VOLUME V - COAL MINES
77.501 Electric Distribution Circuits and Equipment; Repair
"Electrical work," as referred to in this Section, includes the design, installation, maintenance or repair of electric equipment and circuits. Splices and terminations made in electric cables, installation of couplers on the ends of cables, electric machine repairs, electric wiring, pole and line work, work performed inside electrical substations or other areas in proximity to exposed energized electrical parts, work performed inside transformers, switch boxes, switch houses, electric panels or other enclosures of electric equipment and circuits are examples of tasks that are considered to be "electrical work" and are required to be performed by or under the direct supervision of a qualified person.
Examples of duties that are not considered to be "electrical work" and would not be required to be performed by a qualified person or under the direct supervision of a qualified person are, operation of electric equipment, transportation of equipment and cables, operation of control switches, circuit breakers or switch boxes, provided no energized parts are exposed, changing cutting bits, lubrication work, moving of energized trailing cables, or inserting or withdrawing proper cable couplers into or from their receptacles. These tasks are considered to be part of the normal routine operation of electric equipment; therefore, they are not considered to be "electrical work."
The term "direct supervision" shall not be interpreted to mean
that the qualified person be physically present at all times
during the performance of such repairs, but the qualified person
has the following responsibilities:
- The qualified person shall examine and/or test an electric circuit or machine and determine the need for repair or maintenance.
- The qualified person must give specific instructions to the employee assigned to perform this work with respect to the nature and extent of the repairs to be performed and, where necessary, prescribe the manner in which the work is to be performed.
- The qualified person is, at all times, under continuing duty to instruct, advise, or consult with the employee, in the event the work assigned cannot be performed by the employee in the manner prescribed.
- The qualified person must examine and test the completed work before the circuit is energized or the machine is returned to service.
"Suitably tagged" means that a sign with wording such as"Danger - Hands Off - Do Not Close - Miners Working on Line,"shall be attached to the opened disconnecting device. The tag should bear the name of the workman who installed it.
Keys to locks used to lock out switches should be kept by the person working on the circuit or equipment.
77.502 Electric Equipment; Examination, Testing and Maintenance
For purposes of this Section, "electric equipment" shall include all control circuits; control switches or devices; circuit breakers; fuses; conduits; wiring; motors; transformers; lighting equipment; hand-held tools such as drills, wrenches, and saws;etc. The tests, examinations, and proper maintenance required by this Section shall include all items mentioned above and all other such equipment at the mine.
77.503 Electric Conductors
Section 77.503 requires that, "electric conductors shall be sufficient in size and have adequate current carrying capacity and be of such construction that a rise in temperature resulting from normal operation will not damage the insulating materials."Section 77.503-1 outlines the term "sufficient" and states that electric conductors must "meet the minimum current carrying capacity provided for in the National Electric Code, 1968"(emphasis added). While Section 77.503 states general ampacity and conductor size requirements, Section 77.503-1 incorporates the specific minimum requirements of standards promulgated by consensus standards organizations.
Since publication of the 1968 NEC, technological advances in power cable manufacture have been made. Insulated conductors having better grades of insulation and temperature ratings have been developed which far exceed the capabilities of conductors addressed by the 1968 NEC. Therefore, ampacity tables for insulated conductors other than trailing cables used on the surface and manufactured in accordance with minimum NEC standards, or which meet the more general safety test of Section 77.503 are acceptable.
For example, conductors manufactured in accordance with the Insulated Cable Engineers Association (ICEA) standards, which conform to the ICEA ampacity tables or other nationally recognized standards would be acceptable as meeting the requirements of Section 77.503. Enforcement action should not be taken if cables do not meet the specifics of the 1968 NEC ampacity and temperature rating standards, but equal or surpass the minimum level of safety afforded by compliance with the NEC.This enforcement policy will allow the use of newly-designed electric conductors which are not addressed by the 1968 NEC, but which do comply with Section 77.503 and offer equal or greater miner protection.
Trailing cables are required to meet the minimum capacity requirements of the Insulated Power Cable Engineers Association(IPCEA) - National Electric Manufacturers Association (NEMA)standards. This policy does not affect MSHA's treatment of trailing cables.
77.504 Electrical Connections or Splices; Suitability
This Section requires that splices made in electric conductors be made in a workman-like manner and establish sufficient electrical conductivity so that the joined conductors will not heat or spark under load. Because of the different characteristics of devices,such as pressure terminal or pressure splicing connectors and soldering lugs, they shall be suitable for the material of the conductor and shall be properly installed and used. Conductors of dissimilar metals shall not be intermixed in a terminal or splicing connector where physical contact occurs between dissimilar conductors, unless the device is suitable for the purpose and conditions of use. Materials such as solder, fluxes,inhibitors, and compounds, where employed, shall be suitable for the use and shall be of a type which will not adversely affect the conductors, installation, or equipment. Soldered splices in electric conductors shall be joined with suitable connectors and then soldered. Splices made by twisting conductors together,tying knots in conductors, splices that have bare or exposed conductors, splices that heat or arc under load, or splices in multiple conductor cables that do not have the outer jacket replaced would constitute noncompliance.
All connections or splices in insulated conductors shall brbe reinsulated to at least the same degree of protection as the remainder of the conductor. Tape, such as rubber, glass,asbestos, or plastic will be acceptable as insulation. Friction tape is not an acceptable insulation material, but may be used to provide mechanical protection.
77.505 Cable Fittings; Suitability
For the purpose of this Section, a cable, with either single or multiple conductors, is one that has an outer jacket in addition to the insulation provided for each power conductor. An electrical fitting is an accessory such as a clamp or other part of a wiring system that is intended primarily to perform a mechanical rather than an electrical function. The function of a proper electrical fitting for a cable entering a junction box,electrical panel, termination box, or other electrical enclosure is to prevent a strain on the electrical connections and to prevent chafing or other movement of the cable that might allow an energized electrical conductor to fault to the enclosure frame. Proper fittings would permit box connectors, packing glands, strain insulators, strain clamps, or metal or wood clamps, etc.
Electric circuits that are made up of individual insulated wires that enter junction boxes, termination boxes or other electrical enclosures need not have fittings, but must be provided with insulated bushings.
77.506 Electric Equipment and Circuits; Overload and Short-Circuit Protection
Section 77.506 requires "automatic circuit-breaking devices or fuses of the correct type and capacity" on electric equipment and circuits to protect against short circuits and overloads.Section 77.506-1 specifies what was considered to be the correct type and capacity for circuit breakers and fuses at the time the standard was enacted. It requires such devices and fuses to meet the minimum requirements of the 1968 NEC. For certain excavation equipment, requiring strict compliance with the terms of the 1968NEC could prevent the use of circuit protective devices of appropriate type and capacity. Therefore, Section 77.506-1should not be applied to some equipment which meets the requirements of Section 77.506.
Alternating- and direct-current loop (feedback) systems and their controls, which are used on large shovels, drag lines and in-mine hoisting installations are normally designed so that their currents are limited to values below those which would cause a harmful overload condition to circuits or motors. These circuits on the equipment specified above comply with Section 77.506, and will not be required to have short-circuit or overload protective devices to comply with the terms of the NEC.
77.507 Electric Equipment; Switches
The intent of this Section is to require that all control devices be fully enclosed to prevent exposure of bare wires and energized parts. Improvised starting methods such as plug and receptable devices, trolley taps and trolley wire "stingers" that are used to start or stop electric motors are examples of noncompliance with this provision.
77.508 Lightning Arresters, Ungrounded and
Exposed Power Conductors and Telephone Wires
Conductors that are provided with a metallic shield or are jacketed by a grounded metal covering or enclosure, suspended by a grounded messenger wire, installed under grounded metal framework, protected by the umbrella-effect of overhead grounded static line, or are buried in the earth 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 interconnected near the lightning arrester.
To comply with this Section, three-phase alternating-current circuits should be provided either with three separate lightning arresters or with a three-phase arrester which consists of three arresters in one case having a common ground terminal.
77.511 Danger Signs at Electrical Installations
"Major electrical installations" shall include the following:
- All high-voltage installations housing exposed energized parts, such as: fence enclosed substations; skid-mounted transformers; transformers or switchgear located in vaults or rooms; circuit breaker and switch houses; enclosures on board machines, in preparation plants, or shops, that house high-voltage switches, fuses or exposed buss; and high-voltage motor controls.
Examples of high-voltage installations that are not considered to be major are: high-voltage cables serving surface mine equipment; pole-mounted transformers, regulators and capacitors; pole-mounted disconnecting switches; motor generator sets having no exposed high-voltage parts, and high-voltage cables or wiring in conduits passing through enclosures from floor to floor in preparation plants or machines.
- Rooms or areas housing switchboards containing exposed parts energized at more than 40 volts to ground.
Insulating mats or dry wooden platforms are required to be kept in place where a person would normally stand at switchboards or power control switches only if a shock hazard exists.Installations where a shock hazard exists include, but are not limited, to the following:
- all live front switchboards with exposed components energized at more than 40 volts to ground; and
- all overhead high-voltage disconnect switches which are operated from the ground by means of mechanical linkage. (See Section 77.704-9 for requirements for high-voltage switches.)
Grounded metal mats or plates may not be used instead of insulating mats in front of live front switchboards.
Insulating mats or platforms installed at high-voltage installations shall be rated for not less than the phase-to-phase voltage of the circuit. Insulating mats can provide such additional safety if placed at low-voltage line starters,fuse boxes, and other low-voltage switchgear containing renewable parts.
77.516 Electric Wiring and Equipment; Installation and Maintenance
Section 77.516 requires, in addition to compliance with Sections 77.503 and 77.506, that electric equipment installed after June 30, 1971, meet the requirements of the NEC. The NEC has been incorporated into MSHA standards to address wiring and wiring methods for surface facilities and structures not specifically covered in Part 77. The NEC will continue to be applied to surface facilities and structures other than the specified excavation equipment. The NEC contains safety guidelines which are not specifically tailored to surface mine excavation equipment and conductors, but which cover a much broader scope. While Section 77.516 addresses wiring and electric equipment installed after June 30, 1971, on surface mining machines, many provisions of the NEC are not applicable to the wiring methods, types of equipment, and conditions on these machines. For example, certain excavation equipment designed and installed since June 30, 1971, is not compatible with the requirements of the 1968 NEC. Therefore, strict application of and compliance with the NEC for the wiring methods used on surface mine excavation equipment is not practicable.
Wiring and electric equipment installed after June 30, 1971, on-board electric or diesel-powered surface excavation equipment are not required to comply with the NEC under Section 77.516,although mine operators are free to rely on it as a guideline.This policy applies to equipment such as drag lines, shovels,dozers, bucket wheel excavators, mobile drills, mobile cranes,haulage trucks, and end loaders. However, all other relevant provisions in Part 77 will continue to apply to these machines.Equipment and wiring installed after June 30, 1971, will be inspected and enforcement action taken in the same manner as on equipment installed prior to that date. For example, Sections 77.404 and 77.502 requiring examination and proper maintenance will be enforced, along with appropriate conductor ampacities under Section 77.503, short-circuit and overload protection under Section 77.506 and all relevant grounding provisions.
Subpart G .... Trailing Cables
77.600 Trailing Cables; Short-Circuit Protection;Disconnecting Devices
This Section does not permit the use of single element fuses for trailing cable short-circuit protection. Dual element fuses with adequate interrupting capacity may be used to provide short circuit protection for single phase and d.c. trailing cable circuits.
"Adequate interrupting capacity" means that the fuse or circuit breaker is capable of interrupting the maximum short-circuit current the circuit may conduct without destruction to the device.
This Section requires that short-circuit protection be provided for each under grounded power conductor. Therefore, in direct current systems in which neither polarity is grounded, protective elements shall be provided for both the positive and negative lines. This necessitates the use of either a fuse in each polarity or a two-pole circuit breaker.
A visual means of disconnecting power from trailing cables must be provided so that it can readily be determined whether or not the cable is deenergized. Enclosed circuit breakers are not acceptable as visual evidence that power is disconnected. Plugs and receptacles located at the circuit breaker would be acceptable as the visible means of disconnecting the power.
77.603 Clamping of Trailing Cables to Equipment
If a strain clamp is used, it shall be designed and installed to prevent damage to the cable jacket. Cable grips, such as Kellems grips, anchored to the machine, may be used in lieu of strain clamps.
77.606 Energized Trailing Cables; Handling
Other protective devices such as insulated sleeves, insulated jackets, insulated aprons, insulated cable handling hooks, etc.,are required by this Section when they are necessary to prevent the energized trailing cable from contacting the cable handler's body or clothing.
77.606-1 Rubber Gloves; Minimum Requirements
Rubber gloves with a rating of at least 20,000 volts shall be worn while handling energized high-voltage trailing cables,regardless of what other protective devices are used.
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 low-resistance 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:
- Metal waterlines having low-resistance to earth;
- A low-resistance ground field; or
- Any other grounding medium which ensures that there is no difference in potential between the metallic enclosures and earth.