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Need help on understanding why does the "Black Wire" is always burned/damaged periodically?

It's always being replaced. Is this normal? How can I prevent this from happening.

Thank you!

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    Is it always plugged into the same outlet (or extension cord)? If so, the problem is probably in the outlet side. A poor connection, either between outlet and wire or outlet and plug, producing heat.
    – DoxyLover
    Commented Sep 21, 2020 at 7:22
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    Stop running it that way! Do whatever it takes to make it not be warm. What is the load? What is the breaker feeding it? Commented Sep 21, 2020 at 7:45
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    @TacoTruck713 What is plugged into the other end of the black extension cord? Include a picture of the name plate that shows all the electrical "mumbo-jumbo" on it - the electricians here will decipher it for you.
    – FreeMan
    Commented Sep 21, 2020 at 11:32
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    Do you regularly use that setup in a damp/wet outdoor environment like that shown in your 2nd pic? The damage you see in your 1st pic is very likely to occur if some moisture is getting into the connection between the plug & socket.
    – brhans
    Commented Sep 21, 2020 at 14:24
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    This is all kinds of bad news! The wire insulation looks damaged, the plug looks damaged, and the environment is wet. The cord is most definitely being abused. What are you using this cord to power, how long is the cord, where is it running, how long is the cord being left in place? Commented Sep 21, 2020 at 16:10

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Firstly when a plug or socket overheats you should replace BOTH the plug and the socket and also cut-back any heat-damaged wire. Heat damage can cause buildup of poorly conductive materials on contacts and it can cause loosening of the springs in socket contacts. This means that mating a heat-damaged connector with a good connector can cause the problem to spread.

Secondly breakers don't trip the instant a circuit is overloaded and US regulations allow 15A outlets to be installed on 20A circuits. So it is quite possible to overload an extension cord without tripping a breaker. You really need to pay attention to what loads are connected to a cord rather than just relying on breakers.

Thirdly normal domestic plugs and sockets aren't suitable for use in wet conditions. If you are going to leave wiring outdoors then it really needs to be done in a more suitable manner.

I suspect the reason that the hot fails first is simply that it in a normal american plug the hot contact is smaller than the neutral contact.

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There is a defect in the hot side of that plug which is causing it to heat up. The defect may be in the plug or in the wire connected to it. The electrical resistance increased leading to overheating and damage.

The same current flows through the neutral side and there is no damage there. This means the plug is not being overloaded. There is a defect in the hot side.

The original cause might have been as simple as the connection was loose or too many strands of wire were cut when the wire was stripped.

You may be able to rehabilitate the plug by loosening all the connections and pulling off the wire, then cleaning off the hot connection with a small file or emery cloth. Then cut off the end of the wire, strip the wires carefully, then remake the connection.

But the best thing would be to get a new plug and make the connections properly.

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There’s a lot of questions here, just from these two pictures. But the main ones are

How long are these cords?
What are they plugged into? What rating is the extension cord? (When you bought it, you’d likely see 14/2 or 12/2. 10a 15a or 20a(amp) Is it on 24/7 in a controlled/covered environment or always out in the weather? It’s hard to say most definitely.

One of the first things I’ll look for like this is if it’s something with a heavy duty motor that starts often. The inrush can damage things overtime.
Also, any, and I do mean any, possible kinks in the cord. I had one that kept blowing a fuse simply because the foot of some equipment had gotten on it for a short time and it was just enough to break the insulation on just the wire inside, not the rubber jacket outside. Left alone it might have been ok, but it had led to tripped breakers and a short in the plug like yours. Possibly dangerous situation.

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