Where direct current is used underground in mines, the following
rules shall govern: (A) In determining the voltage limit the
difference in potential shall not exceed three hundred twenty-
five volts measured by a meter at the nearest switchboard except
with the written approval of the chief of the division of
mineral resources management. (B) For the protection of
circuits, a switch and circuit breaker shall be installed in the
ungrounded side of the circuit but may be omitted from the
return side. Fuses may be substituted for circuit breakers
transmitting twenty-five kilowatts or less. Each circuit leading
in the underground workings of such mine shall be provided with
a suitable ammeter. Additional switches shall be installed in
the ungrounded side of all branch circuits. (C) One side of
grounded circuits shall be very efficiently insulated from the
earth. (D) All trolley and feed wires shall be placed on the
opposite side of the track from refuge holes or necks of room.
All lines except telephone, shot fir