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In a plug point you can see that the upper hole is bigger than the other holes in it.

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Why is this hole bigger than the other two?

yuritsuki
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Brd
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    What happens to your appliance if the fat ground pin could be inserted into the live hole? – Andy aka Nov 14 '15 at 11:43
  • So that it grounds(connect to earth) the body of the appliance/instrument before it is powered to protect you from shock – MCG Nov 14 '15 at 14:26
  • I like how a 7.2k rep user came by, read this question, and thought "well the tag is off, don't think the title is bad, or the body is bad, or anything else. Nah, just the tags" – yuritsuki Nov 15 '15 at 07:05

2 Answers2

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I suppose since the earth prong is also longer on these BS 546 plugs, if you could partially plug it in the wrong way (say earth to phase), then someone else touching the appliance at the same time could receive a shock. So the earth prong is thicker so that you cannot plug it in even a little bit in the phase/neutral holes. (Rotational asymmetry would prevent you from fully plugging it in the wrong way, but rotational asymmetry alone won't prevent a partial but incorrectly rotated/oriented insertion when one prong is longer than the other two.)

enter image description here

The same design idea is adopted by the newer UK plugs (BS 1363) that use rectangular prongs: the earth prong in these is again both longer and thicker than the other two.

enter image description here

The reason why the earth prong is longer is more obvious: so that you always make the earth connection before phase is applied (and vice-versa when you unplug it.)

Fizz
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    Also, consider [non-standard adapters and extension cords that allow you to insert the plug in inventive ways](https://www.youtube.com/watch?v=hvOTiQKkQMo) ([another example](https://www.youtube.com/watch?v=nB1DlBpyS9w)) – sleblanc Nov 14 '15 at 20:31
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An important concern is that it must be impossible to insert the ground/earth pin into the live/phase/hot receptacle of the socket. One way to do this is to make the ground pin larger, so that it cannot fit. In North America, this is done by making the ground pin round, while the live/phase/hot is flat.

It is also common to make the ground pin longer to guarantee that it makes contact first.

Simon B
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