Change junction boxes for "maintainence free"?

[JohnW2]

I've never seen the problem but then I've never looked very hard. I was hoping EFLImpudence would share his wealth of experience with us

Quite - that's why I asked him (as I said, I personally have never seen it, either) - so I'm waiting with bated breath for a reply from him, or from any other electrician who is able to offer us the benefit of their experience!

Kind Regards, John

 

[maltaron]

It has not been mentioned that the power thro' the box is effectively vibrating at 50 Hz, which could possibly loosen the screws. However, I have not heard of it. What I have seen is Junction box screws coming loose on high power audio installations, where they are subject to wide variations of frequency.

 

[ekmdgrf]

It ahs not been mentioned that the power thro' the box is effectively vibrating at 50 Hz, which could possibly loosen the screws. However, I have not heard of it. What I have seen is Junction box screws coming loose on high power audio installations, where they are subject to wide variations of frequency.

That's an interesting perspective. I'd hazard though that in a high power audio installation, you have the vibration caused by the sound output rather than the signal?

I think the mechanical movement caused by a flow of electrons would be extremely limited due to their tiny mass. I wonder though if the oscillating magnetic field could induce a certain amount of movement?

 

[JohnW2]

I think the mechanical movement caused by a flow of electrons would be extremely limited due to their tiny mass. I wonder though if the oscillating magnetic field could induce a certain amount of movement?

Indeed. It obviously would depend crucially on how the various conductors were arranged within the JB, but I imagine that with some arrangements of conductors there could be appreciable magnetic effects acting on them, particularly at high currents.

Kind Regards, John

 

[EFLImpudence]

I wasn't saying the problem is common but, if it does occur, I think it far more likely that the conductors have become warm from the current, rather than the weather, and expand, however minutely, and, because the can only go sideways, when they cool they will be looser and a slightly different shape.
They won't force the screw to undo will they?

Has no one ever found a screw in a box or socket that can be tightened a bit more?
Whether this is because of the above or the installer being weaker, how could we know?

I suppose it should not happen to the cpc terminals as much but I've never taken much notice.

Shower switches with high current seem to suffer more and once it starts it gets worse.

 

[JohnW2]

I wasn't saying the problem is common but, if it does occur, I think it far more likely that the conductors have become warm from the current, rather than the weather, and expand, however minutely, and, because the can only go sideways, when they cool they will be looser and a slightly different shape.

I wasn't suggesting that you had said that it was common, but I was genuinely interested in discovering how often you (or any other electrician) actually encounter real 'problems' as a result of screw terminals in JBs becoming loose - as I said, in my very limited experience, I cannot recall ever having seen a case. To keep this in context, can you remember roughly how long ago you last saw a case of a serious problem (serious overheating, or even a fire) in a traditional screw-terminal JB?

I agree with what you say above about a theoretically possible/probable mechanism.

They won't force the screw to undo will they?

I wouldn't have thought so. However, as I said before, once the conductor has become loose (maybe/probably by the mechanism you mention above), the screw will more-or-less inevitably also be loose - and hence presumably at risk of 'rattling' even looser.

Has no one ever found a screw in a box or socket that can be tightened a bit more? Whether this is because of the above or the installer being weaker, how could we know?

I would imagine that most, if not all, of us will have experienced that but, as you say, one will never know for sure whether that has 'happened' or whether it was 'always like that'. However, although I've seen plenty of examples of that, I don't think I've ever seen a case in which there was any evidence (signs of overheating etc., let alone more) that any 'problem' had resulted. [FWIW, I would add that, as I've mentioned before, whilst I accept that they are not usually 'simple screw terminals' in the normal sense, if I do up MCB/RCD etc. terminals to the manufacturer's recommended torque, I can nearly always tighten it a fair bit more by hand. In other words, 'left to my own devices' I would probably tighten more than the manufacturer recommends]

I suppose it should not happen to the cpc terminals as much but I've never taken much notice. Shower switches with high current seem to suffer more and once it starts it gets worse.

Yes, (although a bit tangential to the context of this discussion, which is really about non-MF JBs) the seemingly fairly common problem with shower switches (which, again, I've never personally seen) would be consistent with your postulated mechanism and, as you say, it would be further supported if you found that loosening was more common with live conductors than with CPCs. Mind you, psychology/human nature might work against you here, since there could well be a subconscious tendency not to tighten CPCs so conscientiously or 'vigorously'!

Kind Regards, John

 

[333rocky333]

I was genuinely interested in discovering how often you (or any other electrician) actually encounter real 'problems' as a result of screw terminals in JBs becoming loose

Since apprenticeship most of my career has been fault repairs, I think i have found poor connections in every accessory going, but cannot recall ever one in a traditional J/b,

I assume its because there quite generous with the brass and the screw usually fills the terminal

However my freind wired a light and when tested there was no supply and turned out one of the M/F connecters he pushed on was the cause, first time he had used one so possibly the end was stripped too short

 

[JohnW2]

Since apprenticeship most of my career has been fault repairs, I think i have found poor connections in every accessory going, but cannot recall ever one in a traditional J/b, ... I assume its because there quite generous with the brass and the screw usually fills the terminal ... However my freind wired a light and when tested there was no supply and turned out one of the M/F connecters he pushed on was the cause, first time he had used one so possibly the end was stripped too short

Many thanks. That's what sort-of concerns me a little. Despite the theoretical problems of screwed connection in JBs, on which is largely based the worries about 'non-accessible' ones, I have a distinct suspicion that real problems/incidents due to non-accessible traditional JBs may be incredibly rare. If that's true, it would make them a fairly 'hard act to follow', such that it would not take much by way of a 'problem rate' with MF JBs (particularly, as you imply, in the early days whilst people get used to using them correctly) for the well-intentioned (or was it 'knee jerk'?) requirement for inaccessible JBs to be MF to actually result in a net reduction in safety.

Kind Regards, John

 

[piggoy]

Sorry to reignite this old thread. Has anyone considered using threadlock/Loctite on the threads of the grub screw in traditional JBs?

 

[dhutch]

I was genuinely interested in discovering how often you (or any other electrician) actually encounter real 'problems' as a result of screw terminals in JBs becoming loose

Since apprenticeship most of my career has been fault repairs, I think i have found poor connections in every accessory going, but cannot recall ever one in a traditional J/b,

However my friend wired a light and when tested there was no supply and turned out one of the M/F connectors he pushed on was the cause, first time he had used one so possibly the end was stripped too short

Well that is clearly the counter argument.

- The trad junction boxes have been issue free for years, and most likely will for years to come.
- Where as the M/F boxes will be new, and there is a real chance for early-life failures to occur.

Bathtub curve anyone?


Daniel

 

[dhutch]

Sorry to reignite this old thread. Has anyone considered using threadlock/Loctite on the threads of the grub screw in traditional JBs?

No.

 

[JohnW2]

Sorry to reignite this old thread. Has anyone considered using threadlock/Loctite on the threads of the grub screw in traditional JBs?

I don't think it would really help in a lot of cases. As I understand it, it's not usually a case of the screws 'working loose' (within the threads, e.g. due to vibration) but, rather, of the copper of the conductors 'creeping' and hence causing the contact between conductor and screw to deteriorate ('manifested' as a 'loose screw', but not because the screw had moved relative to the thread). Locking the screw would obviously make no difference if/when that were the mechanism.

Kind Regards, John

 

[JohnW2]

Well that is clearly the counter argument.
- The trad junction boxes have been issue free for years, and most likely will for years to come.
- Where as the M/F boxes will be new, and there is a real chance for early-life failures to occur.

Quite - that's exactly the point I made in response to Rocky's comment...

Many thanks. That's what sort-of concerns me a little. Despite the theoretical problems of screwed connection in JBs, on which is largely based the worries about 'non-accessible' ones, I have a distinct suspicion that real problems/incidents due to non-accessible traditional JBs may be incredibly rare. If that's true, it would make them a fairly 'hard act to follow', such that it would not take much by way of a 'problem rate' with MF JBs (particularly, as you imply, in the early days whilst people get used to using them correctly) for the well-intentioned (or was it 'knee jerk'?) requirement for inaccessible JBs to be MF to actually result in a net reduction in safety.

Basically, we just don't know. It is possible that MF JBs will prove, over the next few decades, to be associated with even less problems than traditional ones, but since the traditional ones are 'such a hard act to follow', it is also possible that they will prove to have been a step in the wrong direction. If any of you are still around 'in a few decades' time (unfortunately, I won't!), you'll probably then know the answer!

Kind Regards, John

 

[dhutch]

If any of you are still around 'in a few decades' time, you'll probably then know the answer!

Looking forward to it already!

The only clear thing they do offer, which a conventional box does not, is strain relief on the cable.


Daniel

 

[JohnW2]

The only clear thing they do offer, which a conventional box does not, is strain relief on the cable.

That is true - but it is a feature which obviously could have been added to 'traditional JBs' (very few of which have strain relief) without changing the nature of the terminals. That would have been one definite improvement (with no potential downsides) which didn't involve introducing any (long-term) 'unknowns'.

Kind Regards, John