L3 Category - Detector locations

[David Rooney]

8.2d of 5839 explains the circumstances where detectors need not be installed in rooms opening onto corridors of less than 4 metres in length.

We are dealing with an old converted Georgian Terraced block (in Islington), basically six houses knocked through at each storey level to form a small office block, three storeys high. So there are multiple staircases, and generally routes through to allow good means of escape in all directions.

Being an old conversion, there are no lobbies formed at each staircase landing. Just as in most houses the rooms (now offices) open directly off the staircase.

All walls and doors surrounding each staircase are of fire rated construction, and each starcase will have AFD at each main landing.

The question then, is if you are allowed not to install AFD in rooms off short corridors, could this be stretched to not installing AFD in rooms opening directly onto the enclosed staircase?

Answers on a postcard please....

[Chris Houston]

No.

[David Rooney]

Short, sweet and to the point thankyou!

What about where there is a lobby (could be conceived as a short corridor) between office and the staircase?

Presumably the staircase and corridor should have AFD but the offices do not need protection?

[messy]

Wouldn't constructing multiple lobbies be more expensive than L3 AFD?

[David Rooney]

Sorry I think you mis-understood. There will be no construction. It's an existing building. in some areas there are "lobbies" formed between staircase landings and office doors, and in other areas the office door opens directly on to the staircase.

I suppose the question really is that if it can be justified not to protect a room opening on to the short corridor (in this case the lobby that has AFD) then why can't it be justifiable to not protect the office opening directly on to the staircase (which also has AFD)?

Blimey was that too many nots....

[Graeme]

8.2d of 5839 explains the circumstances where detectors need not be installed in rooms opening onto corridors of less than 4 metres in length.

Provided that that fire resisting construction and fire doors seperates the short corridor from any other escape route.

[David Rooney]

Yep got that - didn't want to bore you with the complete definition..... But why is it ok not to protect the room opening onto the protected corridor, while having to protect the room opening onto the protected staircase ??

[Graeme]

is the staircase the common escape route for other offices?.If so, being enclosed or not you would not want smoke to get a chance to build up in them from an unprotected office which opens on to it.
A short corridor which is enclosed with fr doors etc with afd is protecting the main escape route it joins onto.

[Allen Higginson]

The way I read it is that you would put detection in the lobby and not the room as the room itself is not leading directly out into the stairwell.Remember, the aim is to protect the escape route and the object of detection in the rooms leading on to the route is the door - you could mount the detector above the door frame (not generally recommended tho!!).
If you don't mind me asking - why are you going for what is the bare minimum in detection terms (although you could have went L4 if no one is,for example ,was sleeping in accomodation on the premises).Surely if you are spending so much on the conversion then a P1/L1 system should have been considered?

[David Rooney]

The building was converted by the owners a while ago, and it appears that (as normal) the fire alarm system was literally the last thing on their minds.

The fire officer has paid a visit and has stipulated the L3 Category.

Yes the staircase is common, but then so is the lobby a common entrance to two and sometimes three offices.

I understand the points made regarding preventing the build up of smoke within the escape route, in this case the lobby or the staircase. And in this case the lobby and the staircase both have AFD.

So why protect the room off the staircase, and not the rooms off the lobby??

[Graeme]

an L3 is to give all the occupants other than the ones in the room on fire to escape safely in escape routes that do not have a build up of smoke in them.
If you do not have detection in a room opening onto a escape route and there is a fire in that room,when the door eventually burns through the afd in the escape route will pick it up but by than it's too late.
An L4 is no good if there is only one escape route and the lobby  in an L3 needs to be secured by fire doors and walls etc to warrant not having any detection in the rooms that open into it.
I think i know what you are getting at though. Why have a short corridor with afd and fr doors etc with say three rooms that open on to it that don't have afd.If there is a fire in one room then the other two rooms need to be evacuated before the smoke gets in the short corridor?
is that what you mean?

[David Rooney]

Exactly that Graeme. - I wish I could have said that in the first instance !!

And that is my point. Why is there a difference between the staircase and the short corridor. Both are escape routes for various offices in one way shape or form. Both have AFD and as you say, if the smoke reaches the detector then it may well be too late to use the escape.

I think the "old" definition of every room opening onto an escape being protected was clearer and ultimately safer.

If you take it to the extreme, in small office blocks where "protected" corridors are divided into 4 metre sections then potentially you could have no detection in any of the offices and still meet the L3 definition. Even though, such a system would on face value be L4.

It's a strange world !

[Allen Higginson]

Strange indeed!! There are a few grey areas that are open for interpretation and it depends who is interpreting them!
I still have an issue on why the break glass in the stairwell must be on the point of origin - if someone comes out,say,on the 6th floor after spotting a fire then sudenly remembers to hit the BGU on the 4th the control panel says that the fire is reported on the wrong floor.I know the way round it is to locate the unit inside the corridor before going out on to the stairs but .....I'll get me coat!!!

[Graeme]

think that is for phased evacuation Mr H

[colin todd]

You are making this way too complicated guys. Old Buzzy, as always, has more or less said it. The purpose of the L3 system is to protect the escape routes. Thus it is intended to give a warning to people beyond the room of origin before the escape route (corridor or staircase) is affected by fire. The L3 system was born out of research carried out on behalf of the then Home Office in the 1980s. The research showed that, where there was a long corridor, the corridor could, under some very special circumstances,  be smoke logged before detectors in the corridor, mounted 15m apart, operated. Thus, it was decided that, to provide proper escape route protection, you would need detectors in rooms adjacent to the escape route. Then in 2002, a minor relaxation was created (funnily enough specifically for Georgian house conversions and similar, as it had first been introduced into BS 5839-6 for HMOs of that type in 1995). The point is that, if you have a small lobby of the type you describe, with rooms off, there is really no chance that the occupants of the room cannot escape before the detector in the lobby operates. The lobby is so small the detector is virutally outside the door of the room of fire origin. So, you do not need the detectors in these rooms, and the stiarcase itself will be protected by the additional FRSC door. On the other hand, it is a bit late to warn people when the detector in the staircase operates, so for L3 you need the detectors in the rooms opening onto the staircase. Perhaps your question is more really do you need an L3 system in the first place, which is, of course, another question entirely.