Lift door sensors and smoke

[memnon30]

I have just been asked a question that has me stumped. (Not that hard I grant you).
If a lift was to stop and the doors open onto a heavily smoked logged fire floor, at what stage (if at all) would the smoke be thick enough to make the sensor think that there is an obstruction in the way and not close the doors? In addition, where would I be able to reference this?

[The Colonel]

Not 100% sure but I am lead to believe that in the fire at Dusseldorf Airport some years ago that smoke prevented the doors of a lift closing resulting in deaths. I would suggest contacting a local lift supplier or maintenance company for precise details on the door sensors and settings.

[John Webb]

I recall that early touch-sensitive call buttons could be deceived by smoke and actually could call a lift to a smoke-logged floor. But I think that was with sensors using a high voltage for sensing. Modern ones running at low voltage may be safer in this respect?

[Midland Retty]

Hi Memnon

I'm a little intrigued. Can you expand on how this question came about?

As Im sure you are aware lifts should not be used in the event of fire (unless they are evacuation or firefighting lifts) so it makes me wonder how this situation might arise?

I accept that you could already be travelling in a lift when the fire alarms go off, but the lift should return to ground floor, and you should be able to safely make your escape from there.

If you can't and you get to the ground floor to find the whole floor is untennable, something has gone horribly pair shaped.

I also accept there have been instances as The Colonel mentioned of doors not closing because of smoke, but that was as a result of humans trying to use the lift when they should have used the stairs!

[David Rooney]

I accept that you could already be travelling in a lift when the fire alarms go off, but the lift should return to ground floor, and you should be able to safely make your escape from there.

If you can't and you get to the ground floor to find the whole floor is untennable, something has gone horribly pair shaped.

Unless it's not a fire alarm and it's a "smoke ventillation control system" which is not linked to the lifts. We look after a block like this and have always argued with Islington Council someone could enter the building at ground level and travel up to a fire floor and get killed.

But they passed this system years ago and have no compunction to change it.

[memnon30]

I’m unable to say too much at the moment as its going to court, but I believe the intention is to try and ascertain what could happen if a fire was started in a gnd floor lobby and the lift was used to leave the building either before detection or if there was no detection by people unaware of the fire. Could the smoke from said fire keep open the doors?

[Colin Newman]

Colnel is right, it was definately a feature in the Dusseldorf airport fire.

The lifts were not connected to the fire alarm system andp assengers in the lift were taken to a fire floor.  The doors opened and smoke activated the sensor preventing the lift doors closing.  A number of bodies were discovered in the open lift car.

At the time I was working at Heathrow, and discovered that all of the lifts were sent to ground florr on a fire alarm, but no one had asked "What if the fire is on the ground floor?"

[Psuedonym]

Shouldn't the Building Regs require a Emergency Exit be in the ground lobby area?

[Phoenix]

...... all of the lifts were sent to ground floor on a fire alarm, but no one had asked "What if the fire is on the ground floor?"

Some buildings are clever enough, and it doesn't take a huge brain, to send the lift to another floor, for example to the first floor or the lower ground floor, if the fire happens to be in the normal egress route on the ground floor.

However, this is sometimes over the top.  If the lift is programmed to return to the ground (egress) floor on actuation of the alarm then the short travel time for the lift to reach the ground floor from wherever it is in the building (hopefully in the liftshaft somewhere) means that the fire is incapable of being large enough to compromise the means of escape through the ground floor route. 

For example, the fire might be in the reception desk in the ground floor lobby that also contains the lifts.  It may be that the time for the lift to return to the ground floor is sufficiently short for people to be able to leave the lifts and make their way safely past the incipient fire to escape.  On the other hand, it may be judged that the lift may take too long to reach the lobby or the detection in the lobby might take too long for this route to be guaranteed as tenable in such a fire scenario.

Stu

[memnon30]

Many thanks to all who replied. I will let you know how the case goes when I am able.

[BLEVE]

It all depends on the type of sensor provided. Active and Passive Infrared sensors activate on change of temperature, I would expect that any increase of temperature (thermal radiation from a fire and not just smoke) at the AR/PIR sensor would cause the lift doors to remain open.

In the case of a lift door provided with a photoelectric cell sensor, an infrared beam is directed towards a light dependent resistor. If the optical density of the smoke increases to an extent as to scatter the IR beam, then the doors will remain open.

As others have said, something will have gone pair shaped in order to result in a life-threatening situation i.e. late detection, late evacuation, abnormal fire load in the vicinity of lift lobby etc.

Interestingly, I have seen a number of (IR controlled automatic opening) fire doors positioned in corridor locations. Again, in the event of fire, this type of design would result in the door opening “not the best design concept”.