Siting of smoke detectors for lift shafts

[Tom Sutton]

From BS 5839 pt 1 "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."

Where does the 1.5 m come from and is it the result of research work?

[Wiz]

You'd think there must be a good reason for it, Tom. I don't know of any research that explains this recommendation.

In my mind, I've always thought it was likely to be because smoke rising up a shaft might start losing some bouyancy and not reach the detector at the top of the shaft. So if the smoke then started 'leaking' out of the shaft into the accommodation side on levels below the top of the shaft, you'd want to detect it as quickly as possible, thus the requirement for detection within approximately 1.5m of the shaft rather than 'normal' spacing where it could be up to 7.5m away. Also note the use of the word approximately in this recommendation, I don't think it is used when recommending any other spacing criteria in the BS.

[lancsfirepro]

Can't answer your question Tom but I wouldn't be surprised to find that it's "approximately 1.5m" because it harps back to something someone did years ago and it used to be 5 feet.  Because 5 feet is "approximately" 1.5m.  The study work that came up with that figure (if there was any) has more than likely been lost and rather than spend money re-justifying it they're just running with it.  Good old British Standards.

[John Webb]

I vaguely recall this goes back to the fact that lift doors are not (or were not) smoke tight. Therefore smoke from a fire in the lift shaft will escape through doors, particularly if the lift(s) is(are) still moving up and down creating a 'piston' effect. As lift lobbies are close, usually, to escape routes, I think this particular item is to ensure early detection close to the lift to give as early a warning as possible. I am fairly certain there was no work done at the Fire Research Station on this matter after 1969; that's not to say there wasn't work done before then!

[Fishy]

I vaguely recall this goes back to the fact that lift doors are not (or were not) smoke tight. Therefore smoke from a fire in the lift shaft will escape through doors, particularly if the lift(s) is(are) still moving up and down creating a 'piston' effect. As lift lobbies are close, usually, to escape routes, I think this particular item is to ensure early detection close to the lift to give as early a warning as possible. I am fairly certain there was no work done at the Fire Research Station on this matter after 1969; that's not to say there wasn't work done before then!

You're absolutely right about the doors, John - we tried to find 'S' rated lift landing doors to protect the side of a 'through' evacuation lift that wouldn't be used in an emergency, but after extensive investigations we failed.  They all leak like sieves.  Had to completely redesign the lift configuration (there wasn't room to fit a lobby).