Safety clearances for overhead lines

For my own reference:

National Grid part 1

National Grid part 2

Crane contact

A voltage gradient of 30,000V per cm is needed for electricity to arc through air.  Contact with the conductor and the arc is initiated, a voltage difference of only 20V per cm is needed to maintain the arc.  Therefore, a long arc can be maintained once contact has been made.

Koller et al. 1994 “An important fact is that arcing will cross 2-3 cm for every 10,000 V (Skoog, 1970) so that no direct contact is necessary to trigger arcing at 25,000 V. In addition, once an electrical arc is established, it may extend over several metres (Skoog, 1970). The so-called critical distance is the distance between the power source and another grounded subject which may trigger arcing. In moving subjects, such as in our victims, the arcing can be triggered over a longer distance than in static ones.”

OSHA “mechanical equipment, should remain at least 10 feet (3.05 meters) away from overhead power lines. If the voltage is more than 50,000 volts, the clearance increases by 4 inches (10 centimeters) for each additional 10,000 volts”.

Initiated arc on YouTube.

Greenwald, pointing out that an arc is harder to initiate through moist air.

Look Up! Look Down! Look Out!