Why do junction boxes have to be screwed down?


Postby cycleboy » Thu Apr 21, 2011 5:31 pm

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]
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Postby ericmark » Fri Apr 22, 2011 11:02 am

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.
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Postby cycleboy » Fri Apr 22, 2011 9:38 pm

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.
cycleboy
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Postby ericmark » Sun Apr 24, 2011 8:38 am

That is normal and not a problem.
ericmark
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Location: Mold, North Wales.


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