For lift arriving at a landing, automatic re-levelling of the lift car is required if difference in the levels of the car door sill and landing sill exceeds a certain limit as required by local code. The re-levelling is effected before the car door is opened to avoid boarding/alighting passengers being tripped over due to excessive level difference.
However, when a fully loaded lift car reaches its terminal floor, such as the ground floor of a high rise building, the difference in passenger load between the moments when the first and the last passenger leaves the lift car may result in substantial change in the vertical position of the lift car. The same would happen when an empty car arrives to pick up passengers. These phenomena are due to the elongation/contraction of the lift suspension ropes under different load conditions. The extent of level change depends on the quantity/size of the suspension ropes used and their Young’s Modulus.