The weather is humid and there is electricity leakage.

The insulation of the conductor is aging, the insulation strength is reduced, and the leakage current is not large at ordinary times. When the weather is wet, the insulation strength will be further reduced, so that the leakage current will increase and the leakage relay will operate. The charged conductor sheath means that the insulation strength is reduced, and the water molecules in the humid air are conductive bridges.