Grounding, Bonding, and Electrical Protection for Low Voltage Systems
Grounding versus bonding
Grounding connects to earth; bonding connects metal parts to create a low-impedance fault path. Low-voltage systems still need bonding for safety, surge protection, and a common reference. NEC 250.50 requires a grounding electrode system; 250.94 requires an intersystem bonding termination for communications, CATV, and similar systems. Article 800 and Article 820 provide specific grounding rules for communications and coaxial cable shields.
- Size grounding electrode conductors with Table 250.66 and bonding jumpers with Table 250.102(C)(1).
- Bond metallic raceways, enclosures, and cable trays with listed fittings (250.96).
- Keep signal reference and equipment grounding bonded to the building ground; do not create an isolated earth that is not bonded back.
- Use listed surge protective devices (Article 285) and follow manufacturer grounding for PoE and cameras.
Worked example
A camera pole has its own ground rod, with no bond to the building electrode system. A nearby lightning surge raises the pole potential relative to the building, and the camera data and ground path becomes the fault path. The fix is to bond the remote electrode to the building grounding electrode system per 250.50 and 250.58 and use the intersystem bonding termination at the service.
Traps
Calling a separate ground rod an isolated ground; using conduit as the only equipment ground without bonding bushings; undersizing the grounding electrode conductor; daisy-chaining ground bars instead of a single bonded bus.