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, 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 ten feet of high voltage conductors; and shall post and maintain similar signs on the outside of the equipment in such locations as to be readily visible to other persons engaged in the work operation or in the vicinity of the work operation.