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.