False Ceilings

Nov. 1, 1995
Q: I work at a chemical plant that manufactures plastics, among other chemicals. My network received two lightning hits this summer. Each time we lost many devices attached to our networks. The networks are of different types and sizes--asynchronous; personal computers to front-end Hewlett-Packard processor; asynchronous to front-end processor via modems; multiplexers; and hardwired to and through buildings of different sizes--some single-story and some two-story.

Q: I work at a chemical plant that manufactures plastics, among other chemicals. My network received two lightning hits this summer. Each time we lost many devices attached to our networks. The networks are of different types and sizes--asynchronous; personal computers to front-end Hewlett-Packard processor; asynchronous to front-end processor via modems; multiplexers; and hardwired to and through buildings of different sizes--some single-story and some two-story.

The connections that were blown or destroyed were hardwired to a Gandalf data switch. The switch is protected by an uninterruptible power supply and has a Liebert PDU on the output also, so I don`t think the lightning got to the network via the power supply. Both ends of the connected devices--the PC serial ports and the Gandalf data-switch ports--were hit and went out. Several of the Gandalf ports were on the same card. Some were also scattered throughout the Gandalf data switch. The cabling in the building is a combination of Category 3 and Category 5.

My biggest concern is how I could have prevented this from happening. Is it possible to protect the Category 5 unshielded twisted-pair wiring in a raceway in a typical office building?

Mike M. Hardinger

Solvay Polymers Inc.

Deer Park, TX

A: I can certainly understand your concern, because in Deer Park, TX, you can expect 50 to 70 thunderstorm days per year.

You said that the cable in the buildings is a mixture of Category 3 and Category 5, but what types of media are running between the buildings? If the cable is dielectric, do you have any electrical protection on the cable? If you have protector panels, are the protector units specifically designed to protect your low-voltage data equipment?

I am not an electromagnetic-compatibility engineer. I design cabling systems following the National Electrical Code and TIA standards and am not experiencing similar problems. I do recall a case in the early 1980s where a UTP cable had been placed between a data center in one building and a student lab in an adjacent building. The cable was installed through an abandoned metal pipe and terminated on connecting hardware without protection. This was discovered one morning after a thunderstorm. There were damaged line-drivers and switch cards and many angry users. The metal pipe is less than 40 feet long and at least 15 feet underground between a single- and a three-story building. The cable in that pipe now has electrical protection.

Because you asked about protecting the UTP in a raceway, if you have a metal raceway, is it properly grounded? While it is highly unlikely that your typical office-building raceway was directly energized by a lightning strike, properly grounding a metal raceway will significantly reduce noise on the telecommunications cabling. Contact the raceway or furniture-system manufacturer for grounding instructions.

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