Building Services/Vertical Transportation/Emergency Procedures
As already mentioned in the chapter about traction lifts, lift cables are extremely unlikely to break. Even if this was to happen, all lifts capable of carrying persons are fitted with an overspeed governor. This device works in much the same way as a seat belt in a car by clamping brakes onto the guide rails if the lift exceeds its normal speed. Even if the lift were to drop down the shaft (very unlikely indeed), there are buffers at the bottom of the shaft to soften the drop.
In the event of a power failure, most lifts will stop. The brake is designed to fail open so the lift will stop, probably quite abruptly. This is intended to happen as otherwise the balance between the lift and the counterweight would result in the lift car moving. Ideally, lifts should have a battery or generator backup that will lower the lift to the next floor, open the doors and then shut itself down until the main power is restored. The lighting in lift cars is usually supplied from the mains therefore it is essential that all lift cars have a battery powered emergency light.
Many lifts do not feature any form of backup power so anybody inside the lift will become trapped in the event of a power failure.
Lift cars must also have an alarm system, preferably one that connects to somebody within the building, for example Security or the Estates department. If the lift stops for any reason, such as a power failure or mechanical problem, the persons inside the lift can press the alarm button and seek help. Generally, it is wise to make sure that the alarm cannot be set off accidentally, for example by leaning on the button. Some lifts just feature an alarm that alerts those responsible that there is a problem while more sophisticated systems feature a built-in two way telephone so the persons inside the lift can explain the problem.
When a lift is ‘stuck’ and there are people inside, they must be released. This is either done by trained authorised persons on site or by the fire brigade. The lift doors can be opened using a special key and the general rule of thumb is to move the lift down to the next floor so the occupants can walk out on the level. Before this can be done, the lift must be isolated (turned off) in the machine room, otherwise if the power were to come back on, persons in the machine room could be injured.
Moving the lift generally requires two persons in the machine room – one to release the brake and the other to hand crank the sheave. More modern lifts usually feature a built-in lever to release the brake but older ones require a special tool that is usually kept within the machine room. There is also a special hand wheel that is attached to the end of the sheave axle and turned.
In certain cases where the lift is stuck between floors and cannot be moved, a ladder can be used to evacuate those inside.