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I frequently use an Immersion Rod Water Heater (shown below) and I have noticed that the after unplugging the heater, the neutral pin of the 3 pin plug is quite hot (about 5 degrees more - but not very hot) compared to the live pin. How is this temperature difference possible and why? enter image description here

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    \$\begingroup\$ This may be the result of a bad connection between the neutral pin and the used socket. The socket should be checked and replaced by an electrician. \$\endgroup\$ – Uwe Nov 23 '17 at 10:24
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    \$\begingroup\$ The heating can also come from a bad contact in your wall outlet. Try a different outlet. If the heating comes from the outlet, have it fixed, it can start a fire. \$\endgroup\$ – peufeu Nov 23 '17 at 10:48
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    \$\begingroup\$ Perhaps there is a difference in internal resistance? Since heat comes from dissipated power, which is the result of $I^2R$ losses. Net resistance and net current is relatively constant, but through a particular segment of the conducting material you could find yourself with more local resistance than elsewhere, causing more heat dissipation. This could be a fault or perhaps just a result of the manufacturing process. \$\endgroup\$ – CogitoErgoCogitoSum Nov 23 '17 at 22:29
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    \$\begingroup\$ @CogitoErgoCogitoSum: Use \$ for inline MathJAX and $$ for on-its-own line on this site. \$\endgroup\$ – Transistor Nov 24 '17 at 16:55
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    \$\begingroup\$ Duh!! As the electrons flow from the "hot" wire to the neutral wire they get hot in the heater element. So obviously the neutral connection will be warmer! ;-) \$\endgroup\$ – Hot Licks Nov 25 '17 at 1:23
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I just had one of those last week. The plug was hot, and the socket was hotter. I'm the electrical guy so I popped it off. #12 stranded wire shoved into a smaller #14 backstab hole. Two thirds of the strands had missed the hole!

Dumb things like that happen all the time, and you have to nip 'em in the bud the moment you see them.

Don't use this outlet until it's fixed. It probably feeds thru to other outlets on the same circuit, so check this outlet while using those. Get it fixed ASAP.

If I had used the next outlet over, I would've never found it. It can start a fire. (We use steel junction boxes, that helps.)

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    \$\begingroup\$ I think you're obsessed with backstab failures! It's clearly a BS546 derivative plug (Probably India given the Bajaj brand, but also used in South Africa SANS164-1, GOST, etc). These surely don't offer backstab connections. It's not wrong to suspect the socket or the connection thereto, of course, but it's not going to be a backstab problem. Those things are a North American oddity... \$\endgroup\$ – J... Nov 24 '17 at 19:06
  • \$\begingroup\$ @J... agreed... I'm in India and I too now don't think it's a backstab problem.. This is the only case where the neutral pin seems hot not with chargers, and other appliances. \$\endgroup\$ – ilyas Nov 25 '17 at 2:25
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    \$\begingroup\$ @fluuufffy It's a high wattage appliance - that's why other devices don't heat up. It doesn't mean that there isn't a problem with the wiring to the socket (or a problem with the socket itself), however. The socket won't have backstab connectors so it won't be related to that, but it is probably still a bad connection to the socket (or a bad socket) that is is the problem. A good test would be to operate the heater on a different socket to see if the same behaviour can be reproduced - I expect probably not. \$\endgroup\$ – J... Nov 25 '17 at 2:30
  • \$\begingroup\$ @J... it's a disease, contagious among electrical types, it spreads by fixing backstab problems. Though this wasn't even a backstab problem per se, it was a "what were you thinking" problem. Europe has much better sensibilities, they don't even allow wire nuts. If only that socket heats up, and others do not, it's the socket. Especially if that socket heats up when it's in other sockets. Otherwise it must be the plug. \$\endgroup\$ – Harper Nov 25 '17 at 5:52
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The voltage on each pin, live or neutral, is irrelevant. Also the current flowing through each pin will be the same.

The difference is the resistance of the connection the current is flowing through. This will be caused by the springs in the socket not making good contact with the plug pin.

It's not unusual for heavy current users like kettles to make the pins slightly warm. However a cup heater like this takes very little current, so a warm plug pin is warning that the socket could need replacing. With a heavier current using appliance, it could get hot enough to cause a fire.

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    \$\begingroup\$ I wouldn;t call it very little current. It's 1kW (actually the picture says 1KW but poor use of SI prefixes isn't the issue here); that's the same as a low-power kettle, or 1/2--1/3 a full-power one. It will draw 4-5A assuming that BS546 5A plug is being used at the intended 230V (ish). ~50mΩ would be enough to cause 1W of heating, and that would make the pin quite hot. We're also up to nearly the rated current of the plug (and socket, though many countries that use BS546 plugs use multi-plug sockets) so some heating is to be expected. \$\endgroup\$ – Chris H Nov 24 '17 at 10:02
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To put all that was said in a simpler form, in case a layman stumbles upon this question:

This is an indication that the plug/cord, the socket or both are defective and need to be checked ASAP, since there is a potential fire hazard.

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The high resistance of the plug connection is often due to oxidation on the surface of one of the mating pins/receptacles. Regardless of which one it is, the heat will often cause oxidation on the other surface which also causes undue heating. If this is now plugged in elsewhere, this heated surface encourages oxidation of the new mating surface and so on and so on. It is the same epidemiology as zoological venereal diseases. Terminology is similar, mating, male connector, female ....

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