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