VOLUME V - COAL MINES
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.
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.
2. Second grounding circuit shall connect the frames of the stationary equipment to a low resistance ground field located near the utilization location.
3. The resistance of the grounding resistor and the resistance of the ground field shall be maintained in such a manner that not more than 100 volts will appear between the equipment frame and earth under fault conditions in the event that the grounding conductor should be severed.
75.803-2 Ground Check Systems not Employing Pilot Check Wires; Approval by the Secretary
Wireless ground check circuits shall not be approved unless such circuits are tested and evaluated by Technical Support.
75.804 Underground High-Voltage Cables
This Section does not apply to cables of ungrounded high-voltage circuits that are enclosed in steel armor or rigid steel conduit. All high-voltage cables, whether single or multiple conductor, shall conform to this Section.
If single-conductor high-voltage cables are used, the cables should be installed on a well-supported messenger wire or on cable hangers, and a power grounding conductor and ground check conductor shall extend along with the power conductors. The messenger wire, when paralleled with the cable shielding, can serve as the grounding conductor for the circuit if the impedance of the grounding circuit is low enough to limit the voltage drop to 100 volts or less. Normally, the high resistance of steel messenger wire will limit the distance the circuit can be extended. The shielding shall be connected to the messenger wire at each splice (single-conductor cables) and, at each termination. Inspectors must calculate the parallel resistance of the messenger wire and the three shields when determining compliance with the requirement for a maximum voltage drop of 100 volts.
Cables in systems using "other no less effective device approved by the Secretary or his authorized representative" as provided for in Section 75.803 for assuring grounding circuit continuity are exempted from the requirement to have an insulated ground check conductor.
An existing high-voltage coupler that is not equipped with a pin for the ground check circuit may be used if a locking switch is mounted on the coupler so that it cannot be uncoupled until the key is inserted in the lock and the switch is opened. This operation breaks the ground check conductor first. Several equipment companies are manufacturing conversion kits for installation on existing couplers not provided with ground check pins.
Cable couplers that are mounted on portable substations are considered to be receptacles and should be grounded to the same grounding medium as the substation enclosure. When these portable substations supply power to underground circuits, the grounding conductor and shielding in the cable should be insulated from the coupler housing, and the metallic housings of both the receptacle and the male part of the metallic frame of the portable substation. The grounding conductor shielding shall be connected to the grounded side of the groundings resistor.
75.807 Installation of High-Voltage Transmission Cables
Energized high-voltage cables shall be stored in unused crosscuts or other unused areas away from haulageways or mantrip stations where miners or equipment could contact or damage such cables.
75.808 Disconnecting Devices
A "branch line" means a circuit that is formed by connection to an existing high-voltage circuit for the purpose of feeding branch loads. Cable couplers are acceptable as a disconnecting device only when used with an acceptable device such as a circuit breaker or oil-filled fused cutouts that are used to deenergize the circuit before the cable coupler is uncoupled. If a remote switch in the ground check circuit is used to trip a circuit breaker prior to uncoupling the coupler, visible or audible evidence must be provided to indicate that the circuit breaker has opened before the coupler is uncoupled.
75.809 Identification of Circuit Breakers and Disconnecting Switches
The identifying markers for circuit breakers and disconnecting switches shall be large enough and shall be located so that they can be readily seen if the circuits need to be deenergized quickly.
Either metallic or plastic material may be used for the marker to adequately identify the circuit (e.g., No. 2 Belt Drive, No. 3 Rectifier, 1 Rt. 3 North, etc.).
The identifying markers shall leave no doubt as to which circuit or circuits will be deenergized when the switches are pulled.
Subpart J....Underground Low- and Medium-Voltage Alternating Current Circuits
75.900 Low- and Medium-Voltage Circuits Serving Three-Phase
Alternating Current Equipment; Circuit Breakers
Each of the four protective features required by this Section must be provided for all underground low- and medium-voltage three-phase a.c. circuits with the exception of trailing cables,which must have short-circuit, under voltage, and ground-phase protection. Circuit breakers providing short-circuit protection for trailing cables shall be adjusted so as not to exceed the maximum allow able instantaneous settings specified under Section 75.601-1. Short-circuit and overcurrent protection for circuits shall conform to the National Electrical Code, 1968. Charts listing sizes and proper overload and short-circuit protection for motors and motor circuit conductors can be found in Appendix E of this volume. These tables shall be used to determine compliance with this Section.
"Adequate interrupting capacity" is the ability of a circuit breaker to safely interrupt the maximum amount of current that can flow through its contacts upon occurrence of a short circuit at any point in the circuit without damage to itself.
Low- and medium-voltage three-phase circuits used underground 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 underground and surface loads, both the surface and underground portions of the circuit(s) shall be provided with ground-phase protection.
Grounded-phase protective devices for resistance-ground circuits should be adjusted to operate on as low a value of fault current as practical, preferably not more than 50 percent of the current rating of the neutral grounding resistor.
75.901 Protection of Low- and Medium-Voltage Three-Phase Circuits Used Underground
When two phase conductors of a three-phase resistance grounded circuit are used to power single-phase loads, the equipment frames must be grounded to the grounded side of the grounding resistor. The grounding circuit shall be provided with a ground check circuit as required by Section 75.902 and the circuit shall be protected in accordance with the requirements of Section 75.900.
Ungrounded three-phase low- and medium-voltage circuits may be permitted in an underground coal mine to feed stationary equipment only after an investigation has been made by electrical personnel and after that investigation finds that the use of such circuits in a particular mine does not pose a hazard to the miners.
When two phase conductors of an ungrounded surface three-phase circuit are taken underground for use as a control circuit, this circuit exhibits all the hazards inherent in an ungrounded three-phase circuit and shall be judged accordingly. If an operator wishes to install an ungrounded circuit underground, including the control circuit described above, application should be made to the District Manager. The District Manager shall assign electrical inspectors or electrical engineers to an investigation of the application, and, if it is determined that no hazard is created by the installation of the ungrounded circuit, the District Manager shall notify the operator in writing of the acceptability of the circuit and of any restrictions imposed on the operation of the circuit.
Ungrounded circuits feeding portable distribution boxes or power centers that supply power to mobile equipment are not to be accepted.
In all cases where ungrounded circuits are accepted for use underground, the circuit breaker protecting the circuit must be equipped with grounded-phase protection. Ground-indicating lights that do not trip the circuit breaker when a phase-to-ground fault occurs are not acceptable.
75.902 Low- and Medium-Voltage Ground Check Monitor Circuits
The following criteria shall be used for determining compliance of ground check circuits in low- and medium-voltage systems supplying power only to stationary equipment.
- If a ground check conductor is used, the ground check circuit will trip the circuit breaker when the ground check conductor is broken.
- The ground check circuit will trip the circuit breaker if the ground wire is broken at any point in the grounding circuit. If low resistance parallel paths for fault current and monitoring current are present, the ground check circuit will be acceptable if it is designed to trip the circuit breaker when 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 fault conditions.
- Current flow in the ground check circuit will cause pickup of the ground check relay.
The following criteria shall be used for determining compliance of ground check circuits in low- and medium-voltage systems supplying power to self-propelled equipment:
- If a ground check conductor is used, the ground check circuit will trip the circuit breaker when the ground check conductor is broken.
- The ground check circuit will trip the circuit breaker if the ground wire is broken at any point in the grounding circuit. If low resistance parallel paths for fault current and monitoring current are present, the ground check circuit will be acceptable if it is designed to open the circuit breaker when 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 fault conditions.
- The ground check device shall be of failsafe design. "Failsafe" is interpreted to mean that the failure of any component, other than relay contacts, shall not prevent the ground check circuit from opening the circuit breaker when the conditions described in criteria 1 and 2 occur, unless the ground check circuit is designed to open the circuit breaker when such failure occurs.
When an arc suppression device is installed in a power center,the ground check circuit should be connected on the machine side of the device. Monitoring through an arc suppression device preloads the device and reduces its effectiveness in suppressing intermachine arcing and may also cause false tripping of the ground check circuit.
Any device inserted in a grounding conductor (including an arc suppression device and a parallel-path suppression device) shall have a short-circuit capacity that is not less than that of the grounding conductor in which it is installed. MSHA Technical Support tests such devices to determine their short-circuit capacity.
When an arc suppression or parallel path suppression device fora circuit is installed in a power center or distribution box and the receptacle for the circuit is not insulated from the metal frame of the power center or the distribution box, the circuit grounding conductor must be insulated through the receptacle and the associated plug. This is necessary to prevent shorting out the arc suppression or parallel path suppression device.
Nevertheless, the metal casings of both the plug and receptacle must be grounded. Normally, the receptacle is grounded by bolting it directly to the metal frame of the power center or distribution box. However, the plug must be grounded to the metal frame of the power center of the distribution box by an external grounding shunt or separate internal grounding conductor in the receptacle and plug. The grounding shunt or grounding conductor shall be sized in accordance with Section 75.701-4. In some cases, receptacles are insulated from the metal frames of power centers to prevent shorting out arc suppression or parallel path suppression devices. In such cases, both the receptacles and the associated plugs shall be grounded to the grounding conductors in the cables. In all cases, the metal casings of both halves of in-line cable couplers shall be grounded to the grounding conductors in the cables.
When wireless ground check circuits are used, an interlock circuit shall be provided for all cable couplers (including in-line cable couplers) to ensure that the power circuit will be deenergized before the power conductors are broken when the coupler is uncoupled. Typically, the pilot pins are connected together in the cable couplers to provide an interlock to trip the circuit breaker.
The wiring methods used on power center receptacles and cable couplers shall not result in the ground check circuits becoming ineffective. The pilot pins of the cable couplers shall not be connected together when one of the two pilot pins is connected to the system ground.
The following may be approved by the District Manager as a no less effective method for ensuring the continuity of grounding circuits of permanently installed stationary equipment.
- A second grounding conductor sized in accordance with 30 CFR 75.701-4 and visible for its entire length when practicable shall extend from the power source at the grounded side of the grounding resistor to the frame of the stationary equipment.
- The cable supplying power to the stationary equipment is shielded or steel armored and the shielding or armor is grounded at both ends.
75.902-2 Approved Ground Check Systems Not Employing Pilot Check Wires
This Section requires MSHA approval of all wireless ground check circuits; therefore, only wireless ground check devices bearing an MSHA acceptance number will be acceptable.
75.903 Disconnecting Devices
A connecting plug on the outby end of the trailing cable connected to the power center or distribution box will be accepted as a disconnecting device. Other means, such as switches with visible contacts, may also be acceptable for this purpose. Molded-case circuit breakers are not acceptable as visible disconnecting devices.
Disconnecting devices shall be plainly marked for identification
to reduce the chance of energizing a cable while repairs are
being made on the cable. While identification could take a
variety of forms, one example of compliance with §§ 75.601,
75.903, and 75.904 would be to label the loading machine #1 cable
plug, receptacles, and the circuit breaker through which the loading machine #1 is receiving power as "loader #1."
Consequently, each of these would be labeled alike and easily identified.
75.904 Identification of Circuit Breakers
The circuit breaker must be marked to identify the circuit or machine receiving power through the circuit breaker. For example: A circuit breaker through which "loader 1" is powered, is marked as "loader 1".
Either metal or plastic tags or markers may be used to identify circuit breakers if the tags or markers are attached securely to the circuit breaker enclosure and are large enough to be readily seen. The tag or marker should clearly identify the circuit or machine receiving power through the circuit breaker.
Subpart K....Trolley Wires and Trolley Feeder Wires
75.1001-1 Devices for Overcurrent Protection; Testing and Calibration Requirements; Records
To provide the short-circuit protection required by this Section,trolley systems must be designed, installed, and maintained to assure, at all times, that a short circuit at any location in the trolley system will be cleared by one or more automatic circuit-interrupting devices.
Automatic circuit breakers and fuses used to protect trolley circuits shall have a voltage rating that equals or exceeds the maximum no-load voltage of the trolley circuit and shall be capable of interrupting the maximum fault current that can flow through the interrupting device. Molded-case circuit breakers are sometimes installed in trolley circuits with no-load voltages ranging from 275 to 350 volts d.c. Since most molded case circuit breakers are rated only 250 volts d.c., it is often necessary to connect two or more poles of the circuit breaker in series to achieve an adequate voltage rating. When two or more poles of a molded case circuit breaker are connected in series,the poles of the circuit breaker should be wired so the bottom of one pole is connected to the top of the next pole to decrease the voltage stress between adjacent poles when the circuit breaker opens under load. It is not acceptable to connect fuses in series to achieve a higher voltage rating.
A "short circuit" is defined as an abnormal connection of relatively low resistance, made accidentally or intentionally,between two points of different potential in a circuit.
The setting of an automatic circuit-interrupting device should not exceed 75 percent of the minimum available short-circuit current in the protected circuit to compensate for inaccuracies in the setting and the voltage drop across arcing faults. This safety factor is consistent with accepted engineering practice;however, in determining whether a violation of this Section exists, the safety factor shall not be used.
When a molded-case circuit breaker in the a.c. input leads of a rectifier bridge is used to provide short-circuit protection for a trolley circuit, the required instantaneous setting of the circuit breaker will be less than the required setting of d.c. overload relay located in the output leads of the rectifier bridge. In such instances, the instantaneous setting of the circuit breaker shall not exceed 82 percent of the equivalent d.c. overload relay setting.
Reclosing circuit breakers used to protect trolley systems should be equipped with load-measuring circuits. Reclosing circuit breakers used to protect multiple-feed systems should also be equipped with voltage-differential circuits. Load-measuring and voltage-differential circuits are necessary to prevent the circuit breaker from reclosing on a short circuit or abnormally high load. Load-measuring circuits should be adjusted to as low a value of current as practical. In almost all instances, the system will perform satisfactorily with the load-measuring circuits adjusted at or below 300 amperes; however, there may be instances when slightly higher values are required because of pumps, lights, and other continuous loads on the system.Voltage-differential circuits should be adjusted to values equal to or exceeding 85 percent of the system voltage.
The mine operator should establish a program under which all trolley circuits will be tested and calibrated at least once every 6 months. The circuit breaker may be tested by loading the circuit until the current reaches that amount necessary to cause actuation of the circuit breaker or an external current source may be used. Circuit breakers equipped with solid state controls that are actuated by the voltage drop across a shunt may be tested by disconnecting the control circuit leads from the shunt and applying the proper amount of voltage to the shunt leads to cause actuation. Current actuated relays shall be tested bypassing the necessary amount of current through them to cause actuation. Relays that have a current calibration coil may be calibrated with an external current calibration source, provided that: (1) each relay is installed, adjusted, maintained, tested and calibrated in accordance with the manufacturer's instructions; and (2) the tolerance of the calibration current source and the specified relay operating tolerance shall not exceed 15 percent.
The record required by this Section shall be kept in a book and should contain the following:
- Date of test;
- Name of qualified person making test;
- Indicated current setting; and
- Current required to activate the breaker.
The distance shall be measured by following the shortest distance that air can travel (tight string distance) through crosscuts,entries or other openings.
In longwall mining, the 150-foot distance shall be measured in a straight line from the wire, cable or electric equipment in question to the outby edge of the longwall roof-support system.
Except in longwall mining, the 150-foot distance specified in this Section and Section 75.1002 shall be measured from the wire,cable or electric equipment in question to the nearer of either: 1. The outby edge of the pillar being mined; or 2. The inby edge of the solid pillars immediately outby the previously pillared area.
75.1003 Insulation of Trolley Wires, Trolley Feeder Wires and Bare Signal Wires; Guarding of Trolley Wires and Trolley Feeder Wires
Guarding shall be done with wood, plastic, or other substantial nonconductive material, firmly secured. Guarding shall extend for at least 6 feet from each side of the door or stopping. In advancing sections where the trolley feeder wire is extended beyond the track, or in retreating sections where the track is removed before the trolley or trolley feeder wires, adequate guarding shall be required between the end of such wires and the end of the tracks.