Bad effects of coiling electrical cables

In the article that you linked to, it says in one of the answers,

The most notorious feature of loaded coiled cables is that they potentially generate a lot of heat in a tight space. In most cases it's not an issue, but at high load with little cooling such a coil could be a fire hazard.

And I actually have first hand experience with this. There was this extension cord (50 or 100 feet) that was wrapped around a spool, which I needed to power a small room heater. Still it was a 1500W heater which draws over half of what the outlet could deliver. Which means that it easily qualifies as a high load.

Lucky ending to the story: I could smell the insulation melting which led me to discover the danger and pulled the plug before it caught fire!

(I would have posted as a comment, but my rep < 50 Thought the OP really should have this info)

Is there any bad effects of coiling electrical cables ?

That depends on the cirumstances.

The electrician is correct for the part of electrical engineering that he (probably) deals with. And that is high-power devices (more than say 200 Watt) running on mains (AC) voltage. With high power devices, large currents flow and these large currents through mains wires cause the cable to warm up. That's OK if the cable has enough "breathing space". If you coil up the cable that might not be the case. Especially in devices like these:

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the heat cannot escape if you do not completely unwind all the cable.

If you would only be using this for powering one low power device like a radio or a phone charger, the heat generated will be of no concern, you can leave the cable rolled up.

But if you use it to power a 300 W floodlight and/or a lawnmower and/or a toaster then you do need to unroll the cable completely so that any generated heat can escape into the air.

Fortunately for you USB and nearly all computer cables cables are quite low power so coiling them up is no issue at all.

As winny commented: feel the cables wjhen they have been in use for a while. If you cannot feel that they are getting warm, then there is no issue.

I don't think the other answers are correct in stating that the heat is the only issue, though it is the biggest. I came into my office one Monday morning and found that the door had been forced open by the fire department who had put out a fire in the server room where someone had used a coiled cable to power a server - it had been like that for weeks but updates running on the server caused the power use to be higher for a sustained period of time.

However, power going through a coiled cable will also create a more concentrated electromagnetic field which can cause other problems. The most frequent problem I've experienced caused by this is a distortion in a CRT monitor caused by the power cable being coiled at the back of it, but others would be possible.

For example a long USB cable coiled with a long power cable could result in a significant voltage being induced in the USB cable, potentially destroying the USB port and/or peripheral.