So I'm looking at this old Schuko plug that's been lying at my desk for a while now....and I noticed something.

The rating printed on the face of the plug says
10/16 | 250 (10/16 amps at 250 volts, I assume).

What's with the dual amperage rating? Wouldn't it just make more sense to rate the thing at 16 amps and leave it at that?

10A DC
16A AC

But if there's no DC services anymore, why do they even bother rating the plugs for DC?

Hi...

There are loads of systems, other then services, that still use DC power. Motor drives... lifting magnets... plating systems... emergency battery banks etc. etc. The DC rating would allow the plug to be used in such a device.

[This message has been edited by Rick Kelly (edited 01-19-2006).]

Why would AC or DC be a limiting factor? It's the RMS value of the current that will cause heating of the contacts. Could it be to do with the arc forming when the plug is withdrawn from a live socket?
In any case, using a domestic mains connector on DC is bad and potentially dangerous practice. From what I'm beginning to suspect, it seems to be only in this part of the world where there is an official low voltage DC plug and socket; ie the T configuration Clipsal 402/32, yet I am still horrified at the amount of 240V connectors being used for solar installations, boats, caravans, speaker connectors etc.

You are bang on... it is the interruption of the current that causes the difference.

Interrupting a DC current is a much harder thing to do as there are no "zero crossing points" in the level of the current.

As such... contacts rated for a specific amperage on AC are usually rated lighter on DC. This is true for any device that breaks the flow of current, or that can break it like a plug or a relay contact.

[This message has been edited by Rick Kelly (edited 01-19-2006).]

There used to be some weird round-pin plug that was commonly used for low-voltage DC in Austria but it went out of production.
On a train I once saw a mixture of standard Swiss 10A and BS 1363 used for 24 DC table lights. The Swiss sockets were 24V DC and 220V AC next to each other... thanks heaven I excaped the sysphus task of replacing all of them!

Neither BS1363 nor BS546 connectors have ever carried dual AC/DC ratings in Britain. Mind you, the connectors are rather conservatively rated for either. A BS546 5-amp plug has pins which are pretty similar to the 10/16A Shucko devices.

Were BS546 used with DC though in the past?

The BS1363 plugs, at least the older versions, carried "AC ONLY" warnings on them.

For those of you who have never seen a BS546 plug:

It comes in three sizes (there were other sizes which slipped out of usage)

Triangular pin layout, same as modern UK plugs.

The 5amp version has 2 pins the size of schuko 16A plugs and an earth pin about twice that size. (The sockets, once their shutters are overriden fit Euro plugs pretty much perfectly)

The 15A version pins: 7.05 mm × 21.1 mm. Live and neutral are spaced 25.4 mm apart, and earth is 28.6 mm away from each of them and is signifigantly longer.

I've heard that the old two-pin version of the BS-546 plug were sometimes used for DC.

You'd have one polarity at one part of town and if you moved somewhere else, or took .... a radio or record player to a friend's house....and they had a different polarity n their mains, you'd have to reverse the plug.

If you had a 3-pin plug, you'd have to undo the connections and reverse them.

Again, this is all just stories I've heard. Don't quote me on this.

And those stories are quite correct. BS546 was certainly used with DC supplies in the past, both the 2- and 3-pin versions.

As you say, when a 2-pin plug was used on a live-chassis AC/DC radio or TV, the user had to reverse the plug in the outlet if it didn't work on the first attempt (the tube filaments and dial lamps would glow, but with incorrect polarity there would be no HT/B+ voltage).

Yep -- Which meant that if you had a radio with color-coded cord and lived in a house fed from the negative "outer" pole of the supply, you'd need to deliberately connect red to N and black to L in the plug, the opposite of normal.


I don't recall seeing BS1363 plugs so marked, but some sockets certainly were. I always thought it was primarily where the switch wasn't a quick-break type and thus unsuitable for DC.

So far as I'm aware there was nothing in the original BS1363 specification which limited their application to AC. When BS1363 was introduced in the late 1940s, a good many older urban areas still had DC supplies.

Potentially having the fuse on the neutral would possibly liimt their use though ?

The fuse would still be in whichever side of the line was "hot." As far as the connections in the house wiring were concerned, there was no difference whether the house was fed from the positive or negative pole.

The black/N terminal of all sockets was still connected to the incoming neutral and thus at earth potential. That just meant that the red/L terminal was at +200 to 250V in some houses and -200 to 250V in others, hence the problem with a radio not working if taken to a house on the opposite pole until its wires were swapped in the plug.

Either way, the fuse would still be in the ungrounded conductor.



[This message has been edited by pauluk (edited 01-23-2006).]