A few columns ago, I discussed transmission testing and tossed out the opinion that a protector ground should test a maximum of 25 ohms for proper safety and service. I received a thoughtful email from Henry J. Saam of the Washington County Rural Telephone Cooperative, wondering where I came up with that figure. He pointed out that the National Electrical Code (NEC) doesn't specify maximum ohms value for communication circuits:

"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 me thinking. Where did I 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 of a novice, used that figure to shut me up. Maybe I read it somewhere. But, after some searching, I agree with Henry: NEC Article 800 does not state a maximum acceptable limit for ground ohms. But there are reasons for our assumption. Section 800-31 (b) (5) does state that grounding can be acceptably accomplished to the metallic power ground electrode. Section 250H states the 25 ohm maximum for the power ground electrode.

Ground and Earth
Original telephone circuits in the late 1800s were earth return circuits which consisted of a single transmission wire and two ground rods. The circuit was completed through the earth between the rods. On a ground return system the better the ground, the more noise on the circuit, the worse the ground the poorer the quality of the transmission.

The burgeoning use of AC power transmission exacerbated the problems, so, in 1885, 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 an electrical environment ground is a reference to one side of a circuit. The best example of this is the common automotive electrical system: the positive side of a 12-volt battery is connected to the electrical wiring system, and the negative battery side is connected to the metal frame and engine of the car. This saves both on wire and weight.

I have a 1943 MB Willies jeep with a negative 6-volt system. The negative side is connected to the electrical wiring system and the plus side of the battery is connected to the frame (ground). Here we have positive ground in reference to the negative 6 volts wiring system. Remember, the car is sitting on four rubber tires and is not in electrical contact with earth.

Most field technicians do not have much formal electronics training. When I refer to ground, they bring me a shovel of dirt. In a telephone system, earth and ground are of the same potential (within a volt), but they are two 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 protector ground at the same potential as the central office ground. This is rarely possible due to different moisture, chemicals, temperature, etc., between the two 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 station and cable grounds, with 5 ohms acceptable at the CO ground. (Reference: Practical Grounding: Theory and Design (ABC of the Telephone Series) by Myron L. Brewer, page 48.)

In most areas, common bonding and grounding with multi-grounded neutral power systems is highly recommended. 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); and it allows the station ground to use the power ground spacing throughout the grid, which reduces resistance to the CO ground.

As Article 250 mandates a maximum 25 ohm power ground, and it is recommended that a common ground be found with MGN power, it would follow that such a ground ohm standard is acceptable.

While researching this question, I contacted Bill Von Alven, FCC Manager, and 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 me 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 must be common bonded to the power ground at the premises. 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 #6 gauge wire to assure ground safety and effectiveness.

In any area, good bonding is essential. The high currents produced by lightning through a series resistance bond can produce arcing conditions which will damage pairs no matter how good the ground is.

One bond manufacturer tests the quality of their bonds by using an arc welder. They use a short piece of cable (a foot will do) and attach their standard bond on one end and the competitor's bond on the other end. They connect an arc welder across the bonds and increase the current until something gives (which simulates a lightning strike or an AC power cross). The competitor's bond will fail or the shield will melt before their bond connection fails.

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.

Many telephone personnel are under the assumption that 60 Hz and harmonics will have no effect on ADSL and IPTV because those frequencies are from 60 Hz to 3 kHz, and ADSL and IPTV function at frequencies above 25 kHz. But the fact is that longitudinal currents generated by 60 Hz, 180 Hz, and 540 Hz will first cause vector shifts in equipment center tapped transformers. And when those transformers become saturated, both ADSL and IPTV are affected.

As telcos 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 you will share with me and other readers any interesting cases of trouble, particularly if they involve power influences; it's always a popular topic. Contact me at dmccarty@mccartyinc.com or 831.818.3930. You can visit my site at www.mccartyinc.com.

About the Author - Don McCarty

LET`S TALK Check out our website (www.mccartyinc.com). In addition to providing information about our extensive selection of courses, we are adding an equipment review section and a section on troubleshooting, where we will discuss some interesting cases of trouble. If you have an interesting case, send it along (names withheld by request).

As always, I appreciate hearing from you with questions or comments on any topics that you think are of general interest to outside plant technicians and managers.

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