Lift Overrun

The concept of lift overrun is divided into two primary zones: the top clearance and the bottom (pit) clearance. In a traction system, as the car travels toward the highest floor.

The counterweight descends toward the pit.

The overrun must account for the distance needed for the car to come to a complete stop if the normal terminal limits fail to engage.

Factors influencing the needed distance include the rated speed of the lift, the type of braking system employed.

The compression characteristics of the buffers. For high-speed systems, the kinetic energy involved necessitates a lot deeper pits and higher overhead clearances to make sure safe deceleration.

What it involves

For engineers performing Lift Troubleshooting, diagnosing an "overrun fault" usually involves checking the limit switches.

These electrical devices are placed to cut power to the motor and apply the brakes before the physical limits of the shaft are reached.

If the lift continues to travel past these switches, it enters the overrun zone. Here, it will eventually strike the mechanical buffers.

Top Overrun and Counterweight Interaction

Top overrun is specially the distance between the car roof (including any fixed equipment like the door operator) and the lowest part of the overhead structure when the counterweight is fully compressed on its buffers.

Safety and UK rules

This space is not merely for equipment safety. It is a legal duty for technician survival.

Under BS EN 81-20, a specific volume known as the "refuge space" must be maintained.

This ensures that if the lift is moved upward while a technician is on the roof. There is enough room for the single to survive without being crushed against the ceiling.

Bottom Overrun and Pit Depth

Bottom overrun refers to the clearance in the pit when the car is resting on its fully compressed buffers. The pit must be deep enough to house the buffer equipment.

The tensioning pulleys for the governor rope. Give a refuge space for a technician lying on the pit floor.

In hydraulic systems.

The overrun is governed by the stroke of the ram and the position of the mechanical stops within the cylinder.

If lift overrun occurs in a hydraulic context, it usually points to a failure in the levelling sensors or a malfunction in the down-direction valve solenoids.

The following table outlines the standard needs for clearance and refuge spaces based on modern UK safety rules. Note that these figures are minimums.

Specific manufacturer data from Lift Troubleshooting manuals should always be consulted for bespoke fittings.

Parameter: Car Top Refuge Space; Need (BS EN 81-20): 0.5m x 0.7m x 1.0m (Min). Purpose: Technician safety during upward travel.

Parameter: Pit Refuge Space; Need (BS EN 81-20): 0.5m x 0.7m x 1.0m (Min). Purpose: Technician safety during downward travel.

Parameter: Buffer Stroke; Need (BS EN 81-20): Calculated based on 115% of rated speed; Purpose: Kinetic energy dissipation.

Parameter: Bottom Clearance; Need (BS EN 81-20): Minimum 0.5m below car apron; Purpose: Prevention of crushing hazards.

Managing lift overrun involves a hierarchy of electrical and mechanical systems. Each layer acts as a failsafe for the one preceding it.

If the primary control software fails to stop the car, the following hardware components intervene:

Safety and UK rules

Directional Limit Switches: The first line of defence, usually located in the hoistway, designed to interrupt the safety circuit. Final Limit Switches: Placed slightly beyond the directional limits.

These cut power to the main contactor and the brake coil directly. Buffers: The final mechanical stop.

These are either energy-accumulation type (springs) or energy-dissipation type (oil-filled polyurethanes). Braking Systems: Electromagnetic friction brakes designed to hold the car even at 125% of the rated load.

The Role of the Terminal Limit Switch

Terminal limits are critical in preventing lift overrun. These switches are normally closed (NC) and are physically opened by a cam mounted on the lift car.

In modern electronic systems, these physical switches are often supplemented by absolute encoders that give millimetre-precise positioning to the controller.

But, the physical switch remains a required safety backup in most jurisdictions including the UK.

Buffer Characteristics and Maintenance

Buffers are not intended to be used during regular operation. If the car strikes a buffer, a full safety inspection is needed.

What to check and report

For oil buffers, technicians must check oil levels and make sure the piston has not seized due to corrosion. Often of repeated lift overrun, the buffers themselves may have degraded.

Losing their ability to absorb impact. This can lead to building damage to the car frame.

Identifying why a system is experiencing lift overrun needs a systematic diagnostic approach. Overrun is rarely a standalone issue; it is usually a symptom of a deeper mechanical or electrical failure. Engineers should prioritise the following areas during a site inspection:

1. Brake Slippage and Friction Issues

The most common cause of lift overrun in traction lifts is a failing brake system.

If the brake pads are worn or the springs have lost tension, the car may drift past the floor level after the motor has stopped.

What it involves

This is especially prevalent in "heavy down" or "heavy up" scenarios where the load imbalance exceeds the brake's holding torque.

Diagnostic Step: Do a brake holding test by loading the car to 125% capacity and verifying it remains stationary at the lowest landing.

Check the air gap between the armature and the coil to make sure it meets manufacturer specs.

2. Encoder Discrepancies and Positioning Errors

Modern lift controllers rely on digital encoders to track the car's position in the shaft.

If the encoder wheel slips on the governor rope or if there is electrical noise in the signal cable, the controller may "think" the car is at the floor when it is actually several inches beyond it.

This cumulative error can result in the car entering the lift overrun zone at high speeds.

3. Hydraulic Valve Failure

In hydraulic lifts, lift overrun is often caused by debris in the valve block or a failing "down" valve solenoid.

If the valve does not close fully, hydraulic fluid continues to return to the tank. This allows the car to creep past the floor.

Conversely, a stuck "up" valve can cause the pump to push the car into the top overhead clearance.

Secure the Site: Isolate the lift and make sure no passengers are inside.

Engage the "Out of Service" mode. Check Pit and Overhead Clearances: Measure the physical distance from the car to the overhead slab and the pit floor.

Compare these against the original building schematics. Inspect Limit Switches: Manually trigger the directional and final limit switches. Check for continuity in the safety circuit.

Make sure the cams are securely fastened and not worn. Check Brake Torque: Inspect the brake linings for glazing or oil contamination.

Adjust the tension as per the service manual found on Lift Troubleshooting. Review Controller Logs: Access the motherboard diagnostic menu.

Look for "Terminal Limit Trip" or "Position Loss" error codes. Examine Buffers: Make sure buffers are correctly placed and that oil levels (where applicable) are within the sight glass range.

Working within the lift overrun zones—specially the pit and the car top—is the most dangerous aspect of lift maintenance.

The Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) and Provision and Use of Work Equipment Regulations 1998 (PUWER) dictate strict following safety protocols in these areas.

What to check and report

Technicians must always check the operation of the "Pit Stop" switch and the "Car Top Station" before entering these zones. The lift overrun space is designed to be a survival zone.

However, its efficacy is negated if the technician is standing in an incorrect position or if the refuge space is obstructed by stored materials or debris.

Refuge Space Dimensions

The refuge space is a predefined volume where a person can safely remain while the car is at its extreme limit of travel.

Safety and UK rules

For modern fittings (BS EN 81-20), these spaces are marked clearly on the car top and the pit floor.

Maintaining the integrity of these volumes is a legal duty during any upgrade work or repair project.

 // Example of a Controller Logic Check for Overrun Limits IF (CarPosition > UpperLimitSwitch) { Activate_Emergency_Brake(); Cut_Motor_Power(); Signal_Fault_Code(E42_OVERRUN_ERROR); } 

Engineers must be able to calculate the needed lift overrun when modifying a system or installing new buffers.

The formula for the minimum stroke of an energy-dissipation buffer is based on the gravity stop distance at 115% of the rated speed.

S = 0.0674 × (1.15 × v)²

Where:
S = Stroke in metres
v = Rated speed in metres per second

Applying this formula ensures that if there is a free-fall or uncontrolled descent. The deceleration skilled by the passengers does not exceed 1g, preventing severe injury.

Any reduction in this calculated distance needs the use of a "Reduced Stroke Buffer" system. This must be combined with a certified terminal speed limiting device.

In many older UK buildings, existing shaft dimensions do not meet modern BS EN 81-20 needs for lift overrun. This presents a big challenge during lift replacement.

In these instances, BS EN 81-21 gives other safety measures for lifts installed in existing buildings.

Technological services for reduced overrun include:

Movable Balustrades: Foldable handrails on the car top that reduce the needed overhead height during normal operation but give safety during maintenance. Pre-triggered Safety Systems: Electronic systems that detect a technician's presence and mechanically lock the car before it can enter the reduced refuge zone. Telescopic Aprons: Used in shallow pits to prevent the crushing hazard while maintaining the needed toe-guard protection.

Routine inspections must prioritise the integrity of the lift overrun safety chain. A failure in this system is often "silent".

The lift continues to run normally until an emergency occurs, at which point the lack of overrun protection leads to disaster.

Monthly Inspection Checklist

Safety and UK rules

Clean the pit floor to make sure no debris interferes with buffer compression. Lubricate the guide rails in the overrun zones to prevent safety gear galling.

Check the security of all limit switch mounting brackets. Inspect the buffer strike plates for signs of impact or misalignment.

Annual Testing Needs

What it involves

Once per year, a full speed terminal limit test should be conducted. This involves bypassing the normal slowdown limits (under controlled conditions) to make sure the final limits and buffers do as designed.

This test must be documented in the site logbook to satisfy insurance and LOLER rules.