My post on helmets was just an information one. They were not my words but it may help with an understanding of the rules here in Europe. I've copied a bit more for info.
Every helmet sold in the UK must be tested to ECE 22.05. This involves abrasion, impact, penetration, deformation and retention testing, as well the field of view from the visor. This standard is accepted throughout Europe across 47 countries.
You might find your lid also has a ‘Snell’ marking – this testing is voluntary, and is often used by manufacturers in the development of helmets.
Another marking you might see is ‘DOT’, which is an American standard. This does not replace ECE in Europe and the UK – your helmet must also be tested to ECE 22.05, but it does mean you could legally wear it in the US too. DOT testing is carried out by the manufacturer, and one area it differs to ECE 22.05 is that a DOT helmet can transfer a lot more energy to the wearer’s head in a crash than an ECE lid can.
You might also find a gold ACU sticker – this is the governing body for motorcycle sport, and while no further testing is carried out, only helmets that have been submitted by the manufacturer and awarded this sticker can be used in racing. Off-road racing requires a silver ACU sticker. You should wear an ACU-approved helmet on public track days, though it’s unusual to see helmets in good condition being turned down for use, and with stickers easily bought, a scrutineer would need to have a very good knowledge of helmets to know if one hadn’t been approved by the ACU.
Your helmet will either have a ‘double-D’ fastener, or a ‘micro-metric ratchet’. You can’t choose – it’ll depend on the model.
Double-D is the choice of racers and gives a very secure fit every time you put the lid on, with a really simple mechanism. But it’s hard to use if you have your gloves on.
Most touring lids have a micrometric ratchet – with this, you set the strap to the size you want at first, then it gives around an inch of adjustment every time you put the helmet on, which means you should still get a secure fit. Some people don’t like the fact that it depends on moving parts, but it’s a very secure and simple self-locking design, and a vast improvement on the old seat-belt buckle types that were set to one length only, so tended to work loose.