Inspection & testing
Proof testing explained: what it is, what it is not, and what the certificate means
Updated August 1, 2026 · 3 minute read · All guides
In short
Proof testing is a non-destructive load test applied to a finished lifting assembly before it ships, at a multiple of its working load limit. For alloy chain slings that multiple is twice the working load limit. It verifies workmanship and confirms the assembly, then it is recorded against the assembly serial number and issued as a certificate. Proof testing does not establish the working load limit and it is not a destructive break test.
Key facts
- Proof test load for alloy chain slings is 2 times the marked working load limit, applied without permanent deformation.
- Proof testing is non-destructive: the assembly is sold afterwards, so the load must stay well below yield.
- Working load limit is established by design and material, not by the proof test.
- Breaking strength is typically 4 times the working load limit for alloy chain slings. That ratio is the design factor.
- A proof-test certificate should reference the serial number stamped on the assembly, not just the purchase order.
Three numbers people confuse
Most confusion about testing comes from mixing up three different loads.
| Term | What it is | Typical relationship |
|---|---|---|
| Working load limit (WLL) | The maximum load the assembly may carry in service | The rated figure on the tag |
| Proof load | A one-time test load applied before shipment | 2 times WLL for alloy chain slings |
| Breaking strength | The load at which the assembly fails | Around 4 times WLL for alloy chain slings |
The ratio between breaking strength and working load limit is the design factor. For alloy chain slings it is normally 4 to 1. Different product families use different design factors: below-the-hook devices, wire rope slings and synthetic slings each have their own, which is why you cannot reason across categories from a single rule of thumb.
What actually happens in a proof test
A finished assembly, complete with its master link, chain and end fittings, is placed in a test machine and loaded to its proof load. It is held, released, and then examined.
The test is looking for a small number of specific things:
- Permanent deformation. The assembly must return to its original dimensions. A link that has stretched has yielded, and the assembly fails.
- Workmanship faults. Bad welds, incorrectly assembled couplers, wrong components fitted.
- Fitting behaviour under load. Hooks that open, latches that jam, couplers that shift.
It is a non-destructive test, which is the whole point: the assembly is going to be sold and used. That is also why the proof load has to sit comfortably below the yield point.
What proof testing does not tell you
Three things it is regularly assumed to prove, and does not:
- It does not establish the working load limit. The WLL comes from the chain grade, size and assembly design. Proof testing confirms the assembly is what it claims to be.
- It is not a break test. Nobody sells a chain that has been pulled to destruction. Breaking strength is established by destructive testing on samples from the production batch, not on your assembly.
- It is not an in-service inspection. A proof test on the day of manufacture says nothing about the condition of a sling after two years in a precast yard. That is what periodic inspection is for.
Re-proof testing after repair
If an assembly is repaired, the situation changes. Replacing a hook or a coupling link means the assembly is no longer the item that was originally tested and tagged. Proper practice is:
- Repair using components of the correct grade and rating, following the original manufacturer's instructions.
- Re-rate the assembly to its lowest rated component.
- Re-proof test the repaired assembly.
- Re-tag it with the current rating and a new record.
Field repairs made from a bin of unmarked spares are the most common way a rated assembly quietly stops being rated.
What a good certificate contains
A proof-test certificate is only useful if it can be tied back to a physical item years later. It should state:
- The serial number stamped on the assembly
- The working load limit and the configuration it applies to
- The proof load applied and confirmation the assembly passed
- The standard the assembly was manufactured and rated to
- The date and the issuing party
The serial number is the part that matters. A certificate filed against a purchase order becomes unmatched the moment equipment moves between branches or a second identical sling arrives.
How this shows up in what we supply
Every lifting assembly we supply is proof-tested before shipment, permanently tagged with a serial number, and shipped with a certificate that references that serial. Below-the-hook devices are proof-tested to their design proof load and marked with rated load, device weight and serial. Hoists are proof-tested and supplied with a test certificate per unit.
If a certificate goes missing, we can reissue it from the serial number, because the test record is held against the assembly rather than against the order.