Grommet required in pre-made hole in backbox?

[MarkSmith]

Hi folks,

Replacing a broken surface-mounted backbox (light switch) with a metal, recessed one:http://www.wickes.co.uk/1-gang-metal-box/invt/205261/
It has a pre-made hole in one side, perfect position for the cable (which is under the plaster) but normal grommets don't fit in the hole. Hole is not sharp around the bit in contact with the cable (although it is at the top - it is formed so that the sharp edges face "upwards").

Is a grommet needed here?

Thanks,
Mark

 

[flameport]

A round 20mm grommet should fit in the oval hole on a 16mm backbox.
However if the edges are not sharp, then no grommet required.

A 25mm depth box would be a far better choice http://www.wickes.co.uk/1-gang-metal-box/invt/205262/
because there are plenty of switches which are too deep to fit on a 16mm box, and even those that do will leave virtually no space behind making it much easier to damage cables when fixing the switch in place.

 

[MarkSmith]

Thanks flameport.

Yes normally I'd go for a deeper box but in this case it's a bit of a problem for reasons I won't bore you with. The switch fits easily into the 16mm box so I decided to use that for an easy life.

Thanks,
M.

 

[ebee]

Ditto the 16mm a no non but yes the grommet should still fit OK but just a bit fiddly to get in place. It is best to fit the grommet rather than not because the thickness of the metal means the edge can never be blunt (Some boxes used to have a formed edge so do not need a grommet but I do nt think they do them now

 

[securespark]

Yes, they do!

To avoid doubt, the best thing would be to post a picture of the box in question.

Appleby have what is described as a "smooth cable entry hole" in their catalogue.


Although this is not a link to their catalogue, it shows an Appleby box with said hole:

http://www.tooled-up.com/Product.asp?PID=121321&Referrer=froogle&gclid=CI_UzYiLiq8CFWwntAod0lNg9g

 

[Lectrician]

Most 16mm flush metals have a pre-punched hole in the top, with rolled edge. No grommet required. If you pop-out any other of the knock-outs, you would need to use a grommet - a 20mm grommet will fit the oval hole.

 

[MarkSmith]

Most 16mm flush metals have a pre-punched hole in the top, with rolled edge. No grommet required. If you pop-out any other of the knock-outs, you would need to use a grommet - a 20mm grommet will fit the oval hole.

Thanks. Yes, using pre-punched hole (which 20mm grommet doesn't fit).

 

[17thman]

you could insert a matchstick or 2, where needed, to ensure the cable is not in touch with the metal and squeeze a small amount of silicon into the remaining aperture.

 

[ebee]

Yes, they do!

To avoid doubt, the best thing would be to post a picture of the box in question.

Appleby have what is described as a "smooth cable entry hole" in their catalogue.


Although this is not a link to their catalogue, it shows an Appleby box with said hole:

http://www.tooled-up.com/Product.as...ogle&gclid=CI_UzYiLiq8CFWwntAod0lNg9g[/QUOTE]

Thanks for that secure - I stand corrected

 

[kevindgas]

Most 16mm flush metals have a pre-punched hole in the top, with rolled edge. No grommet required. If you pop-out any other of the knock-outs, you would need to use a grommet - a 20mm grommet will fit the oval hole.

Thanks. Yes, using pre-punched hole (which 20mm grommet doesn't fit).

i just cut the grommet to fit

 

[JohnW2]

If you want to use a grommet and cannot find one to fit, things like these are widely available.

Kind Regards, John.

 

[ebee]

Thing is , if it is the pre punched hole with a rolled edge, then no grommet required.

If you use one of the others then you remove the knock out and add a grommet because the edges are "near sharp".

The rolled edge on one pre prepared hole only appears to be available with 16mm boxes hence elongated slots for holes rather than round holes - unless someone knows different?

Having said that I don`t like 16mm (or "plasterdepth" boxes as they are known) and got rid ofa load of them years ago.

 

[MarkSmith]

To avoid doubt, the best thing would be to post a picture of the box in question.

http://www.wickes.co.uk/1-gang-metal-box/invt/205261/

So the one I have has the same rolled metal hole as the Appleby one.

Thanks all.