Section 21C. The owner, agent or employer responsible for the operation of equipment shall post and maintain in plain view of the operator on each crane, derrick, power shovel, drilling rig, hay loader, hay stacker, pile driver, or similar apparatus, any part of which is capable of vertical, lateral or swinging motion, an approved weather-resistant warning sign legible at twelve feet reading “WARNING—Unlawful to operate this equipment within SIX FEET of high voltage lines.”