Siting Of Optical Beam Detectors

[Wiz]

Firstly a message to Kurnal & Graeme Millar - welcome to my 'house-warming' drink in this my new room in The Home For The Slightly Bewildered. Lucky has heard that a Meccano set is being used somewhere, so I suggest we keep the noise down.

Now for my question. I recently commissioned a fire detection and alarm system containing optical beam smoke detectors. These had been installed at a distance of some 3 metres below the ceiling, but spaced as per the requirements of of BS5839 part 1 2002 22.5. d) for supplementary detection. I pointed out that there was still a requirement for detection within 600mm of the ceiling, but my customer said this was impossible to achieve since there were structual beams in the way. Furthermore, whilst there was actually a 'gap' of 450mm between the ceiling and the beams, the technical specification of the beams stated they must be mounted at least 500mm below the ceiling.

Has anyone any comments or advice?

[lucky]

AW I wanna play.......

[Wiz]

Hi Lucky. How did you manage to find us so quickly? I paid Matron a fair slice of my monthly allowance for a new room and it took you all of 15 minutes  to track me down! OK you can join in as long as you don't break the Meccano set.

[kurnal]

Hi Wiz

When you say the system was for supplementary detection, what was the reason for this? stratification issues?

What was the environment- warehouse, sports centre, shopping centre etc? Theres a lot of potential factors including whereabouts the fire loading is- is there any fire risk above the beam detection, what type of fire are we planning to have, what is the height and shape of the roof and ceiling and the temperature under normal conditions at ceiling height?

If the system was installed as part of an L1  system for life safety then all areas should bave been covered with detection as you correctly state. But more than likely in this case it was probably part of an engineered solution in which some smoke calculations were done to show it unnecessary,  or indeed the beam system may have been installed for other reasons - for property protection or business continuity or to address a specific life safety problem.

In a large,  high building I think I would prefer to rely on a beam system than point detectors at the limit of their operating range. And certainly would if I were also maintaining them!

Anyway lucky and I are just going to have a test flight in a helicopter he has made from the meccano set. We may be able to escape tonight.  We are powering it using elastic hairbands stolen from Matrons boudoir.
If we dont come back dont risk looking for us. We may be gone some time.

[lucky]

Thanks wiz I will be carefull,I slipped me leash and Matron cant find me at the mo,this is very interesting,Kurnal start up the jelly copter..weeeeeeee.

[lucky]

Look out wiz were coming in low,woo,woo........is that matron down there,take it up Kurnal mind the light...........

[Wiz]

Hi Wiz

When you say the system was for supplementary detection, what was the reason for this? stratification issues?

What was the environment- warehouse, sports centre, shopping centre etc? Theres a lot of potential factors including whereabouts the fire loading is- is there any fire risk above the beam detection, what type of fire are we planning to have, what is the height and shape of the roof and ceiling and the temperature under normal conditions at ceiling height?

If the system was installed as part of an L1  system for life safety then all areas should bave been covered with detection as you correctly state. But more than likely in this case it was probably part of an engineered solution in which some smoke calculations were done to show it unnecessary,  or indeed the beam system may have been installed for other reasons - for property protection or business continuity or to address a specific life safety problem.

In a large,  high building I think I would prefer to rely on a beam system than point detectors at the limit of their operating range. And certainly would if I were also maintaining them!

Anyway lucky and I are just going to have a test flight in a helicopter he has made from the meccano set. We may be able to escape tonight.  We are powering it using elastic hairbands stolen from Matrons boudoir.
If we dont come back dont risk looking for us. We may be gone some time.

Thanks for your input on this matter Kurnal.

I never said the detectors were installed for supplementary detection, only that the spacing of the beams calculated from floor level meets the recommendations in BS 5839 Part 1 2002 for optical beam smoke detectors as supplementary detection. However the detection beams are sited some 3m below ceiling level because structural beams extend down to about 1.5m.

The area where the detection beams are installed is a school entrance foyer of about 25m x 18m x 12m high. About a quarter of that area is  a 'balcony' containing a walkway for first floor rooms. Many corridors from other parts of the school lead into this foyer area both on the ground and first floor levels. The ceiling is flat and other than light fittings there is nothing of any sort of fire risk. On the ground floor level there are quite a few obstructions such as display cabinets etc.

The system was designed as part of an L2 system by a consultant working for the local authority. The designer has not 'revealed' any speciifc fire engineering principles in his design for this foyer area only that he didn't think point detectors would be easily accessible for maintenance purposes, so he decided on detection beams and had to mount them well below the ceiling level due to the structural beams. Also he didn't mount them on the other axis because of windows on one wall and the possibility of 'tampering' from persons on the 'balcony' walkway.

Any views?

p.s. Since when did you become such good friends with Lucky? Maybe you have always been working with him undermining my own plans to escape this hellish place. I invite you for a 'house-warming' drink in my new room and before G.M. and I realise  what is happening, you and Lucky use up all the Meccano bits and leave us with nothing to create our planned test rig to carry out our experiments to disprove Kirchoffs First Law and so release the hairy man with the flat cap and whippet, from his intricate calculations.

[lucky]

were going down....look out....crash.....clang,aw sorry wiz I was never very good at flying...anyone wanna buy some slightly damaged meccano............

[lucky]

Kurnal....are you ok......Kurnal speak to me...Im off here comes matron...............

[lucky]

Sorry Matron I crashed the jelly copter when the optical beam blinded me,I think the Kurnal is injured.........

[Graeme]

Firstly a message to Kurnal & Graeme Millar - welcome to my 'house-warming' drink in this my new room in The Home For The Slightly Bewildered. Lucky has heard that a Meccano set is being used somewhere, so I suggest we keep the noise down.

Now for my question. I recently commissioned a fire detection and alarm system containing optical beam smoke detectors. These had been installed at a distance of some 3 metres below the ceiling, but spaced as per the requirements of of BS5839 part 1 2002 22.5. d) for supplementary detection. I pointed out that there was still a requirement for detection within 600mm of the ceiling, but my customer said this was impossible to achieve since there were structual beams in the way. Furthermore, whilst there was actually a 'gap' of 450mm between the ceiling and the beams, the technical specification of the beams stated they must be mounted at least 500mm below the ceiling.

Has anyone any comments or advice?

If they have been spaced as per 22.5 d then this means that they are supplementary to the ones that are within 600mm of the ceiling.

You have to start your detection at the top and work down.The calculations are meant to try and catch the stratification  or the cone of smoke going up hence the very small gaps between detectors.
This works out as 10% of the height the units are from the floor.
It does not mean that you can omit the ones on the ceiling and have them all lower down.

The smoke will end up at ceiling level once the room temperature rises.

[Graeme]

Wiz

You spec would say 500mm away from a wall as per a normal smoke detector.

[kurnal]

Thanks for the extra info Wiz.
Your beam system looks like a bit of a compromise.
The school sounds like a typical Government Building Bulletin design of the 60s or 70s and fire safety was well down the agenda. School buildings were exempt from building Regulations.  As built these places tended to have extensive open plan areas, dead ends, and no fire stopping or cavity barriers anywhere above false ceiling level or within the wall cavities.  Oh and mains powered fire alarms/ lesson change systems with no battery backup. I have done a few jobs in these myself- in fact I shall be doing a risk assessment in one tomorrow.....
Usually the level of risk in a foyer area is fairly low.

All you can really hope to do is leave it safer than you find it. You cant possibly address all the issues involved. And it sounds like this is what you have come across. Your beam system does not conform. As you know if the designer was planning an  L2 system it should have had detection toeffectively cover all parts of the escape routes and rooms leading onto escape routes  and as GM says in between the sips of his lucozade the supplementary detection and spacing is usually  additional  to make up for problems such as stratification- if the main predicted smoke layer is unlikely to make it up to the ceiling. But an L2 system it clearly isn't unless calcs have been done or a justification made .

But if the original design spec was for a manual system only, bearing in mind that a beam detection system will probably give better performance than point detectors at that height where the fire loading is fairly low and the predicted rate of growth slow,  I would accept that it sounds like a reasonable way of improving the level of safety in the building, giving early warning of a fire to enable intervention before a fire becomes out of control, for property protection or perhaps compensation for a design shortcoming elsewhere- if the corridors are not  separated from the common area and from each other? .

We didn't make freedom. Lucky he is not. The fundamental error was not takng theMatrons  hair bands for the helicopter out of her hair before using them. She now has a mohican/ brazilian look to her. So we are stuck here for a while yet. Lucky now wants to dig a hole. He is making a vaulting horse with the remaining lego. Hopefully it will cave on him.

[Wiz]

Thanks for all your input guys. However, at times, I think that our focus on escape plans from this place, is causing some to ignore/misunderstand what I had previously written. Many of the items highlighted in answers were not mentioned by me as problems. also I didn't design or install the system, only commission it, when I spotted a variation from the cop. I was wondering if anyone could think of a 'legitimate' reason for installing the detection beam in the manner it has been. I think Kurnal has a point when he infers that 'anything is better than nothing' in many circumstances, but what I have found doesn't meet the cop and without an Agreed Variation I maintain that this must be listed as 'failure to comply'. I think the designer read the BS and thought the 'supplementary detection' recommendations applied to any beam detectors that couldn't be installed within 600mm. I believe you all agree that this is not the case. I also believe that the designer has done his work without any input from others. I am going to suggest that he contacts the relevant people now and see if he can get them all to 'retrospectively' agree this variation. Based on Kurnal's opinions they are likely to agree that what has been provided is probably the best that could be done in the circumstances.

I would also confirm that the instructions supplied with the beam insisted on it being installed at least 500mm from the ceiling ( I have spoken to the manufacturers and they say this is to prevent too much of the 'cone' of the beam being 'lost' by being so close to the ceiling). With the BS requirement of being within the top 600mm of the ceiling, this leaves a 'window' of just 100mm! I feel this is something we should all be aware of when considering the use of optical beam smoke detectors in system designs.

I notice that Lucky has once again survived a potentially fatal situation. I notice he left Kurnal behind even though he seemed to be injured.  Lucky is obviously brave, resourceful and totally without remorse. He may not know much about fire alarms, or even about anything worthwhile whatsoever (except dinosaurs), but he is obviously a survivor. So, Lucky and I are now planning an expedition to try and get a close look at Matron's new Brazillian that was previously mentioned. It may not be a pretty sight and the atmosphere could be pretty poor, but Lucky tells me he is fully trained in the use of Breathing Apparatus.

If we ever return, we'll buy you all a drink in the mess and recount our experiences.

Tally Ho!

[Graeme]

and as GM says in between the sips of his lucozade

Coke Zero tonight.