(b) Protection of Circuits Leading Underground. (1) In every completely insulated feeder circuit, in excess of twenty-five kilowatts capacity, leading underground and operating at a potential not exceeding the limits of medium voltage, there shall be provided above ground a circuit breaker arranged to open simultaneously each ungrounded conductor. In addition, a positive disconnect means shall be installed¹ by the circuit breaker. Overload protection shall be provided to open the circuit breaker in case of overload on any conductor. Fuses may be substituted for circuit breakers in circuits transmitting twenty-five kilowatts or less. Each power circuit in excess of fifty kilowatts leading underground shall be provided with a suitable ammeter.

(2) Every alternating current feeder circuit leading underground and operating at a potential exceeding the limits of medium voltage shall be provided above ground with a suitable circuit breaker, such breaker to be equipped with automatic overload trip, arranged to open simultaneously each ungrounded power carrying conductor. Each such circuit shall also be provided with a suitable ammeter.

(c) Cables in Shafts, Slopes and Boreholes. (1) All cables passing underground through inclines, boreholes and shafts shall be installed in a manner that will prevent undue strains in sheath, insulation, or conductors and damage by chafing of cables against each other or against the borehole casing or shaft. All power conductors in shafts, boreholes and inclines shall be covered with suitable insulating materials and installed to provide a minimum tensile factor of safety of five. Conductors shall be securely fastened and properly supported out of contact with combustible materials. When the weight, length and construction of a cable are such that suspension from its upper end only would subject the cable to possible damage, it shall be supported at intervals necessary to prevent undue strains in the sheath, insulation and conductors, and to provide a minimum tensile factor of safety of five. Adequate protection shall be provided so that no damage can result from water, electrolysis, moving cages, skips, ice, coal or other falling or moving materials.

(2) Installation of direct current and high voltage alternating current cables in the same borehole shall require approval of the secretary.

(f) Ventilation. (1) In any gassy mine bare power conductors shall not be installed in any air current that has passed through or by the first working place in the air split.

(2) In all mines high voltage transmission cable, high voltage motors and high voltage transformers shall not be installed in any air current that has passed through or by the first working place in the air split.

(g) Cables in Haulage Roads. (1) Where the cables or feed wires, other than trolley wires, in main haulage roads, cannot be kept at least twelve inches from any part of the mine car or locomotive, they shall be specially protected by proper guards.

(2) Cables and wires, except fire resistant, metallic covered or ground shielded cables shall be installed on roof, ribs, walls or timbers by means of efficient insulators. In no instance shall the method of support damage the cable jacket or armor.

(3) When main or other roads are being repaired, or blasting is being carried on, suitable temporary protection from damage shall be given the cables.

(4) All other wires, except telephone, shot-firing and signal wires should be on the same side of the road as the trolley wire.

(5) Haulage block signal circuits and other control circuits powered from the trolley shall be located on the same side of the road as the trolley.

(i) Underground Transformer and Substation Rooms. (1) Underground electric stations for permanent installations of pumps, hoists, compressors, rectifiers, rotary convertors and transformers, excepting room hoists and³ gathering pumps, which will remain in the same location for a period of one year or more, shall be constructed of incombustible materials and be well ventilated on positive intake air.

(2) Battery charging stations shall be separately ventilated with intake air, the return from which does not pass through any active workings.

(3) Permanent underground stations shall be provided with fire doors which close automatically in the event of fire. These stations shall be provided with a well of concrete or metal, or the equivalent, about the oil containing equipment that contains over twenty gallons, that will confine the oil in the event of leakage or explosion. All transformers ordered for underground installations in mines, subsequent to this ruling shall be of the dry air-cooled type or those containing nonflammable and nonexplosive liquid; other electrical devices or switches or combination of devices or switches ordered for underground installations shall not contain more than twenty gallons of inflammable oil.

(4) Transformers commonly called portable transformers, used underground, shall be located only in well ventilated places, and they shall be installed in fireproof enclosures fitted with doors which close automatically in the event of fire.

(5) Switchboards. Main and distribution switch and fuse boards shall be made of incombustible, moisture resistant, insulating material, and be fixed in as dry a situation as practicable, or shall be of suitable metal construction, exposed portions of which shall be effectively grounded. All switches, circuit breakers, rheostats, fuses and instruments used in connection with underground motor-generators, rotary-convertors, high voltage motors, transformers, and low and medium voltage motors of more than fifty horsepower or fifty KVA capacity, shall be installed upon a suitable switchboard or in a metal clad switchgear structure. Similar equipment for low and medium voltage motors of fifty horsepower and less, may be separately installed if mounted upon insulating bases of suitable material or effectively metal clad.

(6) Clearances. (i) In underground stations where switchboards are installed, there shall be a passageway in front of the switchboard not less than three feet in width and, if there are any high voltage connections at the back of the switchboard, any passageway behind the switchboard shall not be less than three feet clear. The floor at the back of the switchboard shall be properly floored and insulated with nonconducting material, accessible from each end, and in the case of high voltage switchboards, shall be kept locked, but the lock shall allow the door being opened from the inside without the use of a key. (ii) Where the supply is at a voltage exceeding the limits of medium voltage, there shall be no live metal work on the front of the main switchboard within seven feet of the floor or platform, and the space provided under subsection (6) (i) of this section shall not be less than four feet in the clear. Insulating floors or mats shall be provided for medium voltage boards where live metal work is on the front.

(7) Transformers. The primary of each underground power transformer shall be protected by a suitable circuit breaker equipped with automatic overload trip arranged to open simultaneously each undergrounded power conductor. The primary of a transformer of less than twenty-five KVA capacity may be protected by proper fuses. When a transformer is the only load on a branch circuit, the branch circuit protection can be considered the transformer protection.

(8) Outgoing Feeder Protection. Main circuits leaving underground substation or transformer stations shall be protected by circuit breakers.

(9) Grounding. All metal buildings, all metallic coverings, metal armoring of cables, and the frames and bedplates of generators, transformers and motors shall be effectively grounded, or equivalent protection provided.

(10) Identification of Hazard. All high voltage machines and apparatus shall be marked to clearly indicate that they are dangerous, by the use of the words “Danger, High Voltage,” conspicuously posted.

(11) Protection of Terminals. All terminals on machines, motors, or equipment over medium voltage underground shall be protected with insulating covers or with metal covers effectively connected to ground.

(12) Unauthorized Persons. No person other than one authorized by the mine foreman or mine electrician shall enter a station or transformer room or interfere with the working of any apparatus connected therewith.

(13) Fire Protection. Fire extinguishers suitable for extinguishing electrical fires shall be kept at electrical stations and transformer rooms, ready for immediate use.