A friend brought me a present with a (reportedly) US-type electrical plug. I bought a (reportedly) US-to-UK adapter on ebay, and this is what I got:

However, when I tried it, the plug wouldn't fit into the socket. Specifically, the plug seems to have one flat prong that is wider than the other.

What is the problem here? Is this not a US plug? Or is this not a suitable US-to-UK adapter? Are there multiple types of two-prong US plugs? Do I need to be looking for a different adapter?

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    Maybe it's a polarised NEMA-1 plug. – Tomas By Nov 10 at 18:13
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    So apparently the wider prong is (also) ground. I guess you need another adapter. – Tomas By Nov 10 at 18:18
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    @TomasBy the tall prong is neutral. It should never be confused with ground. They are not alike. – Harper Nov 10 at 21:05
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    @TomasBy they are not the same and you should extinguish your belief on that subject. That misbelief is responsible for some of the most dangerous wiring mistakes that I see. They are the same inside the main service point, but in any other location they are separated, on purpose, for a litany of reasons. We talk about them regularly over on diy.se, come on over! – Harper Nov 10 at 21:44
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    @TomasBy no, it's to reduce the danger of exposure to one of the legs, which is another way of saying "allowing the appliance to have cheaper insulation around the neutral". A 2-prong appliance is not grounded at all. The other forum covers the topic well, but an obvious difference is return current is intended to flow on neutral, and not on ground. The difference might be moot if everything is working properly. If anything breaks, the careful separation becomes very important to life safety. – Harper Nov 10 at 22:28
up vote 19 down vote accepted

You need a different adapter. Your adapter will accept a standard US non-polarized two-prong plug, but will not — as you discovered — accept a polarized two-prong plug.

Plug adapters are varied. I have a bag of them here, and found five adapters that accept US two-prong plugs, four for the UK and one for France. Of those five adapters, only two would accept a polarized two-prong US plug.

In this photo, the adapter on the left will not accept a polarized plug. The adapter on the right, however, will accept a polarized plug; you can see that the right-hand adapter's left-side slot is slightly wider than the right-side slot.

enter image description here

  • Thank you! I seem to only be able to find adapters with 3 slots on amazon / ebay ... any idea if that will be a problem? Or can I just plug the 2-pin plug to the adapter anyway? – Tasos Papastylianou Nov 10 at 18:45
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    You can just plug in the 2-pin plug. Lots of US appliances only have 2 pin plugs, even though the wall sockets have 3 holes. – Laconic Droid Nov 10 at 18:58
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    @Tasos The 3rd, round, prong/slot is ground on the US. Some connectors (both device plugs and wall sockets) have it, some don't. The ground connection is used for safety and/or EM emissions reduction / acceptance / protect the device from static electricity shock (interference between devices and/or to protect the device from a static shock when a human plugs in a cable). There are adapters from 3 prong to 2, although using them reduces safety and may increase the interference that the device gives off and/or reduce the device's ability to protect itself from interference/static shocks. – Makyen Nov 10 at 19:44
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    @TasosPapastylianou An adapter with three slots is fine, as long as it has the longer second slot for this purpose. But make sure that your device can accept 240 Volts! – Random832 Nov 11 at 5:14

The shape of the plug is keyed for 120V only. (North America has other plug keyings intended for 240V). Does "the present" say in its labeling or instructions that it is able to run on 240V? Many things can, many more cannot. If not, you should not use an adapter like this, which will straight-shot 240V into your device.

They make larger and more complex adapters which convert voltage. Such adapters either have some electronics onboard (which can serve only limited load capacity, in watts), or are a bulky and very dense transformer (the larger, the higher the capacity in watts, clear up to the 1800W max a US device might draw if you don't mind a 15kg transformer).

That plug is a NEMA 1-15 plug that is polarized. That means "the present" is not double-insulated and it is much safer when the wide blade is connected to neutral. Neutral is a worldwide concept (except Philippines), it is a wire (typically 1 of 2) in the supply loop which is manipulated to be near natural earth, so does not present a shock hazard if your body got between neutral and earth. It is not to be confused with the safety ground, that third pin on US and UK plugs.

Whoever sold that to you sells cheap Cheese junk. It is unfit to be marketed as a US to UK adapter, since it lacks the correct keying for polarized plugs, which are very common. This particular one is a highly compromised "universal" adapter meant to plug anything (even UK) to UK. Beware buying such junk, as it can create safety issues of its own, especially if you plug in something with a high power draw.

You are better off seeking a quality unit that focuses on US to UK only, and better off buying it at a local shop, where safety regulations are able to have an effect on the quality of their selection. EBay, Amazon Marketplace and AliExpress sellers do an end-run around government regs by direct shipping to the consumer.

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    "Cheap Chinese Junk" is a good description of that adapter. Big Clive reviewed several similar or identical looking adapters on his Youtube channel: Deadly Chinese plug adapter. and Inside another death-dapter variant, – RedGrittyBrick Nov 10 at 21:59
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    @TomasBy Installations build in the US in the last half century or have a grounded conductor going to every outlet. The system is designed so when everything works right, no current flows in the grounded conductor (also called "earth" or "safety ground"). The round hole in an outlet connects to it. The neutral conductor is nominally 0 volts, but current flows through it in normal operation, so the voltage could be a little above 0 volts. It is connected to the wider of the two slots in an outlet. – Gerard Ashton Nov 10 at 22:38
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    @GerardAshton: maybe you could explain briefly the point of a polarised plug for AC? – Tomas By Nov 10 at 22:46
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    @RedGrittyBrick: The first video might seem frightening to Brits, but people in the rest of the world will find the adapter to be no more dangerous than what they have at home. However the second video is downright frightening. I think that I'm done buying anything that connects to mains from foreign mail order any more. – dotancohen Nov 11 at 16:18
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    @dotancohen good on you. Your friend is the UL listing mark, since UL is a testing lab that will "Big Clive" the gadget on an industrial scale, looking at everything from build quality to plastics flammability and toxicity to actual live testing on a FLIR camera. Or the CSA mark, TUV, ETL, or other competitors. Note that CE is not a testing lab, if the part leads with a CE mark (like Clive's voltmeter) instead of a real testing lab's mark, they are admitting it is junk. – Harper Nov 11 at 17:45

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