New design of mains plug?

Interestingly the report linked to by Roger B used a load current of 20A on leads clearly marked as 13A maximum load. This is an abuse load and seems to have been carefully chosen to be as high as possible without causing a 13A fuse to fail quickly. While a 20A load could clearly be applied to these leads, its a very specifc case. What is clear is that you should not allow plugs to be covered by anything flammable like paper or cloth.
It;s also clear that the hard thermoset plastic bodied plugs are safer. They also end to give off a distinctive smell if they are overloaded.

Robert G8RPI.

I have always used a well known UK brand, moulded from a very hard thermoplastic, Multi Kontact. I am surprised that the test refered to did not examine the internals of those cheap 4 way boards, the use of flimsy metal is an understatement.

I came across this exciting circuit for testing fuses in one of my old books:

With the switch closed so current bypasses the fuse, rheostat R is adjusted until the ammeter reads 100 Amps. Then the fuse is tested by opening the switch.

As 500V × 100A = 50kW, the test fuse is likely to explode and arc. The purpose of this extreme test is to confirm fuse designs can deal a gross overload without the fuse itself causing a fire.

Don't try this at home folks!

Dave

Although wiring a plug top is a subject often joked about I suspect many people who confidently wire one do not observe the care points. When wireing the live terminal care should be taken that the core does not protrude beyond the terminal and cause it to mislocate the fuse carrier. If changing or installing the fuse it should be checked that it fits properly and the spring clips grip firmly and are not misaligned. Without doubt there are poorly designed and manufactured plug tops that may comply with the standard and there are certainly some to be found that definitely do not. The old duraplug rubber plug tops were well known to run hot with a heavy load and this was often to do with the fuse carrier, as they were promoted as a rough service plug then that is what they got, and any strains put on the live pin would affect the fuse location and tended to make it slack. A plug that is often removed and replaced should be checked for loose screws occasionally as a loose terminal will get the plug heating up nicely, portable heaters are perhaps a high risk as they move frequently and pull a heavy load. Plug maintenance is a worthwhile exercise, arm yourself with a bag of 3A fuses because the gremlins just love to fit a 13A fuse if a 3A is not handy Just check screws are tight and the fuse carrier is firm and no signs of distress. To check a terminal is tight loosen it first to check the screw is free to move and retighten, just giving it a bit more will over tighten and damage the cable or shear the screw. The cord grip should grip the full cable outer sheath, not just the cores, ideally the earth core should be long enough to be the last to disconnect if the cable is pulled from the plug top, do not leave so much slack that it impedes assembly of the plug though.

Mike

The Lucy Switchgear company had an ASTA test facility in Oxford, although when we visited no testing was scheduled the test chamber was interesting. It was lined with sleeper sized wood which bore the marks of some dramatic tests, the viewing window had some extremely thick glass and was very small, power was supplied from a very large generator. Lucy used to generate their own electricity and had a variety of diesel generators around the site, they had a one megawatt holdIng furnace for their casting shop.

Mike

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What was the book? That is not a practical test. The 50kW is dissipated in the variable resistor, not the fuse. All the fuse sees is 100A and 500V, either of which could be applied indendently. The critical test for fuse explosions is maximum breaking current which can be thousands of amps even in domestic situations. Typically applied by a capacitor bank, fixed resistance and a large spark gap (or possibly a solid state switch given capability of modern devices). The diagram looks like something thought up by somone with no practical experience, or a gross simplification. You would not use a DC supply unless specfically testing DC fuses. If it does arc you can't stop it easily. With AC in a test like this, any arcing stops at the zero crossing. With a capacitor bank (DC by definition you have a controlled energy limit.
50kW variable resistors do exist, I've used one for testng aircraft ground power units years ago (it was old even then). It was a trailer filled with what looked like slates, but were carbon plates, ther were 6 rows (3 phase, two rows per phase) 6 leaf springs, and a big acme thread pressing on the centre of the springs. you adjusted the resistance by varing he pressure, an effect unique to carbon afik.

Robert G8RPI.

Although plugtops are available with 3A or13A fuses ready fitted it always seems to be 13A in the shop, if they only sold 3A then you would have to uprate for high load appliances as obviously the fuse would blow, maybe no fuse or both fuses should be supplied. Although a variety of fuses are available for plugtops 3A and 13A should be ok for most purposes. I think the table lamp 13A fuse situation arises because tungsten filament lamps often took the fuse out when the lamp failed so a new lamp and fuse would be required, the fuse tin invariably has 13A fuses because all the ones you take out of the plugtops when you put the 3A in go in the tin and you have used all the 3A before. So do you sit in the dark or pop the 13A in the plug until tomorrow when you will get some fuses and swap it over except you forget. In a perfect world you will always have the right fuse and never forget but in real life it doesn’t happen so have an annual maintenance session and prepare to be surprised, I know better but am still guilty of fitting the wrong fuse on occasion and forgetting to revisit it.

Mike

Robert, All modern safety standards have to allow for foreseeable abuse. It is very likely that a four way extension lead will be loaded above 13A so that must be tested for.

Bill, The various fuse values are virtually irrelevant. An appliance that meets European standards and is correctly CE marked will have been designed to be safe when protected by a 16A fuse/circuit breaker (as on the mainland) and so will have a minimum 0.75mm2 lead. There may still be a few old UK appliances around with smaller leads which should have a lower rated fuse.

The harmonised colour coding, Brown, Blue, Green/Yellow, was bought in to deal with a number of safety issues on the biggest being the German use of Red for the earth conductor as they thought it was the most important. As others have said the colours were chosen with colour blindness in mind, especially the bi-colour earth.

While we are locked down let’s check our houses and own up on here how many errors we find

Mike

I can remember when we got the email round at work advising of the new harmonised colour schemes.
Telecoms power normally uses -50v and the old convention was black earth and blue -50v.

New convention is Blue earth and Grey -50v, so the blue changed use from the relevant date.

The exception was System X digital, which used overgrown twinned figure of eight cable (10mm ) for power supplies.
The plastic was all grey with the -50v battery being marked with a blue stripe.
From the relevant date, we had to use the same cable, but with the colours reversed.

Hence a fuse panel rack was likely to have mixed colours of wiring in the same cabinet (with appropriate warning labels of course)

Appropriate for today, the email was dated 1st April, so no-one believed it at first.

Bill

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Yes, why not ridicule yourself with armchair pontificators?

The book is from the Electrical Engineer's Data Books series. It's Vol 2 of 'Manufacture, Design and Laboratory Work' by D.V.Onslow A.M.I.E.E

Not claiming it to be industry best practice today - it was published in 1925! Onslow is quoting an E.R.A specification and it's in the section dealing with the electrical properties of Asbestos.

Although it contains bygone advice, and is built on long gone standards like the 'Home Office Regulations on the Use of Electricity in Factories and Workshops' much of the book still makes sense. It reveals a world in which huge progress had been made, but still had many quaint hangovers, like DC mains. Another colour code example:

Note. - The standard colours adopted for the identification of cables (see Regulation 85) are different from those specified in Regulation 63 N for switchboard connections.

Dave

One day, many years ago, a fellow worker called because The circuit breaker tripped and he didn't know why.

Switched off everything, lights, typewriter etc. reset the trip and switched on one item at a time.

O K until the kettle.

Someone had tripped over the cable and he had rewired the plug.

When I checked, guess where the green earth wire was connected?

Full marks to anyone who said the LIVE pin!

I bent his ear to some effect "If you don't know what you're doing DON'T before you kill someone"

A little knowledge and all that!

A friend who did PAT testing has found (Unbelievable ) a bit of six inch nail in the fuse holder, and missing clamp screws replaced by matchsticks.

Contenders for the Darwin awards, all!

Howard

Even if not a 13A it would likely be a 3A these days (the standard issue options?). I pride myself on fitting an appropriate fuse to everything I have. But do remember the fuse in the plug is protecting the connecting lead, not the appliance.

I was told (25 years ago) that there is at least one plug with conductors not gripped properly (only one layer of insulation outside the plug). Clearly with moulded plugs this error has reduced considerably but it showed the dangers that are simply condoned by some, where electricity is concerned.

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Made a start, so far  1 13A fuse in a standard lamp and loose earth screw, loose earth and neutral in cassette type extension cable, oddly all AV devices seem to have 5A fuses in OEM cables, worrying just how many devices are powered by wall warts with no replaceable fuse, let’s hope that they are properly protected, can I be a cynical optimist? 13A fuse still in lamp as can’t find fuses. Will crack on and confess any more horrors.

Mike

Edited By Mike Poole on 01/04/2020 13:52:14

You need to check that none of these are being used:

In my university days it was not unkown to replace the fuse with the barrel of a 1/4" jack plug (~160A) so you could run a lighting rack from a 13A outlet for a short period. We soon discovered that you got less overheating problems if you installed two neutral pins