Why do junction boxes have to be screwed down? Ask questions and find answers to many subjects relating to electrics and electrical work

[cycleboy]

I've read that the new regs stipulate that the old fashioned junction boxes (ie without strain relief clamps) must be attached to something, like a joist. Why?

Now, I can understand that one might be worried about live wires if you pulled a cable and it came free from the box. But surely, this is [b]more[/b] likely to happen if the box is fixed than if it can move about?

Furthermore, if you were to pull on a cable that is connected to a floating box with cable clamps, you are again [b]more[/b] likely to transfer that pull onto another cable. The result of this would be to run the risk of any cable coming free somewhere else, with the problem of then having to track that distant point down.

I can imagine that a frequently moving cable might cause a terminal screw to work loose, which is obviously not a good thing to happen, but what are the chances? After all, junction boxes are not usually accessible to inquisitive fingers. Besides, any flexing is just as likely to cause the copper conductors to fracture as work a terminal screw loose.

So, what am I missing? What is the reason for this regulation?[/b]

[ericmark]

The regulations are written to allow for all situations. In the main they don't say you must use a junction box with cable restraints but say it must comply with BS EN 60670-22 or BS EN 60947-7 and if anyone bought all the regulations to see what was in BS EN 60670-22 or BS EN 60947-7 then they would have a lot more money then me.

It is hard to find regulations as in book from there is no search button and often the regulation is hidden among other bits.

For example 434.2.1 refers to protective device and distance and effectively limits the length of a spur to 3 meters. But unless you were directed to this regulation likely you would miss it as it comes under "
434 PROTECTION AGAINST FAULT CURRENT" not ring final circuits.

Having many times either found there is not a regulation or the person has miss read the regulation I now always ask where does it say it. Often there is something allowing it as long as something is done. For example a socket marked "Fridge" and supplied with Ali-tube cable does not need to be RCD protected. But often electricians will say "All sockets not need RCD protection" clearly not all but nearly all.

In the main one would clip a cable into and out of the old type round JB and also screw down the JB because it lacks any cable clamp. Because the plastic is so easy to brake in the main these have been replaced with the maintenance free type. These do have cable clamps and do not use screws for cables so can be covered up. I have seen electricians write on floor boards JB below but as soon as the carpet goes down one can't read that.

The inverters to supply extra low voltage are made to fit through the hole cut for the lamp and if fixed you would not be able to remove without lifting floor boards. So some common sense is required. On one hand fitting 50mm pods in a house will brake the distance regulations from combustible material yet the inverter is made so you can fit them. Again some common sense and using common sense you will never fit them anyway.

[cycleboy]

Thanks for the explanation, ericmark. I had a feeling it might be down to interpretation. I have Tricker's "Wiring Regulations in Brief" but find that finding information in it is about as easy as completing a Rubik's cube.

I've just replaced a pendant and ceiling rose with a ceiling mounted light. The rose had terminals and 5 cables whereas the new light just had a light socket, so I needed to use a junction box. Due to the low pitch of the roof and the light being right under the eaves, the access from above was impossible, My solution was to cut a large hole in the ceiling and simply posted the box through the ceiling and cover the hole with the new light. Obviously, there was no possibility of fixing the box anywhere.

[ericmark]

That is normal and not a problem.

[audreygreenwood]

I never really thought of asking this question before. For some reason at my house we've always managed to keep the junction box well hidden in storage amongst the other movings boxes and junk in the garage or the like and as long as it was easily accessible we never really worried to much about it. But I'm sure there's a proper reason for it to be secured down. Maybe to reduce tampering or something? Your guess is as good as mine...

[ericmark]

It has been three years since the post and we now have amendment 1 which I have not got.

The regulations were back in the 14th Edition full of recommendations but when the 16th came out and it became BS7671 then the recommendations were moved into a second book called the guide to the 16th edition and the guide is regarded as the regulations by many but it tries to use layman's terms and as a result is not always spot on.

So if you got a bit of wood and screed a junction box to it without strain relief then also put some cable clips onto the wood the combination would be the same as using a junction box with strain relief. The same applies if that lump of wood is fixed.

In the main a Junction Box with no strain relief uses screws to hold the cables in place and as such it has to be where it can be accessed to maintain it. Screws can with vibration and temperature variations become loose. So it will be where it could be caught up with items being moved and unless fixed could have cores without the sheave showing.

Most maintenance free junction boxes have strain relief built in. Using Wago or similar that is not always the case.

So back to the rules, the strain relief must be on the same device as it is re-leaving that strain for other wise clearly it does not work.