CATV Installer Drives an Isolated Ground Rod at a Retail Service
A 400 A, 120/240 V single-phase service supplies a small retail building. The existing grounding electrode system has two 8-ft copper-clad ground rods 6 ft apart, a #2 AWG copper grounding electrode conductor to the metal water pipe, and #6 copper to the rods. An intersystem bonding termination is mounted at the service. A CATV installer arrives for a new internet service and drives a new 8-ft rod within 3 ft of the CATV entry, about 40 ft from the service. He runs a #10 copper conductor from the CATV ground block to the new rod only and tells the owner this isolated ground will reduce noise on the data line. He does not connect to the intersystem bonding termination. The owner asks you, the low-voltage contractor, whether the install is acceptable and whether the separate rod is a good idea. You must evaluate grounding, bonding, and conductor sizing.
1. Is the isolated ground rod permitted, and must it be bonded to the building grounding electrode system?
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A separate ground rod is permitted, but it cannot be left isolated. The building grounding electrode system must be a bonded network. A new rod is an electrode, and 250.50 requires available electrodes to be bonded together into the grounding electrode system. Communications grounding requirements in Articles 800 and 820 also direct the communications grounding conductor to the building grounding electrode system, not to a remote earth connection that is deliberately kept separate.
An unbonded rod 40 ft from the service creates a difference in potential between the CATV shield/ground block and the electrical service during a fault or surge. That is exactly the condition grounding and bonding are intended to prevent. The installer's rod can remain as a supplementary electrode only if it is bonded to the existing grounding electrode system with a grounding electrode conductor sized per 250.66. For a rod electrode, 250.66(A) permits the bonding conductor to the rod to be no larger than #6 copper, but it still must be at least #6 copper. The #10 conductor is not adequate for bonding the new rod.
2. What is wrong with the #10 conductor and the point of connection?
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The #10 conductor may be acceptable as a communications grounding conductor in some Article 820 installations, but it is not acceptable as the sole bonding jumper for a new grounding electrode. The electrode bonding conductor to a rod must be sized per 250.66 and, for a rod, is not required to exceed #6 copper. A #10 conductor is smaller than that minimum and is not protected from physical damage in the described route.
The point of connection is also wrong. The CATV grounding conductor should connect to the intersystem bonding termination at the service, or to another accessible point on the grounding electrode system as permitted by the communications article. Connecting only to a remote, unbonded rod leaves the CATV system referenced to a different earth potential than the electrical service. Correct the installation by bonding the new rod to the existing grounding electrode system with at least #6 copper and by connecting the CATV ground block to the intersystem bonding termination or the service grounding electrode conductor. Keep the connection accessible.
3. Explain why the noise-reduction claim is technically unsound.
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Grounding is first a safety function: it provides a low-impedance path for fault current and limits voltage differences between conductive parts. An isolated earth electrode does not provide a quiet reference for data. Earth is not an equipotential plane; separated electrodes can sit at different voltages because of lightning, utility faults, or nearby current flow.
When two grounding points are bonded, they are forced to nearly the same potential, which reduces common-mode voltage differences. When they are separated, the CATV shield and the building electrical ground can bounce relative to each other, and that difference can appear as noise or surge current in the data path. If shielding or grounding changes are needed for signal quality, they should be designed as part of a bonded system, not by creating a separate earth. The correct advice is to bond the new rod and the CATV ground block to the building grounding electrode system.