Hot washing of insulators refers to cleaning the insulators in transmission lines when the lines are live. Transmission lines can afford very little downtime. Cleaning the hundreds of insulators has to be carried out when the lines are live with voltage.
Insulators get dirty due to dust, moisture, bird droppings and chemicals from smoke. These deposits will form a layer over the surface and can contribute to a flashover between the conductor and the grounded frame of the transmission tower. Thus, periodic cleaning of the insulators is essential.
Hot washing involves cleaning the insulator surfaces with de-mineralized water. De-mineralized has high resistivity (greater than 50000 ohm cm). The water is pressurized and sprayed in jets from special cleaning machines. These cleaning machines are stationed on the ground or in some cases fixed on helicopters which hover near the lines and clean the insulators.
The Hot washing is usually carried out from the bottom of the insulator. The whole insulator is not made wet at any given point of time. The bottom of the insulator is washed and then the washing proceeds to the middle sections and the then to the top of the insulator.
The water spray from cleaning one insulator should not fall on another insulator. This may cause a flashover. The wind direction should also be taken into account.