UKH

Are buckles usually designed to release under high load so if it comes to it, the helmet will detach rather than strangle the user?

or does that release happen elsewhere in the system?

or not at all?

I need to do a field repair on a helmet buckle hence wondering.

Yes, the chip strap has to be able to hold 50kgs for 2 minutes to meet the standard (I think!)

this site should give you some detail: https://www.hexarmor.com/posts/climbing-style-safety-helmet-standards-you-n...

basically, depends if it’s a climbing or industrial helmet, but all will break away at some point.

Not really - on climbing helmets (specced to EN12492) have strong chin straps intended to keep the helmet on in an impact so that it's still there if there's another one coming.  (Stonefall for example.)  The 50kg mentioned above is a minimum, not a maximum and there's no upper limit.

Industrial helmets (to EN392) go the other way and have straps designed to release so you can't accidentally hang yourself on a crane hook or something.  (From memory I think they're supposed to let go at 50kg but could be wrong about that.)

Some manufacturers make essentially the same helmet to both standards, and last time I looked Petzl (for example) distinguished between them by having a red clip on the industrial ones that are meant to let go, and a black clip on the mountaineering version that isn't.

It's all a bit academic as far as your field repair goes, (especially if you're sewing the strap) probably best just to make it as strong as you can within reason.

Make sure you don't have a loose strap end that could get caught in an abseil device, or that you can release the strap under that circumstance. Many years ago the strap of my Joe Brown helmet caught in my figure of eight . While it wasn't restricting my breath, it was very tight on my neck, and I couldn't release the buckle without taking my weight off the rope, which was difficult on a free abseil...

The Petzl Vertex actually has a switch on the chin strap to select the breakaway strength.

"DUAL chinstrap allows the worker to adjust chinstrap strength in order to adapt the helmet to different environments: work at height (EN 12492) or on the ground (EN 397). The clip has two positions, corresponding to these two uses: high strength, limiting the risk of losing the helmet during a fall, and low strength, limiting the risk of strangulation if the helmet is snagged while the user is on the ground"

Ooh, fancy. 

I'm struggling to imagine how the two could be in close enough proximity without resorting to smutty suggestions