There are several reasons for the cable heating without load
There may be an intermediate joint in the cable. The insulation of the joint is not properly handled, which leads to leakage between phases, but there is no short circuit. This leakage is equivalent to a capacitor without breakdown, and the capacitor has current, so the wire will heat up.
If it is a rubber cable, it may be that the rubber sheath is damaged, there is water or moisture in it, which causes the insulation strength to decline and is close to the short circuit limit without short circuit.
The partial grounding fault of the phase line causes part of the current to flow underground.
Low voltage megger can be used to test the resistance, as long as it is higher than 0.5m Ω, you can continue to use. But I think that since the heat is heating, it must be less than 0.5m Ω. After the power failure, you can touch the wire by hand to find out the hottest place, and then use heat shrinkable tube or PVC insulating tape to wrap it. The wrapping layer should not be less than 15 layers.