Rigging Safety: Working Load Limits and Certification

Rigging hardware carries numbers, and the numbers are not interchangeable. Breaking load, working load limit, proof load and safety factor describe different things, and the failure mode this causes is always the same: someone uses a breaking strength as if it were a working limit, and the margin disappears.

The four numbers

• Breaking load (MBL). The load at which the component or the line fails. For rope this is measured on a new, dry sample under a steady pull. It is a laboratory number and it describes the end of the component's life, not a load to use.
• Working load limit (WLL). The maximum load the component is rated to carry in service. It is derived from the breaking load by dividing by a safety factor. This is the number you plan with.
• Proof load. A test load applied to a sample of production to demonstrate that it holds — typically around twice the WLL. Passing a proof load does not make the component rated for that load.
• Safety factor. The ratio between breaking load and working load limit. Five to one and six to one are common for lifting hardware.

Where the safety factor goes

The safety factor is not padding. It absorbs a specific list of things that the calculation does not include.

1. Wear and corrosion, which reduce the section that carries the load.
2. Uneven load distribution between the legs of a multi-leg arrangement.
3. Dynamic effects — a load that is not lifted smoothly, or that swings.
4. The difference between a laboratory pull and the direction the load actually arrives from in service.
5. The possibility that the person who specified the component did not foresee your particular arrangement.

Angle, again

The angle between sling legs is where a system that was inside its limits becomes outside them, and it is worth repeating because it is so consistently underestimated. At an included angle of 120 degrees, each leg of a two-leg sling carries the entire load. At 150 degrees it carries nearly twice the load. A sling assembly in which every component is rated for the load can still fail at a wide angle.

Certification and what it actually tells you

A certified component is one that has been manufactured and tested to a recognised standard, and marked accordingly. The certification covers the manufacturing process and the batch, and it tells you that the component met its specification when it left the factory. It does not tell you anything about its condition now.

• Certification is about the component. It says nothing about the strong point you attach it to, the splice in the rope, or the geometry of your arrangement.
• Markings matter. A rated component is marked with its working load limit, its size and its grade. An unmarked shackle is an unknown quantity, whatever it cost.
• Inspection is about the condition. A certified component that has been overloaded, corroded or worn is out of service regardless of its paperwork.
• Records connect the two. Certification plus a record of inspection and service history is what lets you make a judgement rather than a guess.

For personal safety equipment, the rules are different

A marine safety harness and its tether are personal protective equipment. They must be certified products used within the manufacturer's instructions, inspected before every use, and inspected more thoroughly at the specified interval. Any harness that has arrested a fall is out of service permanently — not repaired, not re-stitched, not returned to use. Improvised webbing systems are not a substitute, and a harness is the one item on this site where the paperwork genuinely matters.

A short checklist

1. Plan with the working load limit, never the breaking load.
2. Keep the included angle between sling legs below 120 degrees, and below 90 where you can.
3. Match every component — rope, splice, shackle, anchor point — to the same rating.
4. Add a dynamic factor for anything that moves, swings or snatches.
5. Inspect before use and record what you find.
6. Retire on evidence: a shock load or visible damage means out of service, not re-rated.