A number of columns ago, we discussed transmission testing and tossed out the opinion that a Network Interface ground should test a maximum of 25 ohms for proper safety and service. We received a thoughtful letter from Henry J. Saam of the Washington County Rural Telephone Coop of Pekin, Indiana, wondering where we came up with that figure. He pointed out that the NEC doesn’t specify maximum ohms value for communication circuits.

He stated, “Article 800 of the National Electrical Code covers the general grounding requirements for communication circuits but does not specify ground resistance values. Article 250 of the NEC applies exclusively to power circuits. The reference in Article 250 to a 25 ohm ground should not be construed as a mandatory value, and in no way applies to communication grounds.”

That got us thinking. Where did we get that value? Was it one of those things that become a truth simply because we’ve always said it? Maybe some old-timer, irritated at the constant questions from a novice, used that figure to shut us up. Maybe we read it somewhere. But, after some searching, we agree with Henry: NEC Article 800 does not state a maximum acceptable ground ohm. But there are reasons for our assumption: Section 80031 (b) (5) does state that grounding can be acceptably accomplished to the metallic power ground electrode. Section 250H states the 25 ohm maximum for power electrode.

Ground and Earth
Original telephone circuits in the late 1800s were earth return circuits consisting of a single transmission wire and 2 ground rods. The circuit was completed through the earth between the rods. The better the ground at the Interface, the more noise created on the circuit, the worse the ground at the Interface, the worse the transmission.

The burgeoning use of AC power transmission exacerbated the problems, so, in 1883, a tip conductor was introduced. This provided an insulated return path for loop current. It also provided good longitudinal balance to the circuit as long as the tip and ring were of the same capacitive and resistive length. Earth grounds, which were once a transmission facility, now became a protection facility against power and lightning, and a control of line born noise (induced AC).

In the Plain Old Telephone Service (POTS) business, low frequency AC harmonics noised up the line. In today’s Triple Play environment, equipment is affected by AC interferers up to 10 Megahertz.

In a telephone system, earth and ground are of the same potential (within a volt), but they are 2 separate electrical systems. This is where the confusion starts. Ground (tip) is a return to the other side of the negative CO battery. To assure safety, CO earth ground (protection) must be at the same potential as the tip ground and the grounded electrode system.

Ideally, it is the intent of the Telco grounding system to have each Network Interface ground at the same potential as the central office or remote ground. This is rarely possible due to different moisture, chemicals, temperature, etc., between the 2 points. Many sources state that the goal of the industry is to attempt to have all grounds not exceed 3 ohms. Since this is often not possible to obtain, a more realistic 25 ohms of resistance is acceptable for most telephone Network Interface and cable pedestal grounds, with 5 ohms acceptable at the CO and remote ground. (Reference abc of the Telephone Practical Grounding; Theory and Design, by Myron L. Brewer, page 48.)

In most areas, common bonding and grounding with the Multi Grounded Neutral (MGN) of the wye distribution power systems is required. There are several advantages to this hookup: all utilities have the same voltage in case of an electrical surge (prevents arcing between adjacent utility wires); it allows the station ground to use the power ground spacing throughout the grid, which reduces resistance to the CO or remote ground. As Article 250 mandates a maximum 25 ohm power ground and it is required that a common ground be found with MGN power.

While researching this question, we contacted Bill Von Alven, FCC Manager, Telephone Equipment Registration Program. He stated that Part 68 of the FCC requirement does not designate a value for protector ground and that this value is left to the individual operating companies. He referred us to several technical experts, including Mr. Charlie Hansel, an independent consultant on grounding, bonding and safety. Charlie confirmed that a potential difference between the telephone and the power ground is the cause of flashovers in telephone circuits. When this happens, equipment is destroyed; people are injured and sometimes killed.

For this reason, Article 250 part G requires that telephone service Network Interface must be common bonded to the power ground at the premise. Part H states the power ground electrode should measure less than 25 ohms. If it exceeds this value, a second ground must be placed 6 feet from the first ground, connected with a #6 gauge ground wire, to assure ground safety and effectiveness.

AC Induction
Grounding and bonding is essential for reducing the effects of induced AC. Many technicians are under the assumption that buried conductors are not as affected by AC as are aerial cables. This is a misconception. Burying cable does not protect it from induced AC. As we have seen, the earth is conductive and provides no shielding effect whatsoever. Treat AC induction in buried cables as you would with aerial cables.

In a multi grounded power system, there can be considerable separation between the sending and return paths. It is not unusual to find as much as 60 percent of the current returning in the earth and telephone cables, and only 40 percent in the neutral, because the earth and the telephone cables are a much better conductor. It is not the voltage of the power systems that creates the problem, it’s the ground return current that is the primary culprit, especially for low frequency induction problems. Transmission testing at the protector will provide information needed by the transmission team to identify and fix the problems.

As Telco’s reputations are built around safety for both employees and customers, any deviation from company practices on grounding is unacceptable. Confirm the presence of a good protector ground at each station visit by both ohms testing and a thorough visual inspection.

Signing off
I hope this column helps some of you. If so, let me know. If not, let me know what you’d like to hear about. Thanks for reading! Call or text me at 831.818.3930, email me at dmccarty@mccartyinc.com, or join the chat on my site at www.mccartyinc.com. And check out my TALK BACK online column and tell me what you think: www.ospmag.com/columnists/mccarty/talkback.