question about plug terminal

The length of the Earth Pin is more because the earth pin has to make contact first with the earth socket thus ensuring that the equipment is earthed before extending power supply to it. This is for operator safety.

The diameter is more. Why?

First let me explain all the wrong reasons - before anybody gives the same here.

Many times I have heard people saying that the earth pin is made thicker as it has to carry the much higher fault current during a fault. I always counter them by asking where from this fault current is coming? The Phase Pin/Socket! Isn't it? Then why is not the phase pin is thicker?

Let me give an example. Supposing you have a 1000W electric Iron Box connected to a 230V Single Phase Supply. The load current is 5A. The phase pin & the neutral pin diameter in a 5A Plug Top is about 4mm and the diameter of the Earth Pin is about 6mm. Now, let us say, that during an earth fault, 100A fault current flows. So, one would argue that to carry this 100A fault current, the earth pin is made thicker. But, then at the same time the phase pin will be supplying 100A Fault Current Plus 5A Load Current. The, as per the arguement, shouldn't the phase pin also to be sized to a diameter of 6mm as the earth pin, if not more?

Alternately, if the 4mm dia phase pin is capable of tolerating a current of 105A during a fault, then why make the earth pin thicker, at 6mm, involving more expenditure? So, this argument cannot be accepted.

The second most frequent answer I get is easy identification. But, there are more economical means of identifying the earth pin - like painting it with Green Colour, or making the earth pin with a different shape, like a square, or rectangle or traingle, keeping the overall conductor volume same as that of the phase pin - than increasing the thickeness and thus adding to the cost.

The third most frquent answer is to avoid inserting the plug top into the socket invertedly - upside down. But, this also is not possible, as the plug top's pin arrangement is not in an equilateral traingular formation. Rather, it is an isocelous triangle. So, how harder one tries, one cannot insert the plug top into the socket upside down.

What, then, is the reason?

It is to prevent the inadvertant entry of the Earth Pin into the socket. True, one need not try to put it upside down. But, due to lack of concentration, one may try to insert the plug top into the socket in a misaligned position (i.e.) trying to insert the earth pin into the phase socket (with the other two pins further down out of the socket). If the earth pin diameter is kept the same as that of the phase pin, then , in such a scenario, the earth pin will enter the phase socket. True again! the earth pin might not enter the phase socket fully, obstructed by the length of the other two pins. But, alas! as the length of the earth pin is also more (due to some other safety precaution, which cannot be violated) the earth pinwill enter that much distance into the phase socket so as to establish an electrical contcat with the live terminal insidce the phase socket. This cannot be avoided even by changing the earth pin shape or colour.

The only way is to make the earth pin thicker so that whatever may be the attempt, the earth pin would never enter the phase socket. And, that's why the thickness of the earth pin is more than the phase pin/neutral pin.

Hope it is clear!