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Query on evacuation and when to call 999


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We manage an office block of 6 floors - some of the floor levels are unoccupied due to COVID.  Building strategy is full / simultaneous evacuation (approx. 300 people).

At present, the fire alarm is a single knock system - would it be acceptable (subject to risk assessment) to change the system, or just the system on unoccupied floors, to a double-knock style? 

We are worried that faulty detectors in unoccupied levels may result in the whole building evacuating unnecessarily.  The idea is that if a double knock system is introduced, if the alarm panel shows a detection in unoccupied areas, the site security / reception can investigate, and then after 5-6 mins, the alarm goes into full evacuation mode if not switched off?

Also, generally, if the system remains as a 'single knock' and the fire alarm activates as a result of a single detector in the unoccupied areas, but no signs of fire / smoke, should we still ring 999 or should we investigate first??  My worry is that the rest of the building will be evacuating down using the stairs, and yet if the security / reception are told to investigate, they will be going up the stairs, in the opposite direction of flow!  

Are there any particular rules / guidance on whether a single knock or double knock system is needed please?

Sorry if these are silly / overthought questions.

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By double knock I assume you mean coincidence detection where a second device initiates evacuation, double knock is an often misused term that actually refers to the same detector signalling twice.

Unoccupied floors are less likely to false alarm - assuming the system is being correctly maintained - so that wouldn't be an adequate excuse to change the cause & effect.

However many office buildings do use a staged alarm in a safe and compliant way - usually a smoke detector initiates an investigation stage [Stage 1] (usually silent but in some set ups in the past it would have sounded an intermittent alarm to 'stand by') which has a timer that will switch to Stage 2 (evacuate) if left and is also overridden into Stage 2 by any other device activating; and other devices such as heat detectors, sprinkler interfaces and call points (all more likely to be the real thing) going straight to Stage 2.

Traditionally you would call the fire service upon alarm, these days some (not all) fire services will not attend unless there has been an investigation first and fire (or grounds for suspicion other than just the fire alarm system) confirmed - this is particularly the case if the call comes from a remote Alarm Receiving Centre rather than the site direct.


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Yes, apologies for incorrect terminology.  I meant that 2 detectors need to detect the smoke.  

The problem we have with the investigation period is that for periods of the day, there is no reception or security staff.  So the worry is, if the fire alarm for the building activates, and it is discovered that the cause is from an unoccupied floor level, who is the one to investigate??

The tenants are each made to have their own Fire Wardens to check their respective floor areas, and this includes the communal parts on their floors such as toilets.  But we have 2 floors completely empty at the moment, so if a detector creates a fire alarm activation on these floors, and there is no apparant sign of smoke / flames, who should investigate - we wouldn't expect the tenants on the other floors to go into the unoccupied floor levels to investigate (or should we?!).

Thanks again AnthonyB for your help.

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