Access control · BS 7273-4
Fail-safe or fail-secure locks: what is the difference?
What fail-safe and fail-secure mean, which suits an escape route, and how BS 7273-4 categories A, B and C control doors that release in a fire.
The difference in one table
| Fail-safe | Fail-secure | |
|---|---|---|
| When the power is removed | The lock releases and the door can be opened | The lock stays locked |
| Usually | Magnetic locks | Electric strikes |
| Often used on | Escape routes and stairwells | Server rooms and stores |
| The trade-off | Open in a power cut, so the room is not secure | Locked in a power cut, so people need another way out |
The rule that matters most is simple: people must always be able to leave. On an escape route that decides which type you can use, and the fire risk assessment has the final say.
Doors that release when the fire alarm sounds
BS 7273-4 sets how a fire alarm releases doors held shut by locks. It has three categories, which say how the release behaves when something goes wrong:
| Category | What it means |
|---|---|
| A (critical) | Fail-safe for any fault that affects the ability to release in a fire, including any fault on the critical signal path and a power failure |
| B (standard) | Fail-safe for any fault on the critical signal path and for a power failure |
| C (indirect) | Fail-safe for any fault on the critical signal path up to the interface with the access controller, where the signal goes through other equipment |
How quickly must the door release?
- Category A: within 2 minutes of the fire system losing power. Radio is allowed if it releases within 6 minutes of a radio communication failure. Release triggered by sound is not suitable.
- Categories B and C: radio is permitted with release within 120 minutes, and release triggered by sound might be suitable.
This is from BS 7273-4:2015+A2:2023, summarised from fire and rescue service advice and FIA guidance. Check the standard for a job.
Getting the power right
A lock that does not get enough voltage may not work as it should. Long cable runs lose voltage, and a lock needs to see close to its rated voltage. The lock cable calculator checks the voltage at the lock, and the door power supply calculator sizes the supply. Manufacturers state their own tolerances, commonly 5% or 10%, so check the lock's data sheet.
Common questions
What is a fail-safe lock?
A lock that releases when the power is removed, so the door can be opened. Magnetic locks are the usual example.
What is a fail-secure lock?
A lock that stays locked when the power is removed. Electric strikes are the usual example. People need another way out.
Which lock is used on a fire escape route?
People must always be able to leave, which decides what can be used on an escape route. Fail-safe locks release in a power cut. The fire risk assessment has the final say.
What are BS 7273-4 categories A, B and C?
They set how a fire alarm releases doors held shut by locks. Category A is fail-safe for any fault that affects the release. B is fail-safe for a fault on the critical signal path and for power failure. C is fail-safe up to the interface with the access controller.
This guide is general information for UK fire & security companies, not legal or engineering advice. Always work to the current edition of the relevant standard and your own competent-person judgement.
See this on a real certificateKerova builds the register, readings and verdict into the certificate itself.
Book a Demo