I may have found the answer

[phil d]

I was thinking today about loose connections and the consequences,and how nobody seems to know the cause,then something I'd seen fairly recently popped into my head.I'd been somewhere and they were having electrical work done,now like a lot of others I like to watch how people go about things,actually people watching has been a hobby of mine for quite a while,Anyway this young spark(mid 20's) is fitting a CU,he feeds the cables through the back then using his impact screwdriver proceeds to screw it to the board.OK so far,then after stripping the cables back he proceeded to connect to the terminals using his trusty impact driver!Now a thought occured to me at this point,either someone showed him that neat little trick,or he worked it out for himself and has possibly shown others.I know a lot of the younger guys love their power tools,many's the time I've worked with young lads and had to fit say a ceiling rose. "hang on I'll get my impact driver " says he,he scurries off to get it only to find on his return that I have managed to drive 2 inch and a half 8's in using a normal screwdriver.If we take the average tool as having a max torque of around 150Nm threads could easily be stripped without knowing,it's an interesting possibility wouldn't you say?

 

[Manator]

It is a very valid point phil d

 

[phil d]

It is a very valid point phil d

Yes I was discussing it with Tony and we both wondered if anyone had actually examined the threads on things such as CU's when they find one that has overheated,after all overheating wouldn't cause the thread damage as overtightening would.

 

[Marvo]

It's pretty much guaranteed he's going to strip screws sooner or later. If you even look at the poorer quality neutral bar screws funny they'll strip so I'm not even sure how he could be using an impact tool on those.

 

[kerching]

https://www.youtube.com/embed/CmiapKXYs4w?feature=oembed

Just torquing

 

[phil d]

It's pretty much guaranteed he's going to strip screws sooner or later. If you even look at the poorer quality neutral bar screws funny they'll strip so I'm not even sure how he could be using an impact tool on those.

I have to admit I was so amazed and fascinated that I completely forgot I had my phone on me,a picture or video would have been nice.

 

[Sharpend]

2 minutes ago, kerching said:

https://www.youtube.com/embed/CmiapKXYs4w?feature=oembed

:slap :slap  

 

[ProDave]

Yes I was discussing it with Tony and we both wondered if anyone had actually examined the threads on things such as CU's when they find one that has overheated,after all overheating wouldn't cause the thread damage as overtightening would.

I found an overheated neutral bar on an EICR. It hadn't caught fire, but it had melted and doscoloured the plastic.

Before I replaced it (yes I had a spare N bar and the plastic clips in my box of odds and sods, that was lucky) I tried to analyse what had gone wrong.

The connections that got hot were still tight. The screws still turned, but they were tight. None of the cables moved in the termonal block, everything seemed sound and tight, yet it was clearly overheating.

This particular CU was one where the entire N current goes through one neutral bar, where it splits to two feeds to the two halves of the split load CU, it was that bit that was overheating. This is the common arrangemernt in high integrity boards to allow you to have non rcd protected circuits as well.

My only conclusion was in fact that the neutral bar is simply not capable of carrying the full load current of the entire CU.  Blaming poor installers for not tightening it, and sticking a tin box around it just seems to be side stepping the obvious poor engineering.

Here's my radical solution to the problem.  Make the neutral bars twice as wide, and give them two screws per termination. I would put money on that solving CU fires.  but of course that's far too logical to think like that.

Think about it. Look at a henley block. That's what one designer thinks is necessary to properly terminate a cable that can carry up to 100A.  then look at a CU N terminal block. that is another "designers" idea of a termination capable of carrying up to 100A.  Which one of them do you think got it right?

Do I smell a conspiracy theory?

 

[Tony S]

Here's my radical solution to the problem.  Make the neutral bars twice as wide, and give them two screws per termination.

It’s been tried before but didn’t last long.

 

[kerching]

Take a close look at some of the neutral bars in these modern CUs....there is more "hole than brass"

 

[Rob_the_rich]

Do I smell a conspiracy theory?

is it a fishy smell?

 

[Rob_the_rich]

Take a close look at some of the neutral bars in these modern CUs....there is more "hole than brass"

apparently, brass has only 28% the conductivity of copper. There is the CSA of the neutral bar which can easily be worked out, then minus the hole diameter. The screw is irrelevant. The size of the cable in it does matter though, so bigger cables should be nearer the upstream side.

 

[Andy™]

I found an overheated neutral bar on an EICR. It hadn't caught fire, but it had melted and doscoloured the plastic.

Before I replaced it (yes I had a spare N bar and the plastic clips in my box of odds and sods, that was lucky) I tried to analyse what had gone wrong.

The connections that got hot were still tight. The screws still turned, but they were tight. None of the cables moved in the termonal block, everything seemed sound and tight, yet it was clearly overheating.

This particular CU was one where the entire N current goes through one neutral bar, where it splits to two feeds to the two halves of the split load CU, it was that bit that was overheating. This is the common arrangemernt in high integrity boards to allow you to have non rcd protected circuits as well.

My only conclusion was in fact that the neutral bar is simply not capable of carrying the full load current of the entire CU.  Blaming poor installers for not tightening it, and sticking a tin box around it just seems to be side stepping the obvious poor engineering.

another thing with that, the bar that links them is usually round and stiff, so when its tight, it only has a very small area actually in contact with the bar at the back and the screw at the front

 

[ProDave]

It’s been tried before but didn’t last long.

In what way didn't it last long?  Did they have the same problem or did the manufacturer just give up trying that?

I note that CEF are pusing their own brand of CU's on the basis the terminals into the main swich have two screws. They would get a lot more credability with me if they did the same to their neutral bars.

another thing with that, the bar that links them is usually round and stiff, so when its tight, it only has a very small area actually in contact with the bar at the back and the screw at the front

And that situation is even worse on some makes of CU that have solid bars as the interlink conductors. Then you really do just have two small points of contact to the neutral bar.

 

[phil d]

In what way didn't it last long?  Did they have the same problem or did the manufacturer just give up trying that?

I note that CEF are pusing their own brand of CU's on the basis the terminals into the main swich have two screws. They would get a lot more credability with me if they did the same to their neutral bars.

And that situation is even worse on some makes of CU that have solid bars as the interlink conductors. Then you really do just have two small points of contact to the neutral bar.

Yes I've never liked the solid bar idea personally,ok busbars are solid but then they have bolts through them.I think there are too many potential things to go wrong with the design of a modern CU,put them together with poor installation methods and you have a disaster waiting to happen.

 

[Tony S]

In what way didn't it last long?  Did they have the same problem or did the manufacturer just give up trying that?

I was being sarcastic, it lasted very well.

 

[ProDave]

I was being sarcastic, it lasted very well.

Ah yes. Funny how wood never caught fire isn't it.

 

[Blue Duck]

It's very hard wood, a lot harder to ignite than say.... non combustible plastic? 

Those woody's stood up to a lot of tinkering by the unskilled and I'm not talking 5ww either. decorators, plumbers DIY twats and Del from daan tha pub.

 

[Tony S]

In the days of coal fires I’d got a Wylex board that had been in the wars and destined for the bin. I may as well get some heat from it and so chucked the wood on the fire back, three hours later you could still see the frame.

I don’t know what the hell they used but it was damn good.

 

[Evans Electric]

Wasn't it always verbotten to use any kind of power driver on boards ?

Have to say I use a small B&D screwdiver to run the screws either out or in , but finish them all off with the mad Modulo driver. 

A few things contribute to loose terminals I suppose:

What has changed since the 16th edition when they were deemed NOT to start fires.

1)   Training is often at a college now instead of hands on, assisting  qualified sparks every day.

2)   The short course  trainee with no previous engineering type experience.

3)   Youths whose wrists are worn out after a lifetime of texting their mates 1000 times a day.

4)   Fitting meter tails , single terminal screw only, THEN twisting & bending them to the meter ...then NOT returning to the first screws  .

5)   Consumer units are no longer made in Britain .

I leave space for more.