VOLUME V - COAL MINES
75.520 Electric Equipment; Switches
All control devices shall be fully enclosed to prevent exposure of bare wires and energized parts. The use of plugs and receptacles (e.g., Miller plugs), trolley taps, and trolley wire "stingers" to start and stop electric motors are examples of noncompliance with this Section.
75.521 Lightning Arrester; Ungrounded and Exposed Power
and Telephone Wires
Conductors that are: (1) provided with metallic shields; (2) jacketed by a ground metal covering or enclosure; (3) installed under grounded metal framework; (4) buried in the earth; or (5) made of triplex or quadraplex that is supported by a grounded messenger wire, are not considered exposed for the length so protected. If the trolley wire of a d.c. system is paralleled by an exposed feeder cable, one lightning arrester would provide protection for both if they are connected together near the lightning arrester.
Lightning arrester ground fields shall be maintained with as low a resistance to earth as possible, preferably less than 5 ohms and no more than 25 ohms. Lightning arrester ground fields shall be separated from neutral ground fields by at least 25 feet. This distance prevents lightning surges from being transmitted to the neutral ground field where they could momentarily energize the frames of equipment grounded to the neutral ground field.
Mines using single-phase power originating at the power company's secondary and extending underground cannot normally comply with this Section due to the power company's practice of connecting the lightning arrester ground to the grounded neutral which also connects to the center tap of the transformer, unless an isolation transformer is installed or the power company isolates the lightning arrester ground from the center tap ground and a separate neutral ground field is established.
75.523-1 Deenergization of Self-Propelled Electric Face
A machine that can only be operated from a remote location is not required to be provided with a device that will quickly deenergize the tramming motors of the machine in the event of an emergency. However, if at any time the remote controls are placed on the machine for the purpose of tramming the machine, or if for any other reason the operator trams the machine from onboard or alongside the machine, the machine shall be provided with a device that meets the requirements of this Sections and Section 75.523-2.
Any device other than a bar or lever used for deenergizing tramming motors such as a "dead man control" can be accepted only by an approval letter from Technical Support.
75.523-2 Deenergization of Self-Propelled Electric Face
Equipment; Performance Requirements
The actuating bar or lever and its associated linkage should be designed to activate a circuit breaker, control switch, hydraulic valve, or other device that will deenergize the tramming motors when the actuating bar or lever is depressed.
The bar or lever shall extend a sufficient distance in each direction to provide a ready means of deenergizing the tramming motors without requiring the operator to change position or search for a safe stop switch if an emergency occurs.
The actuating bar or lever shall be designed so that a horizontal or vertical (downward) pressure of 15 pounds force shall cause deenergization of the tramming motors before the bar has traveled more than 2 inches.
Subpart G .... Trailing Cables
75.600 Trailing Cables; Flame Resistance
In order to maintain the flame-resistant qualities of the cable, this Section prohibits the use of flammable materials for replacing the outer jacket. Only flame-resistant tapes or other materials that have been accepted by Technical Support as the jacketing material in permanent splice kits and jacket repair kits will be acceptable for the repair or replacement of outer jackets.
75.601 Short-Circuit Protection of Trailing Cables
Section 75.900 prohibits the use of fuses for the short-circuit protection of three-phase a.c. trailing cables. Only fuses that have been tested and approved by Technical Support under Part 28 are acceptable for the short-circuit protection of d.c. and single-phase a.c. trailing cables. Approved fuses are identified by an MESA or MSHA approval number. (See Sections 75.601-2 and 75.601-3.)
A length of cable meeting the requirements of Section 75.600 and installed between a power center or rectifier and a distribution box, and laying on the mine floor will be considered to be a trailing cable and is required to comply with all applicable requirements of this Subpart.
In systems where rectifiers supply ungrounded or neutral-grounded direct-current power to mobile face equipment, short-circuit protection must be provided for both ungrounded conductors of the trailing cable. A properly adjusted two-pole circuit breaker or MSHA approved fuses installed in each ungrounded conductor would constitute compliance.
Adequate current-interrupting capacity means that the fuse or circuit breaker is capable of safely interrupting the current that can flow upon the occurrence of a short circuit at any point in the protected circuit.
Enclosed circuit breakers are not acceptable as visual evidence that the power is disconnected. Plugs and receptacles located at the circuit breaker and trolley nips are acceptable as visual means of disconnecting the power.
A "disconnecting device" is defined as both the trailing cable plug (cathead) and the receptacle. Both the plug and the receptacle must be marked in a similar manner.
75.601-2 Short-Circuit Protection; Use of Fuses;
Approval by the Secretary
The voltage rating of a fuse shall not be less than the maximum voltage of the circuit in which it is installed.
75.602 Trailing Cable Junctions
Plugs of the same size may be used for different cable sizes if dowel pins or other devices are provided to insure that each cable can only be connected to a circuit breaker of the proper size or if the plugs and receptacles are connected by a short length of chain long enough to permit the plug to be inserted into and withdrawn from the proper receptacle only.
75.603 Temporary Splices of Trailing Cables
A splice includes the mechanical joining of a grounding conductor that has been severed.
The connection of the trailing cable made inby the strain clamp on cable reel equipment that does not have provisions for the trailing cable to enter the collector ring compartment shall not be considered a temporary splice. The method of cable attachment accepted in the approval of the equipment will be in compliance with this provision.
The conductors of a temporary splice shall be joined together so that the passage of current will not create excessive heat at the connection. Each power conductor, grounding conductor, and ground-check conductor shall be individually spliced using a proper splicing sleeve, ring, or clamp; each power conductor shall be individually insulated with proper insulating tape, and the outer jacket shall be replaced with flame-resistant tape or other flame-resistant material to provide an outer jacket as thick as the original.
Torn or damaged insulation on a trailing cable shall be reinsulated and does not constitute a splice unless a conductor is severed.
75.604 Permanent Splicing of Trailing Cables
Materials listed by MSHA's Approval and Certification Center as flame resistant for use in making permanent splices in trailing cables shall be used in complete accordance with the manufacturer's instructions. Splice kits shall be applied without substituting or altering any parts. Any deviation would require additional evaluation or testing by MSHA. Without such evaluation, such deviations shall constitute noncompliance with this Section.
75.605 Clamping of Trailing Cables to Equipment
Section 18.40 requires insulated strain clamps or cable grips on the trailing cables of permissible machines, except that mesh-wire cable grips (e.g. Kellems grips) are not permitted on trailing cable reels. Where mesh-wire cable grips are permitted, they shall be installed in accordance with the following:
- Sufficient slack shall be provided in the trailing cable between the machine and the cable grip.
- A hose clamp shall be provided on the outby end of the cable grip to prevent slippage along the cable jacket.
75.606 Protection of Trailing Cables
Trailing cables shall be placed away from roadways and haulage ways where they might be run over or damaged by mobile equipment. Where the method of mining requires that trailing cables cross roadways or haulage ways, the cables shall be securely supported from the mine roof, or if the height of the coal seam does not permit hanging the cables, the cables shall be installed in a trench cut into the mine roof or mine floor. A substantial bridge for the equipment to pass over the cables is also acceptable if, in the opinion of the inspector, the cables are adequately protected.
Subpart H .... Grounding
75.701 Grounding Metallic Frames, Casings, and Other Enclosures of Electric Equipment
When a three-phase resistance grounded system supplies power to a rectifier bridge and to other portable or mobile loads, neither the positive nor negative conductor of the direct-current circuit can be grounded. The frames of the direct-current equipment must be grounded to the alternating-current ground rectifier frame.However, when a power system is being designed so as to provide a separate bank of transformers to supply power to the rectifier bridge and a separate bank of transformers to supply power to thea.c. portable or mobile loads, then one polarity of the rectifier bridge can be grounded. All frames of direct-current equipment are then grounded to the grounded power conductor.
75.702 Protection Other Than Grounding
Portable tools protected by an approved system of double insulation, or its equivalent, need not be grounded. Where such an approved system is employed, the tool will be distinctly marked.
75.703 Grounding Off-Track Direct-Current Machines and Enclosures of Related Detached Components
"Related detached components" refers to associated parts such as contactor compartments, control switches, or rheostats that are not installed on the frame of the machine. The metal frames or enclosures of such components shall be connected to the same grounding medium as the main frame of the machine to which it is related.
Metal frames or housings of pumps, battery chargers, sequence and slippage switches, breaker boxes, remote control switches,rheostats, rock-dusting machines, auxiliary fans, belt drives,belt feeders, contactor compartments, and any other equipment or devices receiving power from direct-current power systems shall be grounded to the grounding medium of the power system feeding such equipment or devices. The grounding medium for d.c. systems is normally the mine track or grounded d.c. feeder. When ungrounded d.c. systems are used, the grounding medium is usually the grounded frame of the rectifier or generator.
The metal battery case of batteries being charged shall be grounded. In instances where batteries are being charged without removing them from mobile equipment, the frame of the machine or battery case shall be grounded to the grounded frame of the charger to prevent the machine from becoming "alive" through failure of insulation in the charger, such as between primary and secondary windings of transformers and leakage through spilled electrolyte from the batteries.
This Section requires that metal battery trays be effectively grounded to the battery charger frame during charging. Technical Support's Mine Electrical Systems Branch conducted tests on two-pole battery connectors to evaluate the effectiveness of the electrical connection between the connector housings as the means of grounding the battery trays. These tests indicate that the tolerance fit between the male and female connector housings does not provide an effective electrical connection, particularly when the connectors are contaminated with water, rock dust, or mud.
This Section also requires that metal battery connector housings be effectively grounded to the battery charger frame during charging. Consequently, provisions must also be made to effectively ground metal battery connector housings during charging.
All grounding conductor connections shall be clamped or bolted connections, shall be capable of carrying any fault current to which it may be exposed, and shall be connected first and disconnected last.
75.703-3 Approved Methods of Grounding Off-Track Mobile,Portable, and Stationary Direct-Current Machines
Diode grounding of equipment is not acceptable on direct-current systems which have both the positive and negative polarities ungrounded.
75.706 Deenergized Underground Power Circuits; Idle Days - Idle Shifts
Circuits supplying power to automatically-operated pumps shall be considered as being in use although the pumps may not be operating continuously.
Subpart I .... Underground High-Voltage Distribution
75.800 High-Voltage Circuits; Circuit Breakers
A suitable circuit breaker is one that is:
- Capable of interrupting the maximum fault current to which it may be subjected without damage to itself;
- Capable of carrying the continuous current imposed upon it without damage to itself; and
- Rated for not less than the voltage of the circuit.
A high-voltage circuit extending underground shall be protected against the harmful effects of a grounded phase in the under-ground circuit and in any surface circuit supplied from the same set of transformer windings. Consequently, if one set of transformer windings supplies resistance-grounded power to both underground and surface loads, the circuit(s) extending to the surface loads must also be provided with grounded-phase protection. Fuses may be used to provide grounded-phase protection only for small control transformers installed in the same substation as the transformers that supply the resistance-grounded circuit. In all other cases, circuit breakers equipped with grounded-phase protective devices must be used to provide the required grounded-phase protection.
Grounded-phase relays should be adjusted to operate on as low a value of current or voltage as practical. In order to provide safe, reliable relaying, settings should not exceed 50 percent of the maximum fault current for current relaying or 50 percent of the phase-to-neutral voltage for potential relaying.
Where an ungrounded high-voltage circuit is accepted for use underground under the provisions of Section 75.802(b), the circuit must be provided with grounded-phase protection.
Grounded-phase indicating lights that do not trip the circuit breaker upon the occurrence of a phase-to-ground fault are not acceptable as compliance with this Section.
Short-circuit protection can be provided by using the instantaneous units of overcurrent relays or by using inverse-time overcurrent relays with minimal time dial settings.
The pickup of the instantaneous unit of an overcurrent relay is independent of the pickup of the inverse-time unit and is determined by the position of the top of the screw on the instantaneous unit.
Overcurrent devices are required in at least two phases of three-phase high-voltage underground distribution circuits.
75.800-3 Testing, Examination, and Maintenance of Circuit Breakers; Procedures
If the circuit breaker located on the surface provides overload,short-circuit, under voltage, and grounded-phase protection for the entire underground circuit, only that circuit breaker is required to be tested by this Section. Circuit breakers installed underground for coordination purposes in such circuits are not required to be tested; however, each independent ground-check circuit must be tested in accordance with this Section.
The tests required by paragraph (b)(2) of this Section may be conducted one of three ways:
- Primary injection test. This test method involves sufficient current to cause the circuit breaker to trip through at least two current transformers associated with the circuit breaker. Since this method requires that test connections be made on high-voltage conductors or terminals, stringent safety procedures must be followed. This method simultaneously tests the current transformer ratio, the current transformer secondary wiring, the operation and calibration of the relays, and the operation of the circuit breaker tripping circuit.
- Secondary injection test. This test method involves passing sufficient current to cause the circuit breaker to trip through at least two of the protective relays associated with the circuit breaker.
This method simultaneously tests the operation and calibration of relays and the operation of the circuit breaker trip circuit.
- Mechanical activation test. This test method involves mechanically activating at least two of the protective relays associated with the circuit breaker with a non-conductive probe. This method tests the operation of the circuit breaker trip circuit.
75.801 Grounding Resistors
Grounding resistors that are manufactured to meet the extended time rating as set forth in IEEE Standard 32-1972, formerly AIEE Standard 32, are acceptable as compliance with this Section. Resistors that are not manufactured in compliance with IEEE Standard 32-1972 shall be "rated for maximum fault current continuously."
75.802 Protection of High-Voltage Circuits Extending Underground
Grounding transformers must be sized to carry the rated ground-fault current of the system continuously. Consequently, the kVA rating of a zigzag grounding transformer or a wye-delta grounding transformer bank shall not be less than the rated phase-to-neutral voltage of the system (VON) in kV times the rated ground-fault current of the system (I) in amperes. If other loads are supplied from a wye-delta grounding transformer bank, the rating of the transformer bank must be adequate to supply the additional loads plus the rated ground-fault current.
Transformer banks connected wye on the primary side and supplied power from a resistance-grounded circuit shall not have the primary neutral grounded.
When used, grounding transformers shall be connected on the line side of circuit breakers so that the system will always be grounded and must be located in/or adjacent to the same substation as the power source transformers.
This Section requires that the grounding resistor be located at the source transformers. As used in this Section, the term "source transformer" means the transformer that supplies power to the electric circuit.
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) . 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.
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 high-voltage 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.