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Current carrying of plugs AC versus DC

Kelly Marie Angel

In a group discussion on Facebook it was stated that 5 amp and 15 amp plugs were designed to carry more than the stated limit because they had to work on DC as well as AC mains whereas 13 amp plug were only ever meant for AC. I cannot see how the current type would make that much difference as I type this it does occur to me that maybe skin effect comes into play but does that really make that much odds? The original discussion was about a 2 bar fire fitted with what looks like a 5 amp plug. And the fact that 2 kilowatts at 240 volts is about 8.4 amps. I was told that at one time it was permitted to run 3 5 amp sockets off one 15 amp fuse 

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    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

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  • 0

    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

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