Grounding, Bonding, and Surge Protection
Grounding, bonding, and the low-voltage system
Grounding is a path to earth; bonding is intentional connection between metal parts to keep them at the same potential. Article 250 provides the general rules. Articles 800.100, 820.100, and 830.100 give the grounding requirements for communications, coaxial, and network-powered broadband circuits. Low-voltage systems still need bonding and grounding where the article requires it, and the grounding electrode system must be one system, not a collection of isolated rods.
Electrodes and intersystem bonding
The grounding electrode conductor connects the service or system to the grounding electrode system. A single ground rod with more than 25 Ω resistance requires a supplemental electrode, and the two must be bonded together. An intersystem bonding termination gives communications, CATV, and other low-voltage systems a common place to connect. Do not use the neutral as a ground on the load side of the service.
Surge protection
Article 285 covers surge protective devices (SPDs). AC SPDs are selected by voltage, configuration, MCOV, and surge current rating. Signal and coax SPDs protect data, CCTV, and control lines; they must be coordinated with the system’s grounding. A surge protector without a low-impedance path to ground does little.
Worked example
A coaxial cable enters a building and terminates at a grounding block. The block must be bonded to the building’s grounding electrode system with a conductor sized per 820.100, commonly 14 AWG copper for the coaxial shield, and located as close as practicable to the point of entry. If the installer drives a separate rod for the block and does not bond it to the building electrode, the two electrodes can sit at different potentials during a surge; the coax shield, equipment, and network then carry the difference. Bond the block to the same electrode system.
Traps
- Treating a ground rod as the only electrode without the 25 Ω supplemental rule.
- Daisy-chaining equipment grounds instead of a single-point or low-impedance bond.
- Connecting the coax ground block to a separate isolated rod.
- Using the neutral or a convenient conduit as the equipment ground.
- Installing an SPD with long, loose leads that add impedance.