Ring Circuit - steel knockout boxes/grommets

[Perry Gunn]

As we're having our lounge replastered, I'm taking the opportunity to extend the ring and add a couple of extra double sockets in convenient places. I've done this before and I'm quite happy with the wiring side of things but, in the past, I've always used the plastic surface-mount boxes to do this.

As I'm going to be adding the sockets between the time the old plaster comes off and the new plaster goes on, I want to use steel knockout boxes cut into the brickwork to give a flush finish matching the other sockets in the room.

Never having used the steel boxes before, I understand that you are supposed to put a grommet on the knockout to protect the cable.

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?

Many Thanks

Perry

[Christian McArdle]

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.

Christian.

[Perry Gunn]

On Tue, 7 Jun 2005 10:31:20 +0100, "Christian McArdle" waxed lyrical about:

Thanks, that makes sense - they've obviously got the wrong image on the site.

Perry

[Dave Plowman (News)]

The other name for closed is 'blind'. It is a disk rather than a ring, as it were. You simply fold it in half and use sidecutters to make a slit while not cutting into the 'ring' part.

Since you'll be making good afterwards, either type will do.

[usenet]

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'.

[Dave Plowman (News)]

IIRC, grommet strip - but you'd need to go to somewhere like RS for this.

They also pull/push out more easily. ;-)

[Christian McArdle]

That's also a term I've heard used for closed, but I don't think it is easier to understand! Blind for me means someone/thing that can't see. And all grommits are somewhat challenged in the seeing department, AFAICT.

Christian.

[Christian McArdle]

TLC do it. Useful for rectangular or unusual shaped cutouts.

Christian.

[Andrew Gabriel]

The sheath stripped from T&E by running a knife down the side of the earth conductor (like what you're not supposed to do;-) makes excellent grommet strip, and is guaranteed not to react with the cable sheath of course. On straight edges, it needs glueing on, but so does real grommet strip. Different sized T&E gives grommet strip suitable for different thickness metalwork.

Another form of grommet strip is to take a grommet for a larger hole and cut a piece out. I usually end up doing this for the smaller holes in architrave back boxes. The trick is to not cut out quite enough, so the grommet is held in place with some compression force. You could glue the ends if you wanted, but I've never found it necessary.

[Dave Plowman (News)]

A blind stops you seeing through a window. You can see through an open grommet, but not a blind one.

[Dave Plowman (News)]

Thought I knew their cat. intimately.;-)

Where about - it's not with the ordinary grommets. Which I notice they call open and blind.

[Christian McArdle]

So you could call it a "curtains" grommit as well? Especially a pre cut one.

Christian.

[Rob Morley]

I think you're going down a blind alley with that one ...

[Christian McArdle]

code SR150. 5.75 for 5m.

Christian.

[Dave Plowman (News)]

Order code SR150. 5.75 for 5m.

Thanks, Christian. But it's about twice the price of RS - and they do other sizes too.

I've generally thought TLC to be good on a basket of prices but caught them out on some other things too. Like their plug in octal relays. Anyone who charges more than RS are taking the p**s. ;-)

[John Rumm]

Unless that is where they buy them!

(Conversation witha supplier the other day... Me: "How much can you do for?", Them: "nnn pounds Sir", Me: "ah, I can get those for nn pounds from xyz", Them: "Oh... ah... well in fact that is where we buy ours from so I guess you may as well go straight to xyz!")