Extension cords and leads

This is the one genuinely dangerous mistake in holiday lighting, and it is common. A cable drum wound with the cable still on it cannot shed heat — the turns insulate each other. Many reels are rated for 13 amps fully unwound and only around 3 amps while coiled, and that derating is printed on the drum where nobody looks.

Unwind the whole cord, every time, even if you only need three meters of it. A coiled reel carrying its full rated load will get hot enough to melt its own insulation.

Every meter of cable has resistance, so voltage at the far end is lower than at the outlet. Over a long run of thin cable this shows up as dimmer lights at the end of the display and heat in the cable itself.

The fix is thicker cable rather than shorter runs, when you cannot shorten the run. In North America that means dropping from 16 AWG to 14 or 12 AWG for anything over about 15 meters. At 230 volts the effect is much smaller for the same wattage, because the current is halved.

A cord rated for outdoor use has insulation formulated for UV and cold, and cold matters more than people expect: ordinary indoor PVC stiffens and cracks below freezing. Look for a W marking in North America or an appropriate IP-rated connector elsewhere.

Keep connections off the ground. Where two cords join outdoors, either use a proper weatherproof enclosure or hang the join so water cannot sit in it. A plastic container over a joint is a folk remedy, not a solution.