When electric power is used for running compressors supplying air for compressed air tunnel work and such power is purchased from a local central station or power company—

(1) There shall be two or more sources of power from the power company's stations to the compressor plant. Such power feeders shall each have a capacity large enough to carry the entire compressor plant load and normal overload. The feeders shall preferably run from separate generating plants or substations and be carried to the compressor plant over separate routes and not through the same duct lines and manholes so that the breakdown of one feeder shall not cause an interruption on the other feeder.

(2) There shall be duplicate feeder bus-bars, and feeder connections to the bus-bars shall be such that either feeder can feed to each separate bus-bar set, individually, or simultaneously to both sets.

(3) There shall be at least two compressors so connected to the bus-bars that they can be operated from either set of buses. The compressors shall be fed from different bus-bar sets, in such a way that a breakdown of a feeder or bus-bar would interrupt the operation of only part of the compressor plant.

(4) Duplicate air feed pipes shall be provided from the compressor plant to a point beyond the lock.