Fall clearance, swing falls and horizontal lifelines

A fall arrest system that stops a worker one metre too low is no better than no system at all. Clearance, the space below the worker into which they fall while the system stops them, is the most frequently overlooked part of fall arrest planning. Swing falls, where a worker falls to the side of the anchor and pendulums into a structure or the ground, are the second. This lesson shows how to calculate required clearance, how to recognize and limit swing falls, and how horizontal lifelines change both.

The parts of a fall

When a worker connected to a fall arrest system falls, their body travels downward through several stages. Each adds distance:

• free fall: the distance the worker falls before the system begins to act. With a lanyard, it depends on the lanyard length and where the anchor is relative to the dorsal D-ring;
• deceleration distance: the distance over which the energy absorber deploys or the self-retracting device brakes;
• harness stretch and D-ring shift: the harness stretches and the D-ring slides up the worker's back as the straps take the load, typically about 0.3 m (1 ft);
• the worker's height below the D-ring: the distance from the dorsal D-ring to the soles of the feet, often taken as about 1.5 m (5 ft);
• a safety margin: extra space so that the worker stops above the next level or obstruction, commonly at least 0.6 to 1.0 m (2 to 3 ft).

The sum of these, measured from the anchor, is the total fall clearance required below the anchor point. Anything the worker could hit in that space, such as the floor, equipment, a lower roof, a scaffold or a vehicle, counts.

Note how much of the clearance comes from the lanyard itself. That is why a 1.8 m (6 ft) energy-absorbing lanyard is a poor choice for work just above a floor, a lower roof or a truck bed. Raising the anchor is the most effective fix: every metre the anchor moves above the D-ring reduces free fall by the same amount.

Anchors below the D-ring

When the anchor is below the dorsal D-ring, for example at foot level on a roof or deck, free fall increases by the distance between the D-ring and the anchor:

Self-retracting devices and clearance

Self-retracting devices with overhead anchors need much less clearance because free fall is short. The manufacturer states a maximum arrest distance for the device, measured from the point where the fall begins. Required clearance below the working surface is then roughly the arrest distance plus harness stretch plus a safety margin, provided the anchor is overhead and the worker is directly below it. With an arrest distance of about 1.07 m (42 in), harness stretch of 0.3 m (1 ft) and a 0.6 m (2 ft) margin, about 2.0 m (6.5 ft) below the working surface may be enough. Always use the manufacturer's clearance charts, which account for worker weight, anchor position and swing.

Swing falls

If a worker falls while to the side of the anchor rather than directly below it, the lifeline swings like a pendulum. The worker drops lower than they would have if they were directly below the anchor, and they swing sideways into anything in their path: a column, a wall, the edge of the building, equipment, or the ground. Swing falls are especially common with self-retracting devices, because the line extends as the worker walks away from the anchor.

To limit swing falls:

• keep the anchor as directly overhead as possible and move it as the work moves;
• respect the manufacturer's limit on working angle or lateral distance from the anchor, often expressed as an angle from vertical;
• use horizontal lifelines or overhead rail systems so that the anchor point travels with the worker;
• add a second anchor or redirect point to keep the line close to vertical;
• check what the worker would hit at the bottom of the swing, and include the swing drop in the clearance calculation.

Horizontal lifelines

A horizontal lifeline is a cable, rope or rail stretched between two or more anchors, to which workers connect with a lanyard or self-retracting device. It lets workers move along a beam, a roof edge or a crane runway while staying connected, and it can keep the anchor point overhead along the whole length of the work. But horizontal lifelines behave very differently from single anchors, for two reasons.

First, the line sags when loaded, which adds to the fall distance. Second, the tension in a nearly straight line under a load in the middle is many times the load itself, so the end anchors must be very strong. The tension in each half of a line loaded at mid-span is:

where \( P \) is the load from the falling worker and \( \theta \) is the angle of the sagging line from the horizontal. The flatter the line, the higher the tension.

Because of these forces, Canadian rules generally require horizontal lifelines to be designed by a professional engineer or to be manufactured systems certified to CSA Z259.13 and installed exactly as the manufacturer specifies. CSA Z259.16 covers the design of active fall protection systems, including horizontal lifelines. The design must state the maximum number of workers, the span, the required clearance including sag, the anchor loads and the installation tension.

Never improvise a horizontal lifeline by tying a rope between two columns. The tension calculation shows why: an apparently strong rope and anchors can be overloaded, and the sag can drop the worker onto the level below.