Extinguishing time delays … probability and impact
BS6266 explains that “The extensive use of fire protection
systems in electronic installations arises not from a high probability of fire,
nor from a significant hazard to life, but from the consequences of fire loss.”
For this reason many, rightfully, implement an automatic fire
However, a focus on keeping the (pre-discharge) time delay short may prevent recognising when more time is essential to achieve the primary objective of safeguarding business: availability and property.
We propose this needs to be longer in specific instances and
example why and perhaps how best to achieve it.
BS 7273-1 clause 7.3.1 tells us ‘A time delay facility .. to allow personnel to evacuate … will depend upon the potential speed of fire spread and the means of escape from the protected space, it should be as short as possible and should not exceed 30 s unless a longer period is agreed with the stakeholders’, and carries a cautionary note that ‘protracted delays will allow for the further development of the fire and …’. Readers will naturally infer good reason to never exceed 30 seconds. Nonetheless, there are many instances you should, but getting stakeholder (understanding leading to) agreement is the challenge.
EN12094 specifies the timing capability of extinguishing control panels and local codes and authorities have their own interpretations.
This typically originates from historic experiences – good and bad. Notwithstanding, we must also appreciate how applications and expectations have changed and evolved.
A hold button and/or pre-discharge
delay is thus to allow:
- Escape of persons within the area
- Cause & effect (e.g. dampers, doors, pressure
- Intervention to avoid an unwanted extinguishing
Past focus was perhaps on the first two points, although nowadays
the third is more the reason look for a longer delay.
Computer rooms and data centres have grown from small cupboards/closets to large rooms and halls to a scale which makes football pitches look tiny. Escape times may thus need to be longer, yet such data facilities will likely have minimal staffing, employ access control and the ‘onion skin’ of security layering which, whilst serving the purpose of making penetration problematic, also impedes emergency response.
For instance, a large DC has the catastrophic failure of a piece of equipment which spontaneously creates a cloud of effluent the detection system regards as a fire. This could be an exploding – yet self-extinguishing – PDU or something like a failed refrigerant pipe in a CRAC air handling unit. Either way a large quantity of airborne ‘effluent’ is likely to trigger first stage alarm and give coincidence to another detector – triggering second stage alarm – almost instantaneously.
The <30 second countdown thus commences.
First and foremost – does the business’ fire strategy / policy allow staff to go towards the ‘fire’? Likelihood is it may on first stage alarm (the first detector operating) but on second stage alarm (coincidence detection) it probably will not; the likely instruction is the extinguishing release is imminent so all personnel should evacuate.
- If policy does allow them to ‘go and look’ / investigate, can your business scramble appropriate personnel in time to properly respond?
- Can they get through (or byapss) all security measures, man-traps, biometrics, etc, in time?
I suspect not.
Once outside the protected space, if they see an event
unfolding they can manually trigger the system; most panels or system have a
means of manual release at the point of entrance or on/near the panel. This does raise the question that if a
responder sees flames through the window, would they feel authorised – and ‘have
the bottle’ – to operate the system themselves, but that’s perhaps a subject
for a different discussion.
So, back to our case, we have a false alarm which would initiate an unwanted operation (discharge) of the extinguishing system, unless someone can intervene.
Here, sensibly, they can intervene. If the fire is known to be in a protected space, they can logically go towards that. If the point of entrance allows them a means to assess if it is safe – e.g. glazed section in the door or remote CCTV – then they could also be allowed to enter.
We thus need a methodology set with enough time for them to do this – taking suitable assessment of risk and their own safety along the way.
Once through the door, personnel can immediately assess the likelihood of a false or unwanted alarm. Sight and smell at that point, at the entrance, make humans exceptionally good fire detectors.
As our [hypothetical] unwanted event requires someone to enter the protected space and operate a hold off button (as BS 7273-1 clause 7.1.4 which says “The device should be located near to the exit from the protected space”), a ‘buddy’ system provides the safest methodology– with one at the button and the second presumably on “seek and search”.
If it transpires there is a need for deploying the
extinguishant, releasing the hold button commences the countdown timer again.
The paradox here is you then want a short timer, as you now know you want the extinguishant releases asap; early deployment will prevent (during an incipient event) or limit the damage of a flaming fire.
Perhaps the answer is an additional timer in the confirmation of the coincidence detection, so the pre-discharge delay timer could be kept short once again.
Businesses need to challenge themselves and honestly
evaluate the means and time required to attend an unfolding event.
- Who can they count on to make the assessments
and respond accordingly?
- How long is this likely to take?
Does this also raise the question on whether it is worthwhile also putting an additional hold off button outside the protected space, i.e. before entry? Possibly so, if access control / restrictions conflict with this scenario.
“Can you scramble appropriate personnel and get them there in time? I suspect not.”
Coincidence is to provide verification of an unfolding event, and also to avoid unwanted fire protection system releases. Normally there is a naturally occurring time delay between incoming (1st and 2nd) alarms.
Where fire growth would be expected
to be slow – yet there is potential for a [near] instantaneous coincidence
operation – there is good reason to make simultaneous coincidence introduce further
delay prior to triggering the second stage of the extinguishing system.
Todays’ EN12094 extinguishing
control panels may not readily be able to execute this logic, and the Standard
may not facilitate this either, so they would need to have a longer (i.e. probably
>30s) pre-discharge delay. The paradox
returns with the downside that after hold button operation, the required extinguishing
release is similarly delayed.
Where an intelligent fire alarm system communicates to an extinguishing control panel, logic can however be programmed into this, to afford time from the coincident event being recognised until initiating of the second stage at the extinguishing control panel. This provides time to respond, keeps the audible alarms on 1st stage (to make those responding certain of the condition), but when responders are attending – if a fire condition is recognised – they can immediately operate the means of release on/at the panel and thus the delay time needs only be kept as short as possible for escape of persons inside the protected space and necessary cause and effect (such as dampers and pressure relief).
The probability of a fire – as BS6266 implies – is perhaps remote. The likelihood of an unwanted alarm can be greater. Nevertheless, the consequences of the system operating is only acceptable when there is a fire.
I firmly believe there are applications where the is good reason to need to change the timing of extinguishing systems. In those cases – where slow fire growth is expected and there are impediments to a fast attendance – protraction of a kind is essential. I hope the above provides reasoning to help you explore this with your stakeholders.
For more www.siemens.co.uk/firesafety