I understand that BS 1363:1947 was not specific about the use of the "13 A plug" for AC or DC use, but by the 1950s, the standard changed so that the "UK Plug" as we  now know it was specified for AC use only.

I also believe a similar thing happened with BS 546, where originally it was specified for either AC or DC use, but later the manufacturer could specify AC only.

The issue really being that DC supplies were in use in certain parts of industrial towns in the UK until the 1960s - we later fully standardized on AC use. Hard to believe as it is, but definitely at one time (before I was born, I hasten to add) you needed to be careful with certain appliances (such as a 'radiogram' ... but really, anything that wasn't simply resistive like incandescent lamps, heaters etc) to purchase the right version (AC or DC) , because otherwise, when you got it home, fitted the plug and connected it, smoke would be the result !

Amps are amps, and AC or DC makes no difference to the current rating. The skin effect is minute at power line frequences and at the modest currents handled by plugs and sockets.

The older type socket outlets were simply better designed and manufactured and could handle at least  twice the design current, and even more than that short term. Misuse of 5 amp sockets for 2 kw  portable heaters was very common and almost never had any serious consequences.

In my mispent youth I used a Baby Belling cooker with a FLC of about 15 amps from a 5 amp plug and socket  which never even got warm. The oven used about 8 amps and was run 24/7 in cold weather for space heating.

More recently I found a 24 volt DC microwave oven with an input of over 30 amps being used from a 15 amp plug and socket. It only got slightly warm  a similar outlet was used for for a 24 volt kettle rated at 25 amps.

And yes it was permitted to install 3 of 5 amp sockets on a 15 a 15 amp circuit., in practice more than 3 were often fitted

Numerous 2 amp sockets were allowed on a 5 amp circuit.

two 15 amp sockets were oncle allowed on a 25 amp circuit.

we now have 13 amp sockets that are unreliable at more than about 8 or 10 amps continually.

broadgage said:

Amps are amps, and AC or DC makes no difference to the current rating.

Except for two things:

1. The fuse itself - BS 1362 fuses are definitely these days AC only.

2. If the standard includes tests for insertion and withdrawal on load, then, if those tests are carried out with AC only (which the current and recent versions of BS 1363-1 for plugs and BS 1363-2 for socket-outlets have done, but which were not included in BS 1363:1947), then the increased arcing effects with DC won't be tested for.

Point 2 especially worth considering, because BS 1363-1 plug pins are these days often hollow (the practice started around 40 years ago if memory serves, roughly at the same time as moulded plugs arrived on the scene).

broadgage said:

Amps are amps, and AC or DC makes no difference to the current rating. The skin effect is minute at power line frequences and at the modest currents handled by plugs and sockets.

Agreed for practical purposes with the relatively small currents found in consumer circuits, but for future readers I'd want to emphasize that for bigger conductors skin effect is significant even at 50Hz, hence switchboard busbar shapes being material considerations, decreasing return in ampacity/mm² for larger conductors, significantly different current ratings (e.g. table 4E3) and the use of ACSR, hollow and Milliken conductors on power systems. Etc.

Also, as Graham has pointed out, switch/disconnector ratings can be markedly different.

Inclined to agree - the problem of DC that requires chunkier pins is only pulling the plug out with the load connected not the static current rating.

There is also a tolerance problem with round holes and pins, that the area of intimate contact  is actually quite small unless the diameters are a very good match indeed - tricks to mop up tolerance errors,like splitting the pins (old 2 pin plugs did this) or splitting the socket tubes into two or four 'fingers'  (modern BS4343 derived iec 60309 do that) are only a part solution.

Rectangular or blade connections are much simpler to make in low tolerance manufacturing, and still get a good contact area, but the easy pre-war fix was to use round parts that were suited to lathe manufacture, and to oversize things a bit.

Mike

Thanks again for all the answers guys. I was right in that there us little difference whether it's DC or AC  except as was said when unplugging with a heavy load. I thought that the 15 amp plugs pins were capable of carrying a lot more than 15 amps the plugs of old were well built to hear of one being used at 25 amps makes me think they were over engineered  and then some! A friend of mine told me they had a workshop in the dockyard that was all 220 volts DC using 15 amp sockets throughout with heavy duty switches mounted above sockets and the cooker in the canteen  all makes sense now. 

gkenyon said:

Hard to believe as it is, but definitely at one time (before I was born, I hasten to add) you needed to be careful with certain appliances

I remember radios and televisions which could be used on AC or DC and across a range of voltages - the appropriate setting had to be selected.

Concerning putting a high load on a BS 546 5 A plug: it might be difficult to fit a large flex in a small plug.

I had an old 405 line tv that had multiple tappings on its dropper resistor everything from 190 to 250 volts in 10 volt steps interestingly the 190 and 200 taps were for AC only  the rest for both

Perhaps because there would be a single phase rectifier and capacitor for the HT, and that would generate a DC of  almost 1.4 times the RMS - so presumably 190V DC minus one diode drop was not enough to get the HT, but 190AC would give 260V DC or so... The droppers would normally only set the current in the (series wired) valve heaters. They have to be within a few % to get the cathode to red/orange heat for optimum emission without shortening the life too much,

The HT is far more tolerant and could probably vary over +/- 20% or so, and the user can compensate for the change in gain by twiddling the width, height and volume knobs.

Mike.