Program Policy Manual - VOLUME V - Mine Safety and Health ...

MSHA PROGRAM POLICY MANUAL VOLUME V
The frames of electric equipment receiving power from a
resistance-grounded system that supplies a circuit extending
underground must be grounded to the grounded side of the grounding
resistor, regardless of whether the equipment is located on the
surface or underground. However, the metallic frames, enclosures,
and supporting structures of all high-voltage equipment and
conductors located inside either a portable or stationary
substation (including the source transformers, control
transformer, grounding resistor, and circuit breakers) must be
grounded to the same grounding medium to prevent hazardous step
and touch potentials from existing within the substation during a
grounded-phase condition or a lightning strike. Therefore, the
metallic frames, enclosures, and supporting structures of all
electric equipment and conductors located inside either a portable
or stationary substation shall be grounded to the substation
grounding medium.
All high-voltage power transformers and other equipment that
receive power from a resistance-grounded system that supplies a
circuit extending underground should be located outside the
substation containing the source transformers. The frames,
enclosures, and supporting structures of such equipment should be
grounded to the grounded side of the grounding resistor. When
equipment receiving power from a resistance-grounded circuit is
installed inside the same substation as the source transformer,
compliance with Section 75.521 and 75.802 is extremely impractical
without creating step and touch potential fault conditions and
lightning strikes.
Voltage regulators and capacitors located on the load side of the source transformer inside
the substation are part of the power source and are not considered part of the load. As
part of the power source, they must be grounded to the substation-grounding medium.
Among other things, capacitors are used to improve the power-factor by reducing reactive
power in the system. Capacitors lower system losses and improve voltage. Voltage
regulators control voltage spread at the utilization equipment under all load conditions.
These components do not receive power from the source transformer, but they regulate
the power supplied to the different loads (equipment). If these components are located
inside the substation, their frames must be grounded to the substation-grounding medium
to prevent step and touch hazards.
Substations are normally designed in accordance with IEEE Std 80-1986 (ANSI) [3].
These designs include reinforcing bars contained on the below–grade foundation
structure. The purpose of these designs is to limit step and touch potentials at substations
to values that are intended to eliminate the risk of dangerous electric-shock
exposure to persons either working within the substation or approaching the substation.
JULY 2006 (Release V-38) 77