Grommets in back boxes

Question

I am adding some extra sockets to the ring main in my bedroom. I have done this before and used open grommets in the back boxes to protect the cables. However I've seen in one diy book that we should only use blind grommets and cut a hole in them just big enough for the cable - apparently this is to hinder fire / smoke spreading in event of a fire. Is this correct?

Mike · Accepted Answer

No there is no need for any grommets at all. The NICEIC and other bodies probably lobbied by Niglon, one of the larger manufacturers of grommets to insist their members fit grommets. Clearly once the cables have passed through the KO box hole with both outer sheath and core conductor sheaths in tact, those cable are never going to move again so no friction, so no abrasion, so no problem.

Nicholas · Answer

Grommets are the least of your worries. 

What you will need is the best part of a grand for a multifunction tester. You need this so that you can carry out the tests described in BS7671 on the circuit you intend to alter to ensure it condition is suitable for the planned alterations. These tests included continuity of conductors, insulation resistance and earth fault loop impedance to name a few.

Then, unless your DIY book explains how, you probably should fork on course that will give you some sort of knowledge to interpret the results you get. I recommend the City & Guilds 2391.

Then after the work is complete you'll have to test the circuit again to ensure it still meets the requirements of BS7671.

It may also be prudent to check with your house insurance provider to make sure you policy covers damage to property from DIY electrical work.

I reccomend getting a registered electrician who will be able to test and certify the work and also know which grommets to use.

I hope this helps!

Jamie · Answer

Hi mark,
No open grommets are just fine. A blind grommet would have very little effect on a metal back box suppressing smoke or fire.

Martin · Answer

The grommets are to stop the cables from being damaged. If there is a fire in a socket a rubber grommet won’t be much help(as opposed to a fire rated grommet which is not usually used) 
So just a standard open grommet.

Mick · Answer

Use open grommets to suit the hole size , they are there for cable protection not to stop fire spread. If you are using in solid walls they will be plastered around, otherwise use drylining boxes for plasterboard walls. I would also get a electrician to fit them as they will need testing

Albie · Answer

I can see where theyre coming from if it's a cavity wall, if it's a solid wall then plaster will stop any smoke or fire spreading. Also the mcb will trip due to bad connections which make the wires heat up in the first place 

I've never used them as I always take extra care with connections

Chris · Answer

Always use grommets as extra protection, as for open or closed grommets, I always use open. 

Which ever you use if there was a fire the rubber is going to melt, logically closed grommets would create more smoke due to more rubber. 

Remember the only reason me use them is to prevent cables making the back box live.

Nigel · Answer

False!! This only applies at the consumer unit.  Open grommets on all accessories are fine.  Also if I’m doubt of regulations you should be getting an electrician to do the work. You do realise your voiding any and all house insurance you have.

Aidan · Answer

Grommets are not intumescent meaning they would simply melt if there was a fire in a socket. So open grommets are fine.

James · Answer

The wiring regs only state that the cable must be protected as it enters the box. Installing blind grommets, to me doesn't seem right as a fire or smoke protection, as this could escape from the sides of the socket.

John · Answer

Using blind grommets is not necessary but good practice. Usually both cables will come into the socket through 1 knockout, by using blind grommets you would have to use 2 knockouts. It is neater,quicker and more reasnable to use open grommets especially if you're keeping the socket on the ring.

Gary · Answer

All depends on what type of wall they are going into, if its a brick wall then plastered in, then I wouldn't bother, I don't know or seen any other electrician do it, and personally I never have done it, never heard of that one!!

However on dry lined walls between rooms and all internal dry lined walls, metal dry lined boxes with in-tumescent pads fitted in the back MUST be used, I use them whenever I can, they are expensive for what they are, it is in the regulations, but not many electricians use them as they should!!

Richard · Answer

Think about it, if there is a fire where would it spread to? The fire retardation method used depends on the nature of the install. It your back box in buried in concrete then the fire would have to travel through the capping or out the side of the faceplate that is fixed/sealed aganst the plastered wall. If it is in studded wall then fire would need to travel and spread passed several sealed cavities. Now if you are doing this is HMO buildings then you could use blind grommets or a fire retardant sealant as used in apartment blocks. The regs states how large a cavity can be before sealing is needed so check the regs and not the DIY book. The books method isn't invalid, however, consideration has to be given as to application.

Nathan · Answer

Hi Mark,

As a minimum use a normal open grommet, in my experiences the blind fire rated grommets are more often used for cable entey on the consumer unit. I am unsure if a fire rated grommet is required at the end of the line as such in the backboxes, but the use of grommets is both good practice and i believe it can be flagged as an issue if an electrical installion condition report takes place, dont quote me on that last part though.

Regards
NJ

Grommets in back boxes

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