Combating electromagnetic interference

Q: We have a number of new communications (voice and data) cable installations and are constantly addressing problems with electricians installing cable and electrical devices adjacent to communications cable runs, causing electromagnetic interference (emi). I am in search of an industry-standard specification stating distances from cable to emi-causing devices. The electrician`s bible, the National Electrical Code (nec), only mentions a 2-inch clearance between power and communications cables.

Q: We have a number of new communications (voice and data) cable installations and are constantly addressing problems with electricians installing cable and electrical devices adjacent to communications cable runs, causing electromagnetic interference (emi). I am in search of an industry-standard specification stating distances from cable to emi-causing devices. The electrician`s bible, the National Electrical Code (nec), only mentions a 2-inch clearance between power and communications cables. Because of this, electricians do not share my concern.

One example of the problems caused is found in a new-building installation using ladder trays for communications cable. Electricians have wired fluorescent lamps through the trays and mounted them immediately below. Please let me know of any publications that can help.

Dennis Leclerc

State of New Hampshire

Administration Services Bureau

Concord, NH

A: It doesn`t matter who was there first. When the channel fails due to emi, the irate user never calls the electrician. While it is unlikely that a bad ballast (no pun intended) would endanger the life of a network user, it could certainly wreak havoc on network connections.

The nec is a safety code and not a telecommunications cabling performance specification. The ansi/tia/eia-568a commercial building wiring standard of the Telecommunications Industry Association (Arlington, VA) is the most widely accepted performance specification in North America: It refers us to ansi/eia/ tia-569, the pathways and spaces standard, for pathway information.

You may be surprised to learn that the requirements in tia-569`s Table 10.4-1 will be missing in the newly published ansi/tia/eia-569a version of the document. Gone also is the interference ceiling of no more than 3.0 volt/meter throughout the usable bandwidth of the telecommunications cabling.

Your best bet on this one is not a code or an industry standard but a methods manual. The Telecommunications Distribution Methods Manual (1996 Edition) of bicsi (Tampa, FL), in Chapter 4--"Horizontal Cabling Systems," has a section on "Avoiding Electromagnetic Interference." To avoid emi, all pathways should provide clearances of at least

- 1.2 meters (4 feet) from motors or transformers,

- 0.3 meter (1 foot) from conduit and cables used for electrical-power distribution,

- 12 centimeters (5 inches) from fluorescent lighting.

Pathways should cross perpendicular to fluorescent lighting and electrical-power cables or conduits.

Avoiding potential sources of emi must remain a primary consideration for telecommunications design professionals. I suggest specifying the separation requirements between telecommunications pathways/cabling and sources of emi in the contract documents of your next project. And not by reference--spell it out in the contract.

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