Why did the UK change from round pin to square pin plugs?

To keep people in work.

I think you might struggle to fit a fuse to a 5 amp plug.

No. Because it is easier to manufacture a true pair of self-wiping contacts that cover a wide area, against a flat bar, than against a round pin.

the 13A design was all about safety, not jobs for the boys. That came later, with the labour governments.

Could have been but flat connections are easier to make than curved ones. Brian

In fact the BS 1363 was introduced by the Labour govt of Attlee.

I've seen plenty of fused 5 amp plugs.

To make the plugs and sockets from two different wiring systems incompatible.

My grandmother's house until she moved out in 1980, had a glorious mixture of 13A, 15A, and 5A sockets. The living room had 13 and 15A wired side by side. She powered her iron from a bayonet adaptor in the back room pendant bulb socket.

Only MK make em and they cost as much as 5 non fused ones. I have never seen one, ever.

Better contacts.

The original round pin design was made on an automatic turret lathe, then the pins sawn and bent outwards to give them some limited spring pressure. The recetacles were made similarly. For square pins, the pin was harder to make (milling is trickier to automate than round turning), but the contact springs were now in the receptacles, formed from a bent flat strip spring. These could give much better contact force. This massively reduced the risk of poor contacts, overheating and fires when used for high currents.

The US had gone a similar route, but went to pins made of folded thin sheet.

Several makers do them, inc Crabtree and Schneider. They even have shrouded pins these days, although they are mostly staggeringly expensive. They're often not needed to be fused, because 5A circuits are wired as spurs and fused at a low current. Where these circuits are fused to a high current (they're effectively rings, same as 13A practice) then a fused plug is still needed, to protect the appliance.

They're becoming _more_ popular now, at least in theatres. Where lighting was previously powered as a lot of separate dimmable supplies up onto a gantry (lots and lots of cable), new lighting is now DMX controlled and can share its main supply. The plugs need to be fused, but there's much less cable. Lots of small theatres and vilage halls have to use DMX lighting over the last decade, because their insurers are demanding it – it reduces time spent up ladders, fitting coloured gels.

I think I recall seeing plugs where the pins *were* the fuses.

Thanks for the link. I did not anticipate the plug being this shape. I was thinking of the more triangular ones.

The original question now seems a very good one. I assume the answer was to avoid people using unfused round pin plugs in ring circuits (fused at 30/32 amps) for decades to come.

There is another problem I have just thought of. A 5 amp plug in a 5 amp socket would be okay as the radial circuit would be fused at 5 amps. However, if you plugged a 5 amp plug into a 15 amp (radial) socket using an adapter, you would then be exposing the appliance to the full 15 amps. Maybe this was the purpose of the fused 5 amp plugs but I have certainly never seen one in use.

The problem there is the adapter. Not perhaps as bad as the many old adapters to run things from lighting circuits, but any adapter like that would need to be fused internally itself, because of just the problem you describe.

I am sure the adapters were not fused in the era when the plugs were not fused. How would you fuse an adapter anyway? The ones I remember had an outlet at the front for a 15 amp plug and outlets on each side for 5 amp plugs. Would three fuses then be needed (one for each 5 amp outlet and one for the adapter itself)?

Was there a day when electricity for lighting was cheaper than electricity for sockets making it cheaper to plug the iron into the light fitting?

D&S -screw in fuses. 13A ciruit.

My memory of the type you describe, just had a single 13 Amp fuse under a plastic flap, (just like the 13A 'blocks' you can still get today do) ?