Cable Tray Institute: Newsletters

Winter 2000

Cable Tray Grounding: Power, Instrumentation, and Telecommunications
Richard J. Buschart, Technical Director-Cable Tray Institute
Grounding has always been a controversial topic. But, with the growth of digital high frequency systems the issues are more complex. Grounding means connected to earth or a conducting body that acts in place of earth. Some international standards refer to grounding as earthing. Bonding is the interconnection of metal parts to establish electrical continuity. These definitions are NEC terminology and apply to power system grounding.

The purpose of grounding is:

Fire Protection
Electrical Shock Protection
Electrical system ground fault protection
Lighting protection-building and electrical system
Electrical Noise and EMI protection
Voltage Stabilization

Power System Grounding
Power circuit grounding of cable trays is explained in CTI Technical Bulletins, Titles No. 8, 11, and 12, and the National Electrical Code Sections 318-3-© and 318-7. It is also covered in NEMA Standard VE-2.

The purpose of power grounding (Article 250) is to minimize the damage from wiring or equipment ground fault. Cable tray systems are in the path of ground fault currents. Cable tray systems are bonded together through their bolting, connectors splice plates, clamps, and bonding jumpers where there are gaps in the cable tray system. Cable tray systems are not required to be mechanically continuous, but shall be electrically continuous.

Cable trays are also bonded to conduit, cable channel or other wiring drops. They must also be bonded back to the power source. All bonding jumpers must be sized (as a minimum) to meet the requirements of equipment grounding conductors. Both side rails of the tray must be bonded together to the next section. Cable trays can be used as the only equipment grounding conductor (EGC), but they must meet certain criteria (only in qualifying facilities, minimum cross-sectional areas, U.L. classified as to suitability, etc., see NEC 318-7).

There are other alternatives-use EGC’s in the cable (U.L. listed cable can be supplied with ECG’s in certain conductor sizes) or a separate EGC in the cable tray that bonds the cable tray sections together and can also be used to tap EGC’s to individual drop-outs from the CT. These two alternatives can be used for non-metallic cable trays. Cables with equipment ground conductors within the cable are an accepted practice in industry. They provide a two-point connection from the power source to the load, however, any conduit, cable tray, or raceway must still be bonded back to the power source.

Some companies do not accept conduit as an EGC.

The EGC system is a critical safety system. Therefore, it is prudent to treat the cable tray system as an equipment grounding conductor in parallel with the ground conductors in the cables or an individual ground conductor.

Cable Tray Grounding-Signal and Communication Circuits
Where cable tray systems contain only signal and communication circuits that operate at low energy levels, power grounding per NEC Section 318-7 is not appropriate, but cable tray grounding for lightning protection, noise, and electromagnetic interference is necessary. For telecommunications circuits TIA/EIA standard 607, Commercial Building Grounding and Bonding Requirements for Telecommunications, provides grounding for these systems. Voltage disturbances, lightning induced voltages, and radiated EMI are the concern. Lightning protection is a concern if cable trays are located on the top of buildings, in an outdoor exposed area, or in the path of lightning currents. An overhead cable system can provide protection. NFPA780, Standard for the Installation of Lightning Protection Systems 1997 Edition, provides the criteria for building lightning protection.

Cable tray designs are also available that are EMI/RFI shielded. The tray is totally enclosed and the gaskets and covers are constructed and tested to meet EMI standards for the protection of the sensitive circuits in the cable tray against external electric and magnetic fields. Solid bottom cable trays also provide some degree shielding as do cable tray covers. Steel provides effective shielding at frequencies up to approximately 100 kilohertz however at higher frequencies, in the megahertz range, aluminum or copper shielding is more effective.
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