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?

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.

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:

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.

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.

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.

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.

Re: 13 amp plug question
 

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).

Re: 13 amp plug question
 

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

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)

Re: 13 amp plug question
 

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

Re: 13 amp plug question
 

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

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!

Re: 13 amp plug question
 

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.

Re: 13 amp plug question
 

Yes, "prohibitive" is being very polite in my opinion.

I also have (had) an MK plug with a little flag under the main securing screw which could be locked in one of 3 positions, to denote what type of fuse should be fitted.

N.

Re: 13 amp plug question
 

The problem with this is you can then easily pull the cover back, exposing the fuse!

Makes no difference to me what colour the case is, I can't check if the terminals are loose by looking through the case!

Matty

Re: 13 amp plug question
 

I have got one or two where the top has been remembered. I have seen one of the 1960s MK rubber ones with the cut where the cover has been forgotten.

Re: 13 amp plug question
 

I don't think the connection of a supplementary lead through the inspection hole in a plug would have been sanctioned by the manufacturers because of the lack of mechanical anchorage. If an additional single core was required alongside a 2-core flex it would have been better to route it though the cable clamp too. One extra function that the hole often served was as a locating keyway for a projection on the cover. The MK 646 has this and two projections near the cable entry to prevent rotation of the cover.

In the 1930s GEC were advertising plugs to BS372, the earlier standard for round-pin plugs displaced by BS546, with visible earth connections. Other connector types with this feature included earthed bayonet adaptors, appliance connectors and the early industrial plugs mentioned above. It was not limited to plugs, see pic below of 3-pin GEC Landor sockets with exposed earth terminal that can be checked for tightness without opening the cover. The associated plugs made to a similar high standard offer the advanced safety feature of insulated pins, like the Sanders products mentioned in a recent thread, and an earth inspection hole although this is not visible in the pic. At the other end of the scale are the Reeves 2A 'Crescent Junior' low cost plugs and sockets, c/w inspection hole in the plug.

Lucien

Re: 13 amp plug question
 

In the case of this plug, which doesn't have the MK-style wraparound posts, the hole is offset to expose the flex.

Re: 13 amp plug question
 

True. But the slot is so much more massively deep than the projection is tall, that the ensuing aperture must be for a reason.

Re: 13 amp plug question
 

Perhaps it is there to relieve the vapor from a blown fuse so that it does not crack the top off exposing the live terminal on the top of the pins.

Re: 13 amp plug question
 

Sure, but if it is indeed only for inspection it only needs to be large enough to see the conductor inside the terminal or the terminal entry hole and the smaller it is, the neater and the less tendency for dirt to enter the plug. Hence the convenience of filling up the unneccessary top 1/8" of the notch with a locating lug on the cover.

Lucien

Re: 13 amp plug question
 

Refugee,

You are quite correct in anticipating high energy dissipation as a result of a fuse blowing but the BS 1362 fuses which must be used in BS1363 plugs are High Rupture Capacity fuses which should not be broken open by the forces generated.

This is why any fuse connected to the mains should be an HRC type.

PMM.

Re: 13 amp plug question
 

I was thinking more of ones that have been badly wired or with a bent fuse holder.

Re: 13 amp plug question
 

I doubt that it is anything to do with fuses. I have some unfused MK 5A and 15A BS546 plugs which also have holes adjacent the earth pin.

Many years ago a non-BS1362 fuse (bought in the early 1960's) fitted to a 13A plug, blew when the cable insulation of my mother's iron broke down. The arc blew holes in both end caps of the fuse, but didn't break the circuit. The escaping metal vapour plated the inside of the plug with condensed copper, and set up a secondary arc inside the plug in series with the arc in the cable, but there was no tendency to blow the plug apart (a Crabtree I think). A pity I didn't keep or photograph the remains as a warning aganst the use of such fuses, the sale of which was I believe subsequently banned in the UK.

Re: 13 amp plug question
 

I must admit to owning up to the butchery of the wretched rubber plug. Only a couple of times but there is only so much a poor guy can take. Fitting one in a dark living room stinkin of cat pee is something you only want to do ONCE! John.

Re: 13 amp plug question
 

Before digital cameras were common the boss tried to help with production.
He got distracted and wired some IEC socket units so that the switch shorted live and neutral.
The 13A fuse had all of one end cap gone and about half of the other one also all gone.
It made a black mark on the wall and scared the guy that was testing the power units.
All of them had to be checked by eye before being plugged in.

Re: 13 amp plug question
 

I always thought the little slot in the top of the plug was to aid ventilation the 15 amp plug on our washing machine had it and it did use to get warm especialy when the heater and wash motor were in use.

Re: 13 amp plug question
 

If you manage to get a 15A plug warm, you must be drawing an awful lot of current. They are very substantial. More likely the terminals were loose or the socket did not grip the pins properly, or the pins/contacts were tarnished.

If the hole was for ventilation it would be above the fuse, which always warms up when connected to a load greater than about 2.5Kw. 13A isn't quite enough, perhaps they should have designed the plugs with 14 or 15 amp fuses instead.

Re: 13 amp plug question
 

I've always believed that is was for connecting an earth wire, such as providing an 'earth' connection for the 'earth' socket on a wireless. Since many wireless set owners would have difficulty in providing the then conventional earth plate in the garden (e.g. flat dwellers), this would provide the necessary 'earth'. (Now we all know that such an earth will be a poor radio earth, but that would not have been well-recognised all those years ago by the average wireless-user: he just wanted an 'earth' for his wireless).

Below is a photo. of an old 13-amp plug and you can see that inside it there is a small step in the plastic to assist and support such an earth wire. By looking at the internal photo, it is also very clear that the hole is not for the purposes of inspecting the internal earth connection, since the 'viewing hole' is well offset from the earth pin screw terminal.

What is not seen in those photos is the writing on the 3-pin side of the plug. It reads:
13A 250V.
NO. 689
EMPIRE MADE

Al.

Re: 13 amp plug question
 

I have a brown 13 amp plug which has been in constant use since 1952. It has a built in on / off switch above where the earth pin is located. Never seen another like it.

Re: 13 amp plug question
 

I've got one of those, currently on a DAC90A (etc) mains lead. These switched plugs aren't exactly common but they do turn up; usually brown but white ones can also be found.

Re: 13 amp plug question
 

Is it like this one on my workshop hoover?

- Joe

Re: 13 amp plug question
 

Mine's the same as that, yes. An MK.

Re: 13 amp plug question
 

Thanks for all your replies to this thread, but as one of the correspondents observes, we still do not have definitive answer and without shelling out for a defunct BS, we are unlikely to answer this important question.

So, in pursuit of nothing less than a comprehensive explanation of our rectilinear friend, I have fired a mail to the God of plug tops: MK Electrical (Technical Support).

I will update you all in due course!

Re: 13 amp plug question
 

We could also have a quick ask here.

http://www.fam-oud.nl/~plugsocket/UnknownSpecimen.html

Re: 13 amp plug question
 

Update: The technical support guy at MK asked his colleagues and none of them know. Undaunted I have suggested that he goes and asks the guys who play cards in the canteen every lunchtime - every company has them and they know everything.

Re: 13 amp plug question
 

I've mailed Oof at http://www.fam-oud.nl/~plugsocket/UnknownSpecimen.html to see if he can shed any light on the issue.

Re: 13 amp plug question
 

I have a cream Bakelite MultiKontact plug the same as that knocking around somewhere. unused and wrapped in brown paper 8-o

Jan :thumbsup:

Re: 13 amp plug question
 

Maybe the little slot was use as a locating slot during manufacturing to assist some peice of automated proccesing

Re: 13 amp plug question
 

The answer is apparently in clause 15 of the original edition of the standard, BS1363:1947, which mandates a visible earth conductor according to
http://en.wikipedia.org/wiki/BS_1363

I have always understood this to be the function although don't specifically recall reading BS1363:1947. It is corroborated by the many catalogues advertising visible earth connections on many types of plug. I cannot think of any plug in which the notch does not reveal the conductor. There is a lot of circumstantial evidence that this is the only 'correct' purpose for the slot.

Lucien

Re: 13 amp plug question
 

Hi,
I've about half a dozen switched MK plugs; brown, cream, and a white one! I've also got an MK plug with spring loaded sleeves as mentioned recently in another thread. On most of the MK plugs I have, the locating piece that fits into the hole next to the earth pin has broken off.
I wonder if the hole was to reduce the chances of damp condensing inside the plug. Something that seems unnecessary these days as modern plugs don't have said hole.
Cheers, Pete:wave:

Re: 13 amp plug question
 

Maybe. These days with central heating being so widespread condensation would be as big a problem.

These holes certainly pre-date BS1363, though that could well have incorporated the then current best practice.

- Joe

Re: 13 amp plug question
 

The answer is apparently in clause 15 of the original edition of the standard, BS1363:1947, which mandates a visible earth conductor

Which is I believe what I stated in post number two

Neil