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Old 7th Sep 2018, 4:37 pm   #1
ekcopyephilips
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Default Neutral screw loose again

Hi All

I've just been renovating a motor from a Kenwood mixer, it's a later type motor with an electronic control board and a triac is used to control the speed of it.

When I had finished and went to replace the mains plug as was an old Duraplug rubber one with no sleeved L & N pins I noticed that the screw on the neutral pin was loose, not just loose but virtually screwed all the way out. It had almost certainly been screwed in at some point as there was a circular dent in the copper neutral conductor when i pulled it out from the pin.

So why does this phenomenon happen? I'm sure I've seen it before and its always the neutral pin that has the loose screw the live pin connection is always nice and tight. Anyone else ever seen this sort of thing?

Cheers

Mike
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Old 7th Sep 2018, 5:00 pm   #2
SiriusHardware
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Default Re: Neutral screw loose again

It happens with all three screws, for a while I was the official PAT tester here and one of the things I did was to check all non-moulded mains plugs for exactly this problem. I would usually find one or two loose on each run through.

As to how it happens, who knows? Maybe when the screw is first tightened the copper wire initially resists, but then over time sags / reforms / flows away under the constant pressure.

I've often assumed, worryingly, that the same thing is likely to happen to the screws keeping the mains wiring secured on the back of mains outlets.
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Old 7th Sep 2018, 5:00 pm   #3
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Default Re: Neutral screw loose again

I've only seen it when the ends were originally both tinned and just the L has been shortened to fit so doesn't suffer solder creep.
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Old 7th Sep 2018, 5:14 pm   #4
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Default Re: Neutral screw loose again

Not confined to neutral connections of course but there is a very well know phenomena associated with screw terminal connections. When you tighten a screw on a new terminal connection and then go back to it perhaps only a few minutes later the screw will tighten another half turn or so. This is because the circular cross section copper wire "slumps" or "flows" under pressure from the screw to fill the cavity in the terminal. If this second tighten is not done the connection can be vulnerable to loosening further from vibration or temperature effects over time.

I've not noticed the neutral terminal in a plug being more likely to come loose than the others and cannot imagine why that should be the case.
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Old 7th Sep 2018, 6:04 pm   #5
emeritus
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Default Re: Neutral screw loose again

This was also a problem with some of the early solid metric T&E installation cable that was flagged up in the correspondence pages of AFAIR the old IEE News. I subsequently discovered that the house my cousin moved to in the 1980's suffered from it when I went round to investigate why some of his 13A sockets didn't work. Everything, including the light switches, needed tightening. In a couple of cases, the socket came away leaving the wires behind. I think they must have changed the composition of the copper, as I have had no problem with my house wiring, installed in 1980.
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Old 7th Sep 2018, 7:26 pm   #6
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Default Re: Neutral screw loose again

Hi

Yes I've often noticed this, the live always seems to be a bit tighter than the rest.
When I used to install sets in the seventies, Philips always had tinned live and neutral wires to go straight into the plug. Bush however, just had a plain cut mains lead that you had to prepare yourself. There was a a note about this in the Bush technical magazine that was sent to dealers, they said 'it is generally considered bad practice to put tinned connections into a mains plug as the connections work loose relatively quickly'. Rank knew something Philips didn't!

SJM.
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Old 7th Sep 2018, 7:55 pm   #7
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Default Re: Neutral screw loose again

Definitely: the problem with tinning the 'tails' is that when fitted to screw-terminals you get a cold-flow effect in the relatively soft solder so the tension applied by the terminal slackens-off in a relatively short period.

If connecting multi-strand flex to any kind of screw-terminal the path-of-least-risk is to fit a 'bootlace ferrule' on to the flex, using a proper controlled-pressure ratchet tool appropriately rated for the conductor-size. Then fit the bootlace into the terminal.
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Old 7th Sep 2018, 8:01 pm   #8
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Default Re: Neutral screw loose again

There is always some small movement of the pins themselves in a conventional 13 amp plug when inserting/withdrawing the plug from the socket.
I would think that the neutral connection has a greater stress on it when plugging in and unplugging the plug, as the wire goes straight to the cable clamp where it is held firmly (hopefully), whereas the live pin terminal is mechanically decoupled to an extent, by the fuse and its mounting clips, from the pin itself.
This means that there is less stress and tendency to movement at the terminal, thus less disturbance of the clamping action of the terminal screw.
A kind of "mechanical decoupling"? is taking place on the 'live' pin. Tony.
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Old 7th Sep 2018, 9:32 pm   #9
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Default Re: Neutral screw loose again

Hi Gents, not a new effect, it was noted in the IET (IEE) "Electronics and Power" journal perhaps 20 years ago.
It was reported to happen much more frequently on the neutral connection than the live and on industrial/ commercial and domestic installation to a similar degree.
No satisfactory conclusion was ever reached.

Ed
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Old 7th Sep 2018, 11:27 pm   #10
MotorBikeLes
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Default Re: Neutral screw loose again

I recall this being discussed in TH mag 30 or more years ago.
I sold a Grundig Tanhauser table (big table) radio about 8 years ago. It appeared here last week, could I fix it, dead. Thermal resistor in the power supply. I was out of HMP solder, but ordered some, and fixed it yesterday. After finishing, I realised I had not checked the plug. I opened it up, "knew" it had not been touched since I sold it, but when I checked the screws, the neutral was in need of almost two turns. Live perfectly OK.
Les.
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Old 8th Sep 2018, 12:23 am   #11
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Default Re: Neutral screw loose again

This was discussed in the now defunct "Television" magazine in the 1990s. The general consensus of opinion was that, as suggested by Tony in post #8, the live connection is isolated from movement of the plug's pins by the fuse holder. I have encountered earth pin screws rattling round loose inside 13A plugs, but even with a 3 core mains lead this doesn't usually cause the appliance to malfunction.
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Old 8th Sep 2018, 1:14 am   #12
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Default Re: Neutral screw loose again

Does this mean that MK plugs and others with pins fixed in the plug body are (were?) less affected by this?
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Old 8th Sep 2018, 10:45 am   #13
G8HQP Dave
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Default Re: Neutral screw loose again

Could it be that the live screw was originally slightly tighter than the neutral, as everyone knows that the electricity comes from the live so it needs a really good connection?
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Old 8th Sep 2018, 8:31 pm   #14
MotorBikeLes
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Default Re: Neutral screw loose again

Not when I tighten them Dave.
For what it is worth, it is commonly known by some that the "live" is positive and the Neutral is negative.
This appeared more than a few times in TV mag.
I my previous, I meant TV magazine not TH magazine of course.
Les
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Old 9th Sep 2018, 1:01 pm   #15
hannahs radios
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Default Re: Neutral screw loose again

I read the same thing in television magazine as well, think it was a discussion in the letters page. One thing members here might know. Did the old 5 and 15 amp plugs suffer similar effects? I would think that with those it would effect live and neutral the same as there is no fuse to offer mechanical buffering.
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Old 10th Sep 2018, 2:10 pm   #16
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Default Re: Neutral screw loose again

It is perhaps more worrying that I had to replace a wall mounted switch for a storage heater as, over time, a screw had come loose and the high current flow had charred the contacts / surrounding moulding (with no external sign). I am not sure whether it was neutral or live, but that perhaps was less the concern. It's probably a pointer to the need for regular system checks.
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Old 10th Sep 2018, 2:28 pm   #17
G8HQP Dave
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Default Re: Neutral screw loose again

I believe the latest wiring regs now insist on screw connections only being used where they can be inspected (e.g. not under a floor). Other connections must use some form of spring.
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Old 10th Sep 2018, 2:55 pm   #18
Lucien Nunes
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Default Re: Neutral screw loose again

Indeed, only terminals rated 'Maintenance Free' to BS5733 may be installed where inaccessible. This includes pressure crimped and live-spring connectors such as the popular 'Wago', but not screw terminals that cannot compensate for cold flow. This is as much in response to bad installation technique and low quality materials as any shortcoming in screw terminals per se.

The subject of loose neutral connections regularly surfaces on electrical forums, usually with respect to installations rather than plugs. In these cases there is no obvious consistent structural difference between the two terminals as there is in a plug. The contributory causes that I propose / concur with are

a) Mechanical or thermal differences. Some devices have different styles of terminal that are more or less susceptible to slackening, or have different thermal loads on various parts. There may be an obscure similarity of differences that is not obvious from the outside (one contributor gave an example of double-pole switching being added to SP accessories) that result in a statistical bias.

b) Human effects. Many people reported tightening the live first (perhaps because conversational convention of 'Live and Neutral'), then the neutral, then checking both in quick succession. The settling time is likely to be less for the neutral so cold-flow less effectively followed up.

c) Reporting bias. A loose live connection is a loose connection. A loose neutral connection is another of those weird loose neutrals.

NB: We should strictly be calling the brown wire 'Line' these days as the definitions have changed over the years.
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Old 23rd Sep 2018, 10:06 pm   #19
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Default Re: Neutral screw loose again

Hi

I remember reading about this problem in Television magazine many years ago and have always been keen to find out what is happening. I agree that the neutral connection doesn't have the same mechanical buffering as the line/live connection, so would seem a possible reason for the problem. Movement of the neutral pin in the plug base also seems a possible reason.
Thermal effects in electrically heavily loaded plugs must also play a part. Also, on cheaper plugs with lower grade materials, conductivity may be less leading to localised heating which could worsen the situation.
I still think MK had the right idea with their robust screw connections. I don't recall seeing any problems with loose connections in those plugs.

Regarding access to inspect screw terminal connections, I assume moulded plugs don't use screw connections but some form of crimp? I've never pulled apart a moulded plug hence the question.

Regards
Symon.
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Old 23rd Sep 2018, 10:33 pm   #20
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Default Re: Neutral screw loose again

A protective leaf contact between the screw and the stranded wire may be of benefit. These are often used in better quality PCB mounted screw terminal blocks which help to avoid damaged to the wire strands.

Regards
Symon.
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