Christian McArdle wrote:
Looking on the screwfix site they have 'open' and 'closed' 20mm PVC
grommets - the images look identical, so what's the difference and is
there an advantage in using one type over the other?

The closed ones are "filled in". You need to puncture a hole for the cable
to go through if there isn't one already. This means that the hole is
exactly the right size for the cable, though.

Funnily enough I took 'open' and 'closed' to mean something rather
different:-

Closed - the grommet is a continuous toroid, but still has a hole
in the middle for the cable.

Open - the grommet is actually just a length of U shaped rubber
but is shaped in a circle so the ends butt together.

Open grommets are much easier to get into the hole in the box than are
closed grommets and, as long as they're the right size, protect the
cable very nearly as well.

I expect you're right but I think (as someone else said) 'blind' would
be easier to understand for 'closed'.

--
Chris Green