Authored by: Edited by Robert Repas Key points: Resources: |
If you find electrical safety standards confusing, you’re not alone. Many machine builders recently grappled with an ambiguous round of changes to NFPA-79. This is the National Fire Protection Association code that outlines electrical standards for industrial machinery that protects operators, equipment, and facilities from fire and electrical hazards. NFPA-79 is cited extensively in the National Electrical Code (NEC) that governs the electrical wiring of industrial machines.
The standard cuts a broad swath across the industrial machinery landscape. It applies to a comprehensive range of machines — everything from machine tools and injection-molding presses to testing machines and packaging lines. It encompasses all of a machine’s electrical and electronic elements operating at 600 V or less. This means that a vast majority of machine power, control, and lighting circuits fall under the scope of NFPA-79.
Machine builders complied with the dictates of NFPA-79 for years. But earlier this year the standard underwent revisions that make it more difficult to specify compliant electrical cabling.
Appliance wire OK’d
The key change to NFPA-79 for 2012 involves the ability to use appliance wiring material (AWM), which had been banned since 2007. AWM can be an economical wiring choice compared to higher-performing UL-listed machine-tool wire (MTW). But there were several sound reasons behind the AWM ban.
For one, some contractors were using AWM incorrectly as part of the building infrastructure. The NEC does not recognize AWM for this type of use.
Also, not all AWM products are created equal when it comes to the quality of their insulation layer. The insulation thickness of low-quality commodity cabling varied such that the wire, while meeting its electrical insulation requirements, was not sufficiently flame resistant for use in industrial machines.
It’s important to stress that not all AWM suffers from these quality issues. Lapp and other reputable cable manufacturers can and do produce AWM products whose insulation wall thickness is both consistent and adequate for the voltage rating of the cable. High-quality AWM can pass the specified vertical flame tests (FT 1) and can safely be used in industrial machines. Keep in mind, however, that AWM lacks the extra layers of PVC-nylon insulation found in MTW. This extra insulation allows some types of MTW to pass flame tests (FT 4) more rigorous than those for even the best AWM.
Reversing the AWM ban reflected the realities of the global machinery marketplace in which AWM remains a widely used cabling choice. The restrictions imposed by the new NFPA standard attempts to negate the potential drawbacks of AWM.
The first restriction states that the AWM must be identified as suitable for the application at hand and must be used in accordance with the machine-manufacturer’s instructions. Other restrictions detail cable construction including minimum conductor count, flame resistance, and wall thickness. Compliant AWM cable must carry a jacket print legend that spells out the AWM style number, voltage, temperature rating, and flame rating.
The reversal on AWM usage also requires more documentation. For example, manufac turers must provide field-installation information related to the AWM wire with each machine’s technical documentation.
Easy compliance
Given all the cable runs on and around today’s complex industrial machines, the need to document AWM usage for each and every machine represents the most onerous of all the NFPA restrictions. In the case of extremely large machines such as bottling lines, machine builders and installation contractors can spend dozens of engineering hours and thousands of dollars complying with the documentation requirements.
These documentation needs, while not insurmountable, must be factored into the cost of wiring the machine. In some cases, the added documentation costs can erode the minor price advantages of some AWM cables.
One way around the concern of documentation is to favor listed MTW. Because it inherently complies with NFPA-79, MTW cabling alleviates the documentation requirement and eliminates any residual confusion over whether a given AWM product is approved and properly labeled. MTW also has a host of technical advantages that derive from its superior flexibility and resistance to flame, oil, chemicals, and mechanical stresses.
Some MTW products carry a UL TC listing for use in tray cables and may be rated for use in exposed runs. These broadly certified MTW cables adapt to the widest range of application requirements, and their use in exposed runs can slash installation costs.
MTW’s technical edge notwithstanding, there are times when AWM may be the best choice. For example, AWM may win out in overseas markets for supply-chain reasons.
If you do go with AWM, make sure the product you buy is of a high quality designed to withstand all of the abuse found in an industrial environment. Most importantly, buy only from reputable suppliers that can help you comply with the complex labeling, documentation, and application hurdles imposed by NFPA-79.
AWM and NFPA compliance One company successfully navigating the shoals of the recent NFPA-79 revisions is Krones Inc., Franklin, Wis., a manufacturer and integrator of packaging lines for food and beverage companies. The company’s engineering team recently found that compliant AWM usage does require a bit of extra effort compared to the automatic compliance found with UL-listed MTW. “Whenever you must implement changes because of an electrical-code change, there’s definitely an engineering-labor factor,” says Mike Nelson, the Krones engineer charged with NFPA-79 compliance. Some of that engineering labor went into researching specific AWM products to see whether they meet the compliance restrictions. Even proper jacket labeling doesn’t answer every compliance question regarding AWM. “You can’t tell whether a product complies just by looking at the cable,” says Nelson. And even more engineering labor went to NFPA-79’s documentation requirements. In all, Nelson estimates that Krones has spent more than 150 engineering man-hours complying with the requirements related to AWM use. And that …figure represents just the work done to formulate a compliance strategy. It does not include the technical documentation and drawing changes needed to incorporate the wire into each and every machine. |
Exposed cable runs for fast installation Another change in the 2012 NFPA-79 standard allows exposed cable runs along the structure of the equipment or in the machine chassis. As long as the exposed cables closely follow the surface and structural members of the machine, the installation does not require conduit, raceways, or any special hardware. Thanks to reductions in installation time and labor, exposed cable runs can reduce costs dramatically compared to traditional installation methods that need conduit or special mounting hardware. Certain UL-listed cables meet exposed run (-ER) speci…fications, which provide an additional level of protection for these types of applications. Cables meeting -ER requirements are subjected to the same crush and impact tests as armored-type or metal-clad (MC) cables, that lets cables leave the machine area and enter a cable tray without conduit. Keep in mind, though, that not all UL-listed cables meet NFPA requirements, especially lower-priced commodity and rigid products. |
Common electrical-compliance questions With both UL requirements and NFPA standards holding sway over different aspects of electrical compliance, it’s always been tough to figure out whether a given cable choice will pass regulatory muster. The job of picking compliant cables recently became even tougher when the latest version of NFPA 79, the main standard governing the electrical safety of industrial machines, drastically changed the compliance picture. Here are some of the most commonly asked compliance questions and answers: Is NFPA-79 a law? Must a machine comply with NFPA-79? Will machine builders and buyers standardize on the new edition of NFPA-79? Who decides which cables can be installed in the field? Are UL listed cables always allowed for use on a machine? What’s special about MTW approval? Are all MTW cables oil resistant? Can I run MTW cable into building infrastructure? Can cables be left exposed when going from the machine to the cable tray? |