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Listen to the Cable Guy

Nov. 19, 2009
The Signal-to-Noise Ratio of a Cabling System Drops as Soon as You Install It
About the Author

Joe Feeley is editor in chief for Control Design and Industrial Networking. Email him at [email protected] or check out his Google+ profile.

Selection of wire and cable for an industrial network should never be taken lightly. That might not always be the practice, however.

"Indifference can be expensive," says Ted Beach, vice president of sales at Northwire. "If specifiers think cable is cable, PVC is PVC, and electrical performance doesn't matter, then they're making a mistake."

Northwire says some specifiers believe that cable really doesn't matter that much and only needs to conduct electricity. "That's clearly not true as evidenced by Foundation fieldbus plants that installed inappropriate cable that soon became the system constraint," observes Beach. "Using unapproved cables in tough environments, including cheap, off-the-self grades in an industrial environment, often leads to problems. Other mistakes include being unaware that not all Ethernet cable can be used at or near 100 m lengths despite its markings or category rating, and not properly grounding cable shields."

Beach says he polled his group about this, and they add that specifiers sometimes don't properly evaluate the environment, including the motion environment where cables are to be installed. Is it stationary? Any vibration? Will there be occasional or constant bending, twisting or rolling? Will there be frequent connects/disconnects?

"The environment the cabling infrastructure will be operating in is the most commonly overlooked criteria when selecting a product," says Eric Bulington, wire and cable chief engineer at Anixter. "It's often thought that plenum-rated cable is more rugged or durable because it has a higher rating. However, the rating does not provide any additional assurance that the cable will perform as expected in the long run in an industrial application."

There are subtle factors that can have a very detrimental effect on commercial-style riser or plenum cables. "Oil used to cut the metal for enclosures and conduits will cause commercial-style jackets to swell and crack allowing moisture ingress," says Bulington. "Sunlight resistance is rarely a concern for commercial networking cables, but often a concern in industrial environments. Temperatures reached in a chemical or power plant easily can exceed the ambient temperatures assumed during electrical performance testing." That makes the stated performance of the cable largely irrelevant, he says.

"The value of a new car drops the moment you drive it out of the show room, and the signal-to-noise ratio of a cabling system drops as soon as you install it," states Pete Lockhart, Anixter's vice president, emerging technologies. He says the performance of a cabling system can be affected by many things including excessive untwisting to make terminations, high temperature environments like plenum ceilings, excessive bending and pulling tension.

"When you have a cabling system that just barely meets the standard required for your application, installation factors can drop it below the requirements and cause issues on your network," says Lockhart. For example, he says it's technically correct that 1 Gigabit Ethernet can run over Cat. 5E cable from a standards perspective, but the results might not always be positive in real-world scenarios. "The bottom line is to give yourself the best cabling system to start, like Cat. 6, which ensures that you still have a quality cabling system that will productively run all your applications even as technology evolves after installation."

Beach says many specifiers also believe that having cable made to suit their needs is more expensive, takes too long and requires large minimum purchases. "That's not necessarily true, and it's worth investigating," he claims.

Beach argues that most companies don't necessarily think about intangibles such as the product-branding opportunities that custom cabling could provide. "A 'made to application or configured' cable with a company's name and website information on it could promote the idea of a 'well and thoroughly designed' application," he believes.

Select cable based on a solid understanding of the application environment, the installation method being used and the cable's actual performance in a real-world application, not how it performs in a lab. "Finally, be conservative and select a cable that will allow headroom for growth or unanticipated challenges," summarizes Bulington.

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