Smoke Detectors within dwellings

[GB]

BS 5839 part 6 states that detetcors if fitted to walls must have the top of their element within 150mm to 300mm of the ceiling. If a property has ornate coving / cornice, how is the 150mm affected. I have a property where the top of the detector is within the 150 - 300mm range from the ceiling level but is only 120mm away from the top of the coving which is around 30mm thick.

[wee brian]

The last research I saw on this subject (from the states) said that anywhere within 500mm of the ceiling will be OK.

Its probably on the NIST website somewhere

[Ken Taylor]

I can understand if it's 150mm to 150mm but if it's anywhere up to 500mm it seems to go against what we were taught about 'dead spaces' in corners - but it was a long time ago! The 30mm cornice edge doesn't sound significant to me.

[wee brian]

Don't confuse codes of practice with actual facts.

[John Webb]

GB: The 'coving/cornice' is above or below the detector - can you clarify, please? Only on reading your post #1 it seems the detector is above the coving, in which case I would not see any problem.
500mm mentioned by Wee Brian seems a bit too low - it means quite a volume below the ceiling would need to fill with smoke before the bottom of the layer reaches the detector. Much better to have the detector on the ceiling if at all possible. You'd get the most reliable operation there.

[Wiz]

Much better to have the detector on the ceiling if at all possible. You'd get the most reliable operation there.

Supporting Mr Webb's good advice, always remember that BS5839 Part 1 recommends that wall mounted detectors are only considered in L3 categories and I undersatnd that the wall mounting option was only grudgingly first allowed because hotel owners didn't like detecors in the middle of guest bedroom ceilings. I realise that your query relates to 'dwellings' but the Code Of Practices generally all indicate the ceiling provides the best operation.

[Wiz]

I've just read GB's original question again and I feel that what he is saying is that the thickness of the bottom edge of the coving from the wall is 30mm and this edge is obviously closer to the detector than the recommendations for the distance of detector from the ceiling.  I am pretty certain that the effect of the edge of some coving of only 30mm thickness can be discounted  as creating much of a 'dead space' where smoke won't reach. In this case it is the distance from ceiling to detecotor that counts and not the distance from edge of a 30mm thick coving  to detector.

[John Webb]

I'd imagine coving is probably going to fill quite a bit of the 'Dead space' at the junction of wall and ceiling so coving might actually help to improve detection by wall-mounted detectors. My observations of many fire tests suggest that the 'dead space' is rarely a problem anyway. If the fire has sufficient energy to get smoke to the ceiling through any warm air layer under the ceiling, it will usually spread across all the ceiling  to the walls and into the corners of the room with little difficulty and with reasonable speed. It usually only needs a few kilowatts output to do this. (ie the fire heat output is a bit more than any other heat producing object in the room.)