A safety component is a part whose failure mode was engineered, certified and documented. That is what you are paying for, and it is what a bargain on the wrong unit takes away.
The ratings, briefly
Two standards describe machine safety functions in Europe, and most datasheets carry both.
EN ISO 13849-1 rates a safety function as a Performance Level from PL a (lowest) to PL e (highest), derived from the architecture (Category B, 1, 2, 3, 4), the component reliability (MTTF_D), the diagnostic coverage (DC) and common-cause failure resistance.
IEC 62061 / IEC 61508 rate the same thing as a Safety Integrity Level, SIL 1 to SIL 3 for machinery. PL d corresponds roughly to SIL 2, PL e to SIL 3.
Three things follow from this that matter when buying:
- The rating belongs to the function, not to the part. A PL e-capable safety relay wired into a single-channel circuit delivers PL c. Buying the right part does not by itself buy the rating.
- The rating depends on numbers the manufacturer publishes — B10d, MTTF_D, proof test interval, mission time. Those numbers are tied to the specific article number and revision.
- Safety components have a mission time, typically twenty years, after which they are to be replaced regardless of condition. A thirty-year-old safety relay is out of its declared life even if it works.
What to look for on the part
- The article number and revision matching the one in the machine's safety calculation, not merely the same model family.
- A TÜV, TÜV Rheinland, TÜV SÜD or equivalent certificate mark, and the EC type-examination certificate number where the component type requires one (light curtains, safety controllers and interlocks generally do).
- The declaration of conformity, ideally with the unit. For a safety component it is part of the product, not an optional extra.
- The manufacturing date, because of mission time.
The three rules for the secondary market
Surplus and refurbished stock is the normal way to keep an older machine running, and safety components are the one category where that needs qualification rather than a blanket yes or no.
1. New or new surplus only, for anything in the safety chain. An unused unit in original packaging with its declaration of conformity is a safety component. A refurbished one with an undocumented repair history is a part that looks like one. The price difference is real; so is the difference in what you can defend.
2. Insist on the documentation. Article number, revision, date code, declaration of conformity. If a supplier cannot supply those for a safety component, buy elsewhere. This is not bureaucracy: without them you cannot update the machine's safety file, and the safety file is what an inspector or an insurer asks for.
3. Re-validate the function after replacement. Swapping a safety relay is not a like-for-like maintenance task. Re-run the validation for the safety function — the stop time measurement for a light curtain, the two-channel discrepancy test for a relay, the STO response for a drive — and record it. A replacement with a different revision may have different reaction times, and reaction time is what the safety distance was calculated from.
Where refurbished is defensible
Refurbished safety components can be appropriate in two narrow cases: when the refurbisher is the original manufacturer or their authorised service partner, and when the unit comes with a test report tied to its serial number and the remaining mission time is documented. Both exist. Neither is the default, and both should be visible in writing before you order.
Drives with Safe Torque Off
STO is the one place where safety and ordinary parts buying overlap most often, because the safety function is inside a drive you are buying for its motor control. Two practical points: the STO rating is tied to the drive's article number and firmware, so a "same model" replacement with different firmware may not carry the same certificate; and STO usually requires the safety wiring to be re-checked after replacement, because the terminals and the dual-channel logic differ between families even within one manufacturer.
The honest summary
For a non-safety part, the cheapest unit that works is usually the right answer. For a safety part, the right answer is the documented unit from a supplier who can produce its paperwork, and the money saved by relaxing that is the smallest saving in the whole spare-parts budget measured against what it puts at risk.