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

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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
MSHA PROGRAM POLICY MANUAL VOLUME V Ungrounded three-phase high-voltage circuits may be permitted to feed stationary equipment only after an investigation has been made to determine that the use of such circuits in a particular mine does not pose a hazard to the miners. Ungrounded circuits shall not be accepted for circuits feeding portable power centers and rectifiers that supply power to mobile equipment. SHC cables (cables having common metallic shield around all conductors) are not to be accepted for use in underground highvoltage circuits. In all cases where ungrounded circuits are accepted for use underground, the circuit breaker protecting the circuit must be equipped with a ground-phase tripping circuit. Ground-indicating lights that do not trip the circuit breaker upon occurrence of a phase-to-ground fault are not acceptable. 75.803 Fail Safe Ground Check Circuits on High-Voltage Resistance Grounded Systems Ground check circuits are required to be designed so as to ensure a safe dependable path for fault current by causing the circuit breaker to open when either of the following occurs: 1. The ground check wire is broken at any point; or 2. The grounding conductor is broken at any point. If low-resistance parallel paths are present that prevent the ground check circuit from actuating the ground check relay when the grounding conductor is broken, the ground check circuit shall be acceptable as compliance with this Section if the ground check circuit is designed to cause the circuit breaker to open when the impedance of the grounding circuit increases beyond the amount necessary to cause a 100-volt drop external to the grounding resistor during fault conditions. The following method may be approved by electrical inspection personnel as an alternate method for ensuring continuity of a safe, dependable path for fault current for resistance-grounded circuits extending to permanently installed, stationary equipment located on the surface: 1. Grounding circuit shall originate at the grounded side of the grounding resistor and shall extend along with the power conductors and shall serve as a grounding conductor for the frames of all equipment receiving power from the circuit. JULY 2006 (Release V-38) 78
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