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

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MSHA PROGRAM POLICY MANUAL VOLUME V Subpart H Grounding 77.700-1 Approved Methods of Grounding Approved methods of grounding the metallic enclosures of electric equipment receiving power from ungrounded alternating-current power systems are contained in Section 77.701-1. Approved methods of grounding the metallic enclosures of electric equipment powered from direct-current power systems are contained in Section 77.701-2. Subpart H does not specify approved methods of grounding the metallic enclosures of stationary equipment receiving power from low- and medium-voltage, solidly grounded alternating-current systems. Nevertheless, Section 77.701 requires that such enclosures be grounded by methods approved by an authorized representative of the Secretary. An inspector shall not approve any method of grounding that does not include a solid connection to a grounding conductor which extends to the grounded point of the power source. The grounded power conductor of a solidly grounded alternating-current power system may serve as the equipment grounding conductor only between the grounded point of the power source and the grounded enclosure of the service disconnecting means for a building or other stationary facility. The grounded point of the power source and the metallic enclosure of each service disconnecting means shall be connected to an acceptable grounding medium such as metal waterlines having lowresistance to earth, a low-resistance ground field, etc. Subpart H does not specify approved methods of grounding the metallic enclosures of portable or mobile equipment receiving power from high-voltage systems since the requirements for grounding such enclosures are specifically contained in Section 77.802. This Section specifies approved methods of grounding metallic sheaths, armors and conduits enclosing power conductors. The approved methods of grounding listed in this Section are similar in that they each require a solid connection to an acceptable grounding medium. In resistance-grounded systems, the only acceptable grounding medium is the grounded neutral of the system. In other systems, an acceptable ground medium includes: 1. Metal waterlines having low-resistance to earth; 2. A low-resistance ground field; or 3. Any other grounding medium which ensures that there is no difference in potential between the metallic enclosures February 2003 (Release V-33) 185
MSHA PROGRAM POLICY MANUAL VOLUME V and earth. It should be emphasized that the metallic enclosures of all circuits supplied power from the same system must be solidly connected to the same grounding medium. 77.701 Grounding Metallic Frames, Casings, and Other Enclosures of Electric Equipment Certain moveable electric equipment, e.g., rail-mounted and pivoting coal stackers, traveling shop cranes on track rails, small traveling hoists on I beams, etc., cannot be strictly classified as portable, mobile or stationary equipment. For the purposes of frame grounding, such equipment shall be considered stationary. Consequently, the grounding requirements of Subpart H apply to such equipment. This Section requires that metallic frames of electric equipment be grounded by methods approved by an authorized representative of the Secretary. Therefore, rail-mounted and pivoting coal stackers, traveling shop cranes on track rails, small traveling hoists on I beams, and similar equipment shall be grounded in accordance with the following: All tracks shall be bonded or welded at each joint, and each individual track rail or I beam shall be solidly grounded through a grounding conductor which meets the requirements of Section 77.701-3 to an acceptable grounding medium. In instances where the conveyor, hoisting and/or tramming motors receive power through a trailing cable, the moving frame shall be grounded to an acceptable grounding medium through a proper size grounding conductor inside the cable. In instances where the conveyor, hoisting and/or tramming motors receive power through a trolley system, the moving frame shall be grounded to an acceptable grounding medium through an additional grounding trolley contact. 77.701-1 Approved Methods of Grounding of Equipment Receiving Power From Ungrounded Alternating-Current Power Systems The metallic enclosures of all equipment supplied power from the same system must be solidly connected to the same grounding medium. 77.701-2 Approved Methods of Grounding Metallic Frames, Casings and Other Enclosures of Electric Equipment Receiving Power from a Direct-Current Power System Paragraph (b) requires that metal battery trays be solidly connected to the grounded frame of the battery charger while the batteries are being charged. 77.702 Protection Other Than Grounding February 2003 (Release V-33) 186
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Program Policy Manual - VOLUME V - Mine Safety and Health ...

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