Don't Forget to Shut the Back Door

To look only at the alternating current (AC) voltage as a source of potential power quality (PQ) related problems is not a wise approach in most integrated systems. It is rare to find any information technology (IT) or process-control equipment operating without some communications connection. More often, it is connected to a local area network through Ethernet cables or the telecommunications systems with twisted pairs, which allows for sending and receiving data both internally and externally to the facility. A PQ disturbance, such as noise, transients and electromagnetic interference, can find a way into the electronic equipment and cause misoperation or damage if not properly protected, even when the AC side is operating properly.

Just as the electronic equipment can become more vulnerable to PQ disturbances as the operating speeds increase and the internal direct current (DC) power supply voltages decrease, the communications systems generally follow the same paradigm. The serial communications standard RS-232 has been in use for nearly 50 years. Original signal levels were +/–15 volts (V), and data rates were lower than 1,000 bits per second. Today’s USB 2.0 serial communications systems are nearly 500,000 times faster and operate at a fraction of those voltage levels. Valid gigabit per second signals can be as low as 350 millivolts at an Ethernet receiver. The receiver circuitry must be able to reject noise and other EMI/RFI/ESD sources of data corruption, and it must be protected against induced or directly coupled transients in the signal lines, adjacent power cables and the grounding system.

Assuming the facility has proper protection for the AC voltage sources as well as an equipotential-grounding system, one method of the communications system protection is to install devices wherever the communications cables enter or leave the facility, which will clamp and divert those nasty transients. Given the high-frequency nature of the disturbances and the communications signals, filters generally aren’t applicable. Properly rated and listed surge-protection devices (SPD) should be installed as close to the equipment and with as short and straight a grounding cable connection as possible. The above graphic is an example of such a device. Note that a braided cable is used for the ground connection. Braided cables have superior characteristics for diverting transients and high-frequency noise than a solid wire.
NFPA 70, the National Electrical Code (NEC), and Underwriters’ Laboratories (UL) both address this subject.

• NEC Article 645, Information Technology Equipment, “covers equipment, power-supply wiring, equipment interconnecting wiring, and grounding of information technology equipment and systems, including terminal units, in an information technology equipment room.”

• NEC Article 647, Electronic Equipment, “covers the installation and wiring of separately derived systems operating at 120 volts line-to-line and 60 volts to ground for sensitive electronic equipment” and addresses wiring for “reducing objectionable noise in sensitive electronic equipment locations”

• NEC Article 800, Communications Circuits, covers communications circuits and equipment, including installation of listed primary and secondary protectors, separation of communications wires and cables from electric light or power conductors, and grounding methods.

• UL 497, Protectors for Paired-Conductor Communications Circuits, covers primary protector devices required by NEC Article 800 at the building entrances of the communications circuits.

• UL 497A, Secondary Protectors for Communication Circuits, covers the secondary protectors located between the building entrance/primary protector and the equipment intended to be protected, which are intended to be used in the protected side of telecommunications networks that have an operating rms voltage-to-ground of less than 150V and are installed or used in accordance with the NEC.

• UL 497B, Protectors for Data Communications and Fire-Alarm Circuits, covers the data communications circuit protectors and fire alarm circuit protectors, which are intended to protect equipment, wiring and personnel against the effects of excessive potentials and currents caused by lightning in communications alarm-initiating or alarm-indicating loop circuits.

A fine print note (FPN) below Article 800.90(B) in the 2008 NEC brings up a key point: “Selecting a primary protector location to achieve the shortest practicable primary protector grounding conductor helps limit potential differences between communications circuits and other metallic systems.” This is a concern among all of the equipment in the communications system, which often goes between buildings in a campus environment. Earth or ground potential rise occurs when a large current flows to earth through an earth-grid impedance, a lightning strike or utility system fault. Communications equipment in the two locations can then be at different voltage potentials, high enough to damage inadequately protected equipment. One way to minimize any such differences in potential between the two grounding systems is to use fiber optic cable, which is virtually immune to PQ disturbances (provided their power sources are properly protected).

Both the voltage sources (AC and/or DC) and the communications equipment and circuits must be properly installed and protected. Surge protective devices do OK, and preventive maintenance programs should replace devices that need to be.

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.

About the Author

Richard P. Bingham

Power Quality Columnist
Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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