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Old 2nd Oct 2012, 12:44 pm   #1
ssaunders
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Default 13 amp plug question

I recently purchased a radio complete with a 1950-60's 13 amp plug fitted. It reminded me of a life long unanswered question: what was the small rectangular hole in the top of the plug (as though it were in the wall) near the earth pin for?
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Old 2nd Oct 2012, 12:50 pm   #2
newlite4
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Default Re: 13 amp plug question

I think it was there just to allow visual inspection of the earth connection without having to dismantle the plug.
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Old 2nd Oct 2012, 12:50 pm   #3
mike_newcomb
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Default Re: 13 amp plug question

Hi,

I have seen it used to connect an external (additional) earth for a device that that was missing such.

Regards - Mike
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Old 2nd Oct 2012, 8:39 pm   #4
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Default Re: 13 amp plug question

ISTR our 5A and 15A plugs had them too. My dad worked for the LEB (he started with MetEsCo after the war) and he wired our house in Herne Hill. No 13A stuff for him. He got staff discount on the bills so maybe he designed the wiring scheme for heavy use.
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Old 2nd Oct 2012, 11:46 pm   #5
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Default Re: 13 amp plug question

The purpose of the hole in old plugs has puzzled me, too.

The GEC catalogue for 1911-12 has this note on the pages for its "Factory" earthed plugs:

Quote:
The S679, S700 & S701 are provided with earthing connections on both socket and plug, the terminals for attaching the earth wire being fixed in a position on the outside where they can be seen, as called for in Home Office Regulations , Form 928, issued February 1910, Regulation 13, pages 33 and 34.
No earthed connectors for domestic use were listed at this date, but perhaps the requirement for the earth terminals of industrial connectors to be actally located on the outside to facilitate inspection was initially carried over in modified form to the early earthed domestic connectors. I used to have two almost identical 5A 3 pin surface mounted sockets to BS546 gauge. One, presumably the earlier one, had the earth terminal protruding beyond the lower rim of the case where it was clearly visible when installed, whereas the earth terminal of the other was located within the casing as per current practice. However, my experience is that, with old plugs, the hole adjacent the earth pin is much too small to allow the earth connection to be inspected properly.

Last edited by emeritus; 2nd Oct 2012 at 11:59 pm. Reason: typo correction
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Old 3rd Oct 2012, 1:21 am   #6
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Default Re: 13 amp plug question

It's not big enough to inspect the earth connection through, and the pin hole orientation makes added wires difficult. I always assumed it was for probing for some sort of test.

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Old 3rd Oct 2012, 9:14 am   #7
Mike Phelan
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Default Re: 13 amp plug question

I always thought it was as per Mike's post #3.
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Old 3rd Oct 2012, 9:59 am   #8
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Default Re: 13 amp plug question

I allways understood that the hole was to permit of visual inspection of the earth connection.
It was too small for a thorough inspection, but at least if the earth wire had completly pulled out this would be obvious.

Amateurs when connecting 3 core flex to a plug tend to cut all three cores to the same length, rather than leaving the earth longer.
Any excesive strain or loose cord grip would then leave the earth wire as the most vulnerable to pulling out.
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Old 3rd Oct 2012, 10:05 am   #9
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Default Re: 13 amp plug question

Unsurprisingly, this question crops up a lot here.

What is remarkable though, is that the definitive answer has never been found; the reason for its presence seems to have been forgotten, even though it's only a shade over 50 years that square-pin plugs were introduced.

Nick.
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Old 3rd Oct 2012, 1:12 pm   #10
kalee20
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Default Re: 13 amp plug question

Yes I'm surprised too that the answer is elusive.

It was by no means universal - I recall in the 1960's rubber plugs existed with covers which you had to thread onto the cable before connecting the wires to the pins. Such plugs had no little slot hole alongside the earth pin.
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Old 3rd Oct 2012, 2:49 pm   #11
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Default Re: 13 amp plug question

According to Wikipedia the hole was for inspection of the earth connection (see caption under picture on right), and was a requirement of the original 1947 version BS 1363. Presumably dropped in a later revision.
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Old 3rd Oct 2012, 3:00 pm   #12
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Default Re: 13 amp plug question

I don't know what you do in the UK, but in Aussieland with our mains plugs there are often slots next to each pin. This is so you can check the insulation colour of the wire to each pin and tell if the plug is correctly wired. (Green/yellow is earth, for example.)

You can also check that there actually is an earth and the cable is not of the two-wire type. In addition, you may be able to determine that the wires are correctly threaded past the strain-relief moulding. All without opening the plug.

Peter
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Old 3rd Oct 2012, 3:09 pm   #13
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Default Re: 13 amp plug question

Ah yes, the infamous "Duraplug". Most usually found with a slot rudely butchered into the tubular part of the cover by a hapless technician who discovered only after he's wired the plug up that he's not threaded the cover onto the cable.

Quote:
Originally Posted by kalee20 View Post
Yes I'm surprised too that the answer is elusive.
It was by no means universal - I recall in the 1960's rubber plugs existed with covers which you had to thread onto the cable before connecting the wires to the pins. Such plugs had no little slot hole alongside the earth pin.
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Old 3rd Oct 2012, 3:14 pm   #14
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Default Re: 13 amp plug question

Quote:
Originally Posted by dinkydi View Post
I don't know what you do in the UK, but in Aussieland with our mains plugs there are often slots next to each pin. This is so you can check the insulation colour of the wire to each pin and tell if the plug is correctly wired. (Green/yellow is earth, for example.)
We had those too, Peter, on one brand (Delta??) back in the eighties, but I haven't seen them for a long while. The modern equivalent seems to be the all-transparent plug which is apparently favoured by PAT testers (so the manufacturers say, anyway).
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Old 3rd Oct 2012, 3:17 pm   #15
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Default Re: 13 amp plug question

Quote:
Originally Posted by G8HQP Dave View Post
According to Wikipedia the hole was for inspection of the earth connection (see caption under picture on right), and was a requirement of the original 1947 version BS 1363. Presumably dropped in a later revision.
Well spotted, Dave. It's a shame historic BSs aren't available online, as they'd make interesting reading for us.

Nick.
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Old 3rd Oct 2012, 4:47 pm   #16
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Default Re: 13 amp plug question

There was also a plug available that had a little hole on the faceplate so you could see the fuse colour fitted, and then youd know what was in it ( red=3a etc)
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Old 3rd Oct 2012, 5:07 pm   #17
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Default Re: 13 amp plug question

I have seen them with a window below the fuse so that the value could be checked.
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Old 3rd Oct 2012, 7:20 pm   #18
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Default Re: 13 amp plug question

Quote:
It's a shame historic BSs aren't available online, as they'd make interesting reading for us.

Nick.
Nick,

I had occasion to try to get hold of a copy of the original version of BS 1363 in connection with a campaign against these unnecessary plug covers sold, allegedly, to make sockets safe (www.fatallyflawed.org.uk) where you will find quite a lot on the history of this standard.

I called BSI and was told that old versions of the standards are available but are chargeable, I think the price quoted was half the cost of the current standard. I'm afraid that my view is that the cost of standards is prohibitive and so counterproductive. It's something of an anomaly that statutes often quote British Standards and are freely available but the standards are made largely inaccessible to the general public. Even municipal libraries do not always keep a full range because of cost.

The Home Office regulations quoted by Emeritus should be in the National Archives if anyone lives nearby and is interested enough. The GEC catalogue, and other editions, are available in the IET (IEE) archives I believe.

PMM
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Old 3rd Oct 2012, 7:30 pm   #19
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Default Re: 13 amp plug question

I've got several rather-early-1970s VOLEX/TemCo plugs and one Ever-Ready with these little windows. Modern moulded-on plugs often have such a tell-tale window in the plastic fuse-retainer. But many do not!

Quote:
Originally Posted by Refugee View Post
I have seen them with a window below the fuse so that the value could be checked.
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Old 3rd Oct 2012, 8:00 pm   #20
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Default Re: 13 amp plug question

Quote:
Originally Posted by G6Tanuki View Post
Ah yes, the infamous "Duraplug". Most usually found with a slot rudely butchered into the tubular part of the cover by a hapless technician who discovered only after he's wired the plug up that he's not threaded the cover onto the cable.
Oi! Less of the rudely butchered. Surgically removed, more like, using a medium sized pair of sidecutters. At least they changed the design for the current version.
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