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-   -   13 amp plug question (https://www.vintage-radio.net/forum/showthread.php?t=88778)

ssaunders 2nd Oct 2012 11:44 am

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?

newlite4 2nd Oct 2012 11:50 am

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.
Neil

mike_newcomb 2nd Oct 2012 11:50 am

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

contrex 2nd Oct 2012 7:39 pm

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.

emeritus 2nd Oct 2012 10:46 pm

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.

Radio Wrangler 3rd Oct 2012 12:21 am

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.

David

Mike Phelan 3rd Oct 2012 8:14 am

Re: 13 amp plug question
 
I always thought it was as per Mike's post #3.

broadgage 3rd Oct 2012 8:59 am

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.

Nickthedentist 3rd Oct 2012 9:05 am

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.

kalee20 3rd Oct 2012 12:12 pm

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.

G8HQP Dave 3rd Oct 2012 1:49 pm

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.

dinkydi 3rd Oct 2012 2:00 pm

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

G6Tanuki 3rd Oct 2012 2:09 pm

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 (Post 560049)
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.


Nickthedentist 3rd Oct 2012 2:14 pm

Re: 13 amp plug question
 
Quote:

Originally Posted by dinkydi (Post 560063)
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).

Nickthedentist 3rd Oct 2012 2:17 pm

Re: 13 amp plug question
 
Quote:

Originally Posted by G8HQP Dave (Post 560061)
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.

ianj 3rd Oct 2012 3:47 pm

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)

Refugee 3rd Oct 2012 4:07 pm

Re: 13 amp plug question
 
I have seen them with a window below the fuse so that the value could be checked.

pmmunro 3rd Oct 2012 6:20 pm

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

G6Tanuki 3rd Oct 2012 6:30 pm

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 (Post 560092)
I have seen them with a window below the fuse so that the value could be checked.


Herald1360 3rd Oct 2012 7:00 pm

Re: 13 amp plug question
 
Quote:

Originally Posted by G6Tanuki (Post 560064)
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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