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 !

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

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.