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Old 24th Jul 2021, 7:35 pm   #1
Julesomega
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Default Faulty mains plug?

A visitor here recently was impressed with the length of hedges around the garden, about 150yds, and offered a telescopic pole hedge trimmer to save me climbing on steps to reach the tops at about 8', said they couldn't use it due to arthritis. I don't know if they had ever used it, but it looked really useful so today I plugged it in, whereupon the thermal cutout on the extension reel tripped. I reset it and tried the cutter which worked just brilliant. Tried another cut and the cutout tripped again so I reset it and tried again. This time there was no response, the motor would not run

The Screwstation catalogue has a really nice one for about 60, but before I got the car out I took a few screws out of the gift unit and the switch worked fine and the Class-X capacitor was ok (measured 1.0MΩ) so I measured the connection from the plug to the switch. Open circuit! Easy fix, just replace the fuse! As I was holding the continuity tester I prodded it across the Bussman 13A fuse, which was perfectly ok

Must be the cable fractured just outside the plug, we've all seen that. Still holding the continuity tester I quickly disproved that theory but went on to find an O/C between the 'L' pin of the plug and the internal fuseholder

Is this a common problem with moulded-on plugs? Anyway, it was then cut off and an elderly Duraplug was fitted and now it all works fine. If I were really interested I'd test for intermittent shorting between the Live and Neutral pins when the plug is waggled

I'll save my trip until I need some more heavy duty plug tops, when I'll be down to Toolstation who do some very nice modern ones
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Old 24th Jul 2021, 7:45 pm   #2
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Default Re: Faulty mains plug?

That is not the first time I have heard of that happening as it happened to me too.

I had a plug fail with a fault between the fuseholder and the live pin, it was on a kettle and the side of the plug melted exposing live metal.

The socket which the plug was inserted into needed replacing too.
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Old 24th Jul 2021, 8:18 pm   #3
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Default Re: Faulty mains plug?

The mains lead for this laptop computer failed with an open-circuit live wire. I cut it in half, found the half with the socket to fit the PSU was the good bit, so put a Duraplug on that and have had no further problems.

I would be worried that the earth lead might fail and you'd not realise it until it was too late. Since this can happen at any time, an annual PAT test might not be enough. Nor would testing it everytime you start using it (as if anyone does that). Is it any surprise that I will not use moulded mains leads for anything high-power, safety critical, or similar?
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Old 24th Jul 2021, 8:22 pm   #4
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Default Re: Faulty mains plug?

I’ve seen a few fail at the strain relief part. You could try stripping the live and see if the core pulls out, if it does then the length removed can be used to work out where the failure has occurred.

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Old 24th Jul 2021, 8:23 pm   #5
Dave Moll
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Default Re: Faulty mains plug?

Quote:
Originally Posted by TonyDuell View Post
I would be worried that the earth lead might fail and you'd not realise it until it was too late.
Looking at the images of the amputated plug, there doesn't appear to be an earth lead to fail.
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Old 24th Jul 2021, 8:25 pm   #6
TonyDuell
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Default Re: Faulty mains plug?

Maybe not for the hedge trimmer (most such tools are double-insulated) but my laptop mains lead has an earth wire. So do a lot of other things around here that I have replaced cable and plug on before they failed.
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Old 24th Jul 2021, 8:28 pm   #7
tony brady
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Default Re: Faulty mains plug?

the plug is not exactly overflowing with safety approval marks either apart from a smudged BS number
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Old 24th Jul 2021, 8:52 pm   #8
Julesomega
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Default Re: Faulty mains plug?

@W-S, I/m glad I'm not tho only one to have seen this, I sometimes question my own sanity...

Quote:
Originally Posted by Julesomega View Post
Is this a common problem with moulded-on plugs? Anyway, it was then cut off and an elderly Duraplug was fitted and now it all works fine. If I were really interested I'd test for intermittent shorting between the Live and Neutral pins when the plug is waggled
I see that was inadvertently ambiguous: The excised plug has been binned without testing by wiggling, leaving a reliable replacement is service. The missing Earth wire is one less thing to worry about
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Old 24th Jul 2021, 9:43 pm   #9
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Default Re: Faulty mains plug?

Yes, this is happening ever more so with moulded on plugs.
The font in which FUSED is written on that plug is one that rings caution bells straight away with me.
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Old 24th Jul 2021, 10:42 pm   #10
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Default Re: Faulty mains plug?

I've had the neutral on a Vax vacuum cleaner fail at the plug. I'd noticed that the plug felt warm for some time then when it failed checked the continuity of the cable. Cut the plug off and fitted a new 13A plug and continued with the vacuuming.

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Old 25th Jul 2021, 1:12 pm   #11
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Default Re: Faulty mains plug?

Quote:
Originally Posted by TonyDuell View Post
Is it any surprise that I will not use moulded mains leads for anything high-power, safety critical, or similar?
It's no surprise at all, I have had problems with a moulded plug on a 3KW heater, it started making a burning smell before overheating the 13A fuse, that stopped the overheating!
It will eventually have a nice long 3 metre cable fitted with an old skool MK plug (binding post type), these plugs can happily supply full load without combusting.
In due course, I will be buying a box of 10 rubber type plugs, these will be useful for certain garage stuff, think drop & smash!
It's still surprising to find 0.75mm cable used on 2KW heaters, the plug fuse could do with being max 10A at best of times, 5A preferable.
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Old 25th Jul 2021, 2:48 pm   #12
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Default Re: Faulty mains plug?

I saved a cable because it had a bullet switch on it.
The plug was broken and not made of solid plastic.
This allows us to share how the wires are connected to the pins and fuse holder.
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Old 25th Jul 2021, 5:51 pm   #13
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Default Re: Faulty mains plug?

I'm rather surprised to see a traditional screwed cord-grip in a molded plug.
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Old 26th Jul 2021, 1:11 pm   #14
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Default Re: Faulty mains plug?

I've had a couple of cases where moulded-on plugs fitted to power-tools and 'industrial' extension-leads have failed in the way you describe - it seems to be associated with one of the pins being subjected to a healthy whack against something solid which snaps the thin part of the pin where it's fixed firmly to the plug-body. The insulation on the pin-shaft is sufficiently flexible to deform a bit if the pin gets whacked, but the inner end/terminal is fixed more-firmly.

Brass is brittle!

For outdoor/industrial stuff I much prefer the nice blue 16/32A 'Commando' plugs/sockets. Much safer, though even they are fatally damaged if you inadvertently drive the car over one!
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Old 26th Jul 2021, 3:39 pm   #15
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Default Re: Faulty mains plug?

When I was at university in Birmingham in the 1990s, there were always loads of pristine MK Toughplugs to be salvaged off scrapped medical equipment in the (QE) hospital skips.

The reason for their fitment to everything, apparently, was that hospital policy dictated that moulded-on plugs were unreliable and couldn't be visually inspected internally, so they had to be chopped off and replaced with a "traditional" plug.

How daft, I thought! Moulded plugs never go wrong! I also assumed that the wires would have been soldered to the terminals. But this forum has taught me otherwise.

Either way, 25 years later, I'm still using (and re-using) the plugs I salvaged, many with "Hospital Property" embossed into their tops.
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Old 26th Jul 2021, 4:41 pm   #16
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Default Re: Faulty mains plug?

Yep, have replaced the plugs on many of the vacuums at work at some point, and they are decent quality Numatics, had to do the same with our hetty as well.
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Old 26th Jul 2021, 6:25 pm   #17
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Default Re: Faulty mains plug?

A while ago I had the whole-house RCD trip in the middle of the night.
I traced it to the kitchen ring and finally to the "Lec" fridge/freezer, It was now 3am so I ran an extension lead to a socket on a different circuit so it could wait till the morning, expecting it to be the compressor or something equally terminal.

But no, in the light of day there was about 100k of leakage between the earth and live pin of the plug even with the plugs fuse removed. The plug and cable looked substantial and good quality, nevertheless out came the snips and one of my stock of MK plugs with the Granada logo on the cover is now in its place.

Google told me that this problem of leaky mains plugs is not unknown on Lec and some other appliance makes.
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Old 26th Jul 2021, 6:37 pm   #18
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Default Re: Faulty mains plug?

I also had a rewireable Leco? plug fail on a microwave, the plug started smoking and putting a meter on it showed leakage between the live and earth? pins even with no wires attached to the plug.
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Old 26th Jul 2021, 7:28 pm   #19
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Default Re: Faulty mains plug?

Quote:
Originally Posted by The Philpott View Post
Yes, this is happening ever more so with moulded on plugs.
The font in which FUSED is written on that plug is one that rings caution bells straight away with me.
Dave
There is also no BS1363 approval stamped.....in fact there don't appear to be ANY approval stamps.....
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Old 26th Jul 2021, 8:41 pm   #20
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Default Re: Faulty mains plug?

The only major fault I usually find with these type plugs is the wire breaking where it goes into it, it's supposed to be a strain relief but acts as a strain breaker instead.
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