In addition to simply routing and protecting cables a cable tray system must provide protection to life and property against faults caused by electrical disturbances, lightening, failures which are part of the system, and failures of equipment that is connected to the system. These protections can be obtained through proper bonding and grounding. Article 250 of the National Electric Code (NEC) provides the minimum requirements for grounding and bonding. Article 392 provides guidance and minimum requirements specific to cable trays.

To understand these Code requirements adequately, one should always be familiar with defined terms related to the subject matter. Understanding how the defined terms are used within the Code will provide users an improved understanding of how the rules apply to installations.

Definition of Terms

• Ground (Grounded) (Grounding), Earthing

Connected to earth or a conducting body that acts in place of earth

• Equipment Ground Conductor. (EGC)

The conductive path installed to connect normally non‐current carrying metal parts of equipment together and to the system ground conductor or to the grounding electrode conductor, or both.

• Bond (Bounded) (Bounding)

Connected to establish electrical continuity and conductivity

Electrical Conductor Requirements
Article 392.60 of the NEC requires metallic cable trays that support electrical conductors shall be grounded as required for conductor enclosures in accordance with 250.96 and Section IV of Article 250.

The purpose of power grounding (Article 250) is to minimize the damage from wiring or equipment ground faults.

In general, when routing electrical conductors, the cable tray systems are in the path of ground fault currents. The cable tray system is considered to be a non-current carrying parts of equipment that are likely to become energized. Therefore, the cable tray system is required to be bonded, to insure electrical continuity, and have the capacity to conduct safely any fault current likely to be imposed on them to ground. This is accomplished through an equipment ground conductor (EGC), which is bonded to the cable tray system. Metal cable trays should also be bonded to conduit, channel trays 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 ground conductors as defined in NEC table 250.122. Proper bonding of cable trays is further discussed in NEMA VE2.

Steel or aluminum cable trays are permitted to be used as an equipment ground conductor (EGC), but they must meet certain criteria (only in qualifying facilities, minimum cross –sectional area, U.L. classified as suitability, etc., see NEC 392.60(B) (1-4). The use of cable trays as an EGC is also further discussed in CTI Technical Bulletin 11.

The most common alternative for an EGC is to utilize UL listed multi-conductor cables which contain an integral EGC. Note: UL requires many types of tray rated multi-conductor cables to contain an integral EGC.

A separate EGC can also be bonded in or on the cable tray. A separate EGC should be bonded to the cable tray with a bonding / grounding clamp. A separate EGC is commonly selected when single conductor cables are used. Either alternative can be used for non-metallic cable trays. These alternatives 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.

The EGC system is a critical safety system. Therefore, many designers feel it is prudent to treat the cable tray as an equipment ground in parallel with the ground conductor in multi-conductor cables or a separate ground conductor.

Non-Power Conductor Requirements
Metal cable trays containing only non-power conductors are required by NEC only to be electrically continuous Article 392.60 (A), through approved connections or the use of a bonding jumper. When containing only approved low-energy circuits an EGC is not appropriate nor required under Article 392.60(A). Examples of non-power or low energy circuits include: nonconductive optical fiber cables, Class 2 and 3 remote control signaling and power limiting circuits. However, to protect against lighting, radio frequency interference, and electromagnetic interference proper grounding of cable trays is still necessary.

Lighting Protection is a concern if cable trays are located on the top of buildings, in an outdoor exposed area, or in any way are in the path of lighting currents. In such situations the cable tray system should always be properly grounded. NFPA780, Standard for the Installation of Lighting Protection Systems, provides criteria for building lighting protection.

Cable tray systems that contain signal and communication circuits should be grounded and, in some situations shielded from external electrical and magnetic disturbances. This is accomplished, in accordance with NEC 392.60 (A), by insuring the cable tray system is electrically continuous and properly grounded i.e. connected to the facility ground network.

Grounding/ Earthing Cable Trays
The ground network consists of all metal parts of a building connected together: beams, conduits, cable trays, metal frames or devises, all parts which must be connected together to guarantee the equipotentiality of the ground network. To meet this requirement some manufacturers recommend that the cable tray system be bonded to the facility ground system every 50-60 feet. By bonding the tray system every 50’ -60’ the tray will maintain a low potential to ground which reduces external electrical and magnetic disturbances and provides a continuous path for stay currents.

In most installations bonding to the facility ground network is achieved through the cable tray support structure. Steel trapeze or other steel supports securely clamp to the building steel usually provide a solid bond. Where spans not inherently bonded to the facility ground exceed 60’ a separate EGC or bonding jumper should be installed.

Methods of Grounding:
Effective grounding must be permanent / continuous and have ample capacity to safely conduct any current likely to be imposed on the system. It should also have impedance sufficiently low to limit the potential above ground and facilitate the operation of over current devices in the circuit. A continuous, underground metallic water supply system is acknowledged to be the best electrical ground. Other suitable methods include continuous metallic steam and gas piping systems, the grounded metal frame of a building or structure, or an artificial electrode such as driven steel pipe, galvanized or otherwise protected from corrosion, or a buried metallic plate.

Informational note: Wherever multiple grounds are used, it is important that they be tied together in order to avoid any difference in potential between the various parts of the tray system.

Conclusion:
When installed correctly cable tray systems can be a valuable part of a site’s ground network capable of transmitting to ground any fault currents imposed on the system.