Because every type of machine has unique requirements for wiring methods, operator safety depends on your understanding the differences between rules in the NEC and methods outlined in NFPA 79.
Every type of machine has unique requirements when it comes to operator safety. From an electrical standpoint, industrial machine equipment and tools - from drill presses to multi-motored automatic machines - can present special fire and shock hazards. NFPA 79:
Electrical Standard for Industrial Machinery, helps you ensure fire safety by addressing the electrical considerations specific to equipment, apparatus, and systems used in industrial manufacturing processes.
If you're working in an industrial setting, it's important for you to understand that wiring methods and practices outlined in NFPA 79 vary from the rules in the NEC. What are the differences between the two? Let's take a closer look at some specific requirements of NFPA 79 to find out.
Conductors (NFPA 79, Sec. 16.1.1). You must identify conductors at each termination so they match the identification markings on the diagrams attached to the machine.
Equipment-grounding conductors (NFPA 79, Sec. 16.1.2 and 16.1.3). You must use the color green, with or without one or more yellow stripes, to identify the equipment-grounding conductor (where insulated or covered). International and European standards require the use of a bicolor green-and-yellow for this purpose (see IEC 204-1 for specific requirements). You can use conductors of other colors, provided the insulation or cover is appropriately identified at all access points.
For grounded control circuits, you may use a green (with or without one or more yellow stripes) or a bare conductor to connect the transformer terminal to a grounding terminal on the control panel.
Sec. 16.1.3 allows you to use other colors for the purpose of identification as follows:
• Black represents ungrounded line, load and control conductors at line voltage.
• Red represents ungrounded AC control conductors, at less than line voltage.
• Blue represents ungrounded DC control conductors.
• Yellow represents ungrounded control circuit conductors that may remain energized when the main disconnecting means is in the OFF position. These conductors must be yellow throughout the entire circuit, including wiring in the control panel and the external field wiring. International and European Standards require you to use orange for this purpose (see IEC 204-1 for specific requirements).
• White or natural gray represents a grounded circuit conductor.
• White with blue stripe represents a grounded DC current-carrying circuit conductor. International and European standards require you to use light blue for the neutral conductor (see IEC 204-1 for specific requirements).
• White with yellow stripe represents grounded AC current-carrying control circuit conductors that remain energized when the disconnecting means is in the OFF position. For additional circuits powered from different sources that remain energized when the main disconnecting means is in the OFF position, you must use striping colors other than green, yellow or blue to uniquely identify the grounded conductors.
Exceptions to Sec. 16.1.3 allow internal wiring on purchased wiring devices (or where multiconductor cable is used) to deviate from this color scheme. Where the insulation used is not available in the colors required (such as high temperature insulation, or chemically resistant insulation) the identification of conductors is not required.
Splices (NFPA 79, Sec. 16.1.4). You are required to run conductors and cables from terminal to terminal without splices. However, an exception allows you to install a splice in leads attached to electrical equipment, such as motors and solenoids.
Panel wiring (NFPA 79, Sec. 16.2). This section requires you to support conductors in panels to keep them in place. You're permitted to use wiring channels if they're made of a flame-retardant insulating material. If you're working with back-connected control panels, you must provide access doors or swing out panels that swing about a vertical axis. Multiple-device control panels must have terminal blocks or attachment plugs and receptacles to terminate and connect all outgoing control conductors.
Machine wiring (NFPA 79, Secs. 16 and 17). You must totally enclose conductors and their connection external to the control panel enclosure in suitable raceways or enclosures. Unless used for flexible connections involving small or infrequent movements, or connections to normally stationary motors, limit switches and other externally mounted devices, fittings used with raceways or multiconductor cables must be liquidtight.
Wire connectors and connections (NFPA 79, Sec. 16.4.1). You are required to use pressure connectors to connect conductors to devices with lug-type terminals that are not equipped with saddle straps or equivalent means of retaining conductor strands. Under certain conditions of use, the standard allows you to use solder connections and wire-wrapped connections, per Exceptions No. 1 and 2, Sec. 16.4.1.
Raceway fill (NFPA 79, Sec. 17.2). The combined cross-sectional area of all conductors and cables is not permitted to exceed 50% of the interior cross-sectional area of the raceway. The fill provisions are based on the actual dimensions of the conductors or cables used.
Junction and pull boxes (NFPA 79, Sec. 17.12). When constructing junction boxes and pull boxes, you must be careful to exclude materials such as dust, flyings, oil and coolant. After you complete all wiring operations, you must seal all unused knockouts or openings.
Conclusion. Modern electrical machine tool equipment may vary from that of a single-motor machine (such as a drill press that performs simple, repetitive operations) to very large, multimotored automatic machines, which contain highly complex electrical control systems. Typically, these machines are especially designed, factory-wired, tested by the builder and then erected in the plant.
You must install, support, and protect interconnection wiring from the supply circuit and between machines in accordance with the NEC and NFPA 79.
