Detector siting

[col10]

In BS 5395 (22.2c):
 "Other than in Category L4, L5 and P2 systems, if any flue-like structure, open
stairway, shaft for a lift, escalator or hoist, or any enclosed chute, penetrates
one or more ceilings, a fire detector should be sited at the top of the shaft or
enclosure and, on each level in the accommodation area, within approximately
1.5 m of the penetration."
Does that mean within 1.5m of all parts of the penetration or just one detector within 1.5m of some part of the flue like structure, eg open stairway?

[David Rooney]

I would suggest one detector near the opening ... if we started "surrounding" a staircase or similar going through several floors the costs could get silly.

I think we would have to expect a reasonable amount of "even spread" of smoke under a flat ceiling unless there is a reason not to.

[col10]

How about chimney action / convection currents rising up the shaft creating a movement towards the opening.  Potentially there could be a 7.5m hallway at the side of a stairway so you are expecting smoke to travel up to 7.5m in a horizontal direction at the side of an opening and not be dragged up the shaft away from the detector.

[Tom Sutton]

If the smoke travels up the shaft to the top of the shaft then the fire detector at the top of the shaft should detect it, the smoke percolates through the lift or staircase door/opening then the one detector located 1.5 from that opening will detect it, that is how I read it.

[kurnal]

We have covered this before, I can't find the link but I remember the educated conclusion was that 1.5m is an arbitrary figure, the standard reflects this by use of the word approximately and a single detector approx 1.5m from the shaft at each level will suffice.

[David Rooney]

How about chimney action / convection currents rising up the shaft creating a movement towards the opening.  Potentially there could be a 7.5m hallway at the side of a stairway so you are expecting smoke to travel up to 7.5m in a horizontal direction at the side of an opening and not be dragged up the shaft away from the detector.

I'd say you have to do the best with what you've got and with the application of some common sense.

The nearest detector is going to be within 1.5 m ish of the opening not 7.5m so surely smoke would be expected to be drawn across this detector?

And how on earth is any lowly fire alarm contractor going to know or have access to potential smoke movement models when all he's got (if he's lucky) is an A1 plan in front of him and a spec that says design it to L3 ?

[Fishy]

A single detector within 1.5 m (horizontally) of any opening from the shaft/void is how I've interpreted this in the past - so that's the same as the last sentence in the OP?  Oh, and it's BS 5839-1 we're talking about, I think (not BS 5395)?