There is a lot of confusion around the life span of hard hats, or when a hard hat needs to be replaced. Hopefully this excerpt below from Standards Australia will help clarify the requirements for you.
Hard hats do not have an expiry date at the point of manufacture. AS1800:1998 states:-
At the time of issue to the wearer, the helmet should be marked with the issue date.
Field tests have shown that generally , helmet shells have a life of at least 3 years from the time of issue. Components of harnesses may deteriorate more rapidly in service and harnesses should, therefore, be replaced at intervals not longer than 2 years.
For helmets that are used infrequently and stored away from sunlight, dirt and temperature extremes, this guideline/recommendation may not be applicable. The user should examine the helmet regularly and discard if any damage is evident.
Extract of Australian Standard AS1800:1998
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