a radial, a ring, it's a kitchen thing!

[joe-90]

joe-90, you appear to be a bit of a cock.

But right all the same.

 

[Scientist]

Well, interesting one this.

In general, a pure metal will conduct better than an alloy. The electrons that carry the electricity move more easily in a lattice of metal atoms of uniform size, than they do in a mixture. Imagine dodging your way between traffic cones on a bicycle (slaloming between them like we used to do for cycling proficiency badges); much easier if they're all the same distance apart. Therefore copper makes a better conductor than does Brass (a copper/Zinc alloy).

Brass on the other hand is not as soft as copper, so makes for a more mechanically sound plug socket/plug pin.

Therefore the brass bus has to have a greater cross sectional area than the copper T&E.

Fuses are particularly clever. I cannot remember, off the top of my head, the metals. But if you pull a 13AMP fuse to pieces, you should see that the wire has a blob in the middle. The blob is a different element to the rest of the wire. If you overheat the fuse (overload it) the blob diffuses into the wire. You then end up with a eutectic* mixture, with a lower melting point than the rest of the wire, so it melts at the point of the blob.

_______________________

*eutectic mixture: Imagine two metals, A with a melting point of 600 degrees, the other, B, with a melting point of 800 degrees. Imagine then alloying the two metals. Would you expect a 50:50 mixture to have a 700 degree melting point? You would be wrong. As soon as you add a bit of B to pure A, you reduce the melting point BELOW 600. Likewise, adding a little bit of A to pure B reduces the melting point BELOW 800. At some point around the middle, the melting point is the minimum (called the eutectic mixture) which might be, say, 400 degrees. Clever, eh?

 

[bernardgreen]

Oh dear, getting all hot and bothered are we? Take a 13 amp fuse and crack it open. Take a look at the thickness of the wire.

Crack open a socket and look at the bus-bar.

Which of those will handle the heavier current?

And then look at the cross section of the areas of contact between the metal of the pin and the metal of the sprung contact in the socket.

THAT is the main limiting factor as there is a contact resistance there which generates a bit of heat.

 

[bernardgreen]

Sockets can be subject to current in excess of 13A, right?

How? 13 amp fuse is the maximum allowed isn't it?

Yes, and the back of a double socket says MAX 13 amp for the imcoming cables so that means 13 amps shared between the two sockets.

In the 1970s the GPO (the phone company before BT) derated all "13" amp plugs down to 9 amps maximum for safety reasons. Only one brand was permitted to be used for the rare occasions when more than 2 kilowatt was needed.

 

[Adam_151]

All MK socket-outlets are manufactured to comply with BS1363 part 2: 1995 and are rated at 13A per unit. Double socket-outlets have been manufactured and tested to exceed this rating by margin that allows electrical safety and reduces the risk of heat and mechanical damage to components due to overloading. It should be noted that BS1363 part 2: 1995 does not allow double sockets to operate at twice the permissable maximum loading and it should be remembered that double socket-outlets are not manufactured to be able to withstand a 26A load for sustained periods of time.

Research by ourselves and third party organisations has shown that all MK double sockets can safely withstand a continuous load of 19.5A for an indefinite period. Increasing the load slightly will begin to cause heat and mechanical stresses on the components in a relatively short period. Testing showed that a load of 22.3A was sufficient to cause heat stress that would cause a browning of the faceplates and sufficient heat to cause insulation damage to cable cores. A load of 24A for 43 hours was sufficient to cause significant heat damage to the material in which the socket-oulet was situated and within 75 hours sufficient to cause significant damage that would lead to the very real potential of fire.

MK recommend that users of their sockets consult professional design Engineers when designing installations to avoid the possibility of heat and mechanical stress to components and installations caused by overloading of MK socket-outlets

Happy now, Joe? Good!, case closed!, its gone your bedtime anyway...

 

[joe-90]

No I'm not happy. That would defeat the whole object of having a 'fail-safe' system - as it catches fire rather than failing safe. If that be the case then the double socket should have a limiting fuse as does every other socket that has the potential to draw large currents.

 

[RF Lighting]

I just killed an MK socked in a controlled experiment

 

[suffolklad]

surely if a double socket can only take 13 amps we should only use 6 amp fuses if both sockets are used to plug appliances in

 

[suffolklad]

rf by the way its 2007 now

 

[RF Lighting]

Bugger.

 

[Ejaz]

lads, lads c'mon! leave a few leccies alone and look what happens!!

getting back to my last post:

wow thanks for the quick response! yep the 20A radial is wired in 2.5T+E. So my initial thoughts are to run the cooker (runs off 13amp socket) and the fridge/freezer of the ring main with one double socket above counter.

Then run the washing machine and 3 additional double sockets of the 20A radial, does that sound reasonable?

Not sure about RCD for the fridge/freezer will have to think about that one.

I did have a brief read of the "Electricians Guide to Part P" it's the small green guide that was helpful, might get a copy.

Forgot about the cooker hood, i'll supply that off the ring main also.

 

[davy_owen_88]

lads, lads c'mon! leave a few leccies alone and look what happens!!

getting back to my last post:

wow thanks for the quick response! yep the 20A radial is wired in 2.5T+E. So my initial thoughts are to run the cooker (runs off 13amp socket) and the fridge/freezer of the ring main with one double socket above counter.

Then run the washing machine and 3 additional double sockets of the 20A radial, does that sound reasonable?

Not sure about RCD for the fridge/freezer will have to think about that one.

I did have a brief read of the "Electricians Guide to Part P" it's the small green guide that was helpful, might get a copy.

Forgot about the cooker hood, i'll supply that off the ring main also.

Sorry, its just annoying when people seem to want to argue with fact (and common sense).

Anyway, what you suggest is fine but there are some other options.

Do you have an RCD? If you do then I would use the radial for the cooker, fridge and freezer and have that on the non-RCD side. And then extend the ring to power the rest of your sockets. Or better still, remove the ring from the kitchen and add another 4mm^2 radial to the RCD side to power all the other sockets- this gives you a full 32A just for the kitchen.

Davy

 

[securespark]

Sockets can be subject to current in excess of 13A, right?

How? 13 amp fuse is the maximum allowed isn't it?

Yes.

And how many of these plugs can a double socket accept?

No you are doing yourself out of an argument!

On the one hand you're saying that a 13A socket could not be overloaded because the 13A fuse would fail if a greater load were drawn, and on the other you fail to remember that 2 of these 13A loads can be placed on one 13A accessory.

Oh, not forgetting the 3rd hand which says that sockets should be able to take a load far in excess of 13A because of the CSA of the internals....

Make your mind up joe!!

 

[joe-90]

Oh dear, getting all hot and bothered are we? Take a 13 amp fuse and crack it open. Take a look at the thickness of the wire.

Crack open a socket and look at the bus-bar.

Which of those will handle the heavier current?

And then look at the cross section of the areas of contact between the metal of the pin and the metal of the sprung contact in the socket.

THAT is the main limiting factor as there is a contact resistance there which generates a bit of heat.

Exactly my point Bernard. Both single and double sockets use the same mechanism, therefore, if you can run one of the sockets at 13 amps then common sense tells you that you can run two as the mechanism and contact resistance is identical. Ambient temperature doesn't come into the equation. That is why they are both rated at 13 amps. That is why a triple socket is fused and a double is not. A double is designed ( or should be) to handle the load.

 

[joe-90]

Sockets can be subject to current in excess of 13A, right?

How? 13 amp fuse is the maximum allowed isn't it?

Yes, and the back of a double socket says MAX 13 amp for the imcoming cables so that means 13 amps shared between the two sockets.

In the 1970s the GPO (the phone company before BT) derated all "13" amp plugs down to 9 amps maximum for safety reasons. Only one brand was permitted to be used for the rare occasions when more than 2 kilowatt was needed.

Bernard. Putting the maximum rating on the inside of a socket is about as useful as putting the address on the inside of an envelope.

As for your down-rating scenario - supposing that one side of the double socket was drawing 13 amps and the other side 5 amps (same compound load)- taking into account the contact resistance that you mentioned in the other post - is that OK? If not - why not? And if the maximum allowable (according to the inside of the socket) was 13 amps - why were BT allowing 18 amps to be drawn? Hope you can clear these points up for me Bernard.