What is an interface unit?
An interface unit connects the fire alarm system to other building plant, so that an alarm can release door holders, recall lifts, shut down ventilation or trigger smoke control.
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<img src="https://fireoptix.co.uk/glossary/interface-unit/diagram.svg" alt="interface unit — diagram from FireOptix" width="880" height="620" loading="lazy">
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<a href="https://fireoptix.co.uk/glossary/interface-unit">What is an interface unit?</a> — diagram by
<a href="https://fireoptix.co.uk">FireOptix</a>
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</figure>It is how the alarm stops being only a noise. The actions that actually protect people — closing doors that were held open, stopping air handling that would move smoke — happen through interfaces.
They are the part of the system most likely to be forgotten in testing, because proving one means operating the plant it controls rather than listening for a sounder. A cause-and-effect test is the exercise that proves them.
They are also where alterations bite. Plant replaced under a separate contract may not be reconnected to the alarm, and nothing on the panel says so.
In practice
An alarm that does not release the door hold-opens.
The alarm works, the doors work, and the interface between them failed quietly some time ago.
Interfaces to dampers, hold-opens, lifts, shutters and ventilation are the parts of a system that are tested least and fail silently.
The weekly test exercises the sounders and proves nothing about any of them.
What goes wrong in practice
- Cause and effect never tested, so interfaces are assumed rather than proven.
- Plant replaced without the interface reinstated.
- Door hold-open devices interfaced but never tested with the alarm.