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

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MSHA PROGRAM POLICY MANUAL VOLUME V The frames supporting structures and enclosures of stationary high-voltage equipment receiving power from a wye-connected power system in which the neutral of the source is solidly grounded shall be considered effectively grounded provided: 1. such frames, structures and enclosures are solidly connected to a neutral conductor which originates at the neutral point of the power source, extends along with the power conductors and serves as the grounding conductor for the frames, supporting structures and enclosures of all high-voltage equipment supplied power from the system; and 2. the neutral conductor is solidly connected to a ground field at the power source and at each utilization location. February 2003 (Release V-33) 193
MSHA PROGRAM POLICY MANUAL VOLUME V Subpart J Low- and Medium-Voltage Alternating Current Circuits 77.900 Low- and Medium-Voltage Circuits Serving Portable or Mobile Three-Phase Alternating-Current Equipment; Circuit Breakers Low- and medium-voltage circuits supplying power to portable or mobile three-phase alternating-current equipment shall be protected against the harmful effects of a grounded phase in any circuit connected to the same transformer secondary. Consequently, if one bank of transformers supplies power to both stationary and portable or mobile loads, the circuits, extending to the stationary loads, as well as the circuits extending to the portable or mobile loads, shall be provided with grounded phase protection. Grounded phase protection for resistance-grounded circuits should be adjusted to operate on as low a value of fault current as practical; preferably not more than 40 percent of the current rating of the neutral grounding resistor. 77.902 Low- and Medium-Voltage Ground Check Monitor Circuits The ground check circuit required by this Section shall cause the circuit breaker to trip when any of the following occur: 1. The ground check wire is broken. Existing low- and medium-voltage cable couplers that are not provided with a terminal contact for the ground check conductor may be used if suitable means are provided for breaking the ground check conductor first when the coupler is being uncoupled. 2. Either the ground wire is broken at any point, or the impedance of the grounding circuit increases beyond the amount necessary to cause a 40-volt drop in the grounding circuit external to the grounding resistor under ground fault conditions. 77.903 Disconnecting Devices "Visual evidence," as used in this Section, means that a physical separation of the current-carrying parts of the disconnecting device can actually be seen. Consequently, molded case circuit breakers are not acceptable as disconnecting devices. A connecting plug on the outby end of a cable with which the cable is connected to the power source box shall be accepted as a disconnecting device. Other visible disconnecting means, such as switches with visibly opened contacts, will also meet the requirements of this Section. 77.904 Identification of Circuit Breakers February 2003 (Release V-33) 194
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Program Policy Manual - VOLUME V - Mine Safety and Health ...

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