When we recently covered the topic of high voltage safety with respect to mains powered equipment, we attracted a huge number of your comments but left out a key piece of the puzzle. We take our mains plugs and sockets for granted as part of the everyday background of our lives, but have we ever considered them in detail? Their various features, and their astonishing and sometimes baffling diversity across the world.
When you announce that you are going to talk in detail about global mains connectors, it is difficult not to have an air of Sheldon Cooper’s Fun With Flags about you. But jokes and the lack of a co-starring Mayim Bialik aside, there is a tale to be told about their history and diversity, and there are also lessons to be taken on board about their safety.
In the earliest days of domestic electricity, by far its most common use was in lighting. Just as they are today, lighting circuits were hard-wired to receptacles designed for incandescent bulbs. When the first portable domestic appliances appeared they thus only had access to power through lighting sockets, and some of the earliest mains plugs simply replicated the base of an incandescent bulb. As appliances proliferated a variety of plug and socket standards were created to satisfy the need for both electrical safety and convenient access to a mains supply.
Throughout the twentieth century these early sockets were refined, improved, and in some cases superseded by new designs. National standards for voltage and AC frequency were adopted and standardised by the electrical safety bodies of individual countries, and further safety refinements such as earth contacts, partially insulated pins, or shuttered socket receptacles were incorporated into updated versions of each standard. If you are a global traveller or you are selling mains-powered equipment internationally you’d probably be tempted to describe the resulting proliferation as something of a mess.
That’s the Great Thing About Standards…
Trying to make sense of the world’s mains standards is not for the faint-hearted. You can group them into broadly similar populations, but there are always edge cases. Consider the 230V-ish world. Most countries for example have sockets with an almost-compatible circular form and live and neutral pins, but multiple variants of the earthing arrangements and socket keying mean that you will see plugs bearing the features of multiple standards in an effort to ensure compatibility. Meanwhile the UK and Ireland have their own 3-pin fused plug, the Italians have a three-pin-inline socket and the Swiss socket has three slightly offset pins. And we’ve not yet traveled into Africa and Asia, wherein lie a plethora of other sockets, some versions of current and older European or American standards and others home-grown. Against that background the 110V-ish parts of the world with their mostly-standard NEMA sockets look rather fortunate.
A Plug in Full: The BS1363
Bewilderment at the variety of sockets aside, it’s worth looking at the features of a modern mains plug and socket, examining their purpose, and taking a look at their wiring. An article on how to wire a plug might seem a little basic for Hackaday, but since it’s the first line of defence in mains appliance safety it’s vital that you ensure every piece of equipment that crosses your bench has a properly wired connection so it’s an important area to study. Ask any grey-haired electrician for their horror story tales of badly wired plugs they will have encountered over their career, and maybe then you’ll understand.
So we’ll now consider a mains plug in detail. Because this is being written in the UK we’ll illustrate it with a British 13A 240V BS1363 plug and socket, but the safety features are the point here rather than the individual standard. The BS1363 has all the features you would expect of a modern high-voltage mains connector, so consider them equivalent when shown here to the corresponding features on your local plugs. As an aside, is it a matter of national pride that people consider their own national standards to be better than those of their neighbours? For Brits that feeling probably ends abruptly when they first step on an upturned BS1363 in bare feet. Let’s just say if the pins on your plugs don’t protrude at 90 degrees to the flex, your feet are very lucky.
The BS1363 socket has three rectangular receptacles for pins. At the top is the earth, below left the neutral, and below right the live. There is always a shutter covering the live and neutral pins, opened by the insertion of the earth pin. This is a safety feature missing from earlier British sockets, which were open to whatever could be inserted by a curious but foolhardy child.
The plug has the corresponding three rectangular pins, with the earth pin significantly longer than the other two. Our second safety feature, this ensures both that the earth connection is the first to be made and last to be broken, and that the shutter is opened only as the live and neutral pins are entering their receptacles.
The live and neutral pins are partially insulated, with only the section towards their tips exposed. This is another safety feature that ensures that you can’t accidentally contact live pins when unplugging. The earliest BS1363 plugs lacked this insulation.
Removing the cover of the plug, on the right is the BS1363’s feature that makes it unique among the rest of the world’s mains plugs; it contains a replaceable fuse in the live conductor. This is a safety feature not to protect the user from shock or the appliance from damage, but to protect the cable from fire in the event of a short circuit. The value of the fuse should thus be chosen to match the current rating of the cable, though sadly the majority of BS1363s seem to contain a 13A fuse whatever the cable.
At the bottom of a BS1363 plug is a cord grip, usually a plastic clip, or a band with two screws. This safety feature secures the cord and takes the strain to ensure that the conductors can not easily be pulled out.
The final safety feature of a properly-installed plug comes through the work of the person who attaches it to a cable. If you imagine that the cord grip fails and the wires can be pulled from their terminals, you might understand that the first conductor to be disconnected would need to be the live, followed by the neutral, and finally the earth. Thus the installer should carefully cut the conductors to length to ensure that the live wire – brown in UK appliances – is shortest, followed by the blue neutral wire and finally by the green and yellow earth wire which should be the longest. Your plug should come with instructions with the appropriate lengths for each wire.
In a world moving towards moulded cables and plug-top power supplies it’s true to say that familiarity with the internals of your mains connectors is a less universal skill than it might once have been. But while the Average Joe might have to wire a plug a little less often than they used to it’s likely that you as a Hackaday reader will still find yourself with a mains lead or two to fix. It’s thus been worth taking a little time to consider the humble mains plug, and we hope you’ll go away and look afresh at your own connectors. We know you’ll all have your own favourites and bugbears from the worldwide selection of plugs and sockets, so make your case in the comments.
[Banner image source: United States Patent 7070460]