line Units:

  • short circuit isolators
  • input/output interfaces


Short-circuit isolators are used to limit the consequences of low parallel resistance faults between the lines of a transmission path of a fire detection and fire alarm system. The transmission path is normally in a loop configuration which is separated into sections by the short circuit isolators, such that a detected fault in a particular section will isolate that particular section allowing other components in other sections on the loop to function correctly.

Input/output devices are intended for different applications, and may therefore have different functions. Their functions are specified by the manufacturer and they are tested and approved against this specification.

LPCB offers certification of Line units to:-

  • EN 54-17: 2005 Short-circuit isolators
  • EN 54-18: 2005 Input/output devices
  • EN 54-25: 2008 Components using radio links

The initial approval and continued approval processes for Line Units are outlined in scheme document SD038.

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