Destructive surges

Thunderstorms often rage across the Thousand Islands area of upstate New York, served by Township Telephone. During one storm, 28 line cards in the telco's Chaumont central office suddenly failed, leaving many customers without service. Somehow, unusual high-voltage current surges coming from cables in the outside plant had defeated the electrical protection devices in the CO and zapped delicate circuitry on the line cards.

Initially, the company attributed the surges and card failures to lightning strikes in the area. Telco technicians had not yet heard of the phenomenon called ground potential rise (GPR), or of ways to safeguard telecommunications lines in areas exposed to GPR situations.

Although CO technicians quickly restored service outages by replacing the failed line cards with spares, the company still had another bullet to bite: the bill for repairing circuit and component damages on the 28 cards was roughly $24,000.

Imagine the company's shock when the same 28 replacement cards were knocked out by similar surge currents just a few months later! Once again, the telco had to deal with service outages on the same lines and ante up another $24,000 for card repairs. This time, however, technicians realized it was highly unlikely for lightning to strike twice and produce identical line card damage incidents. Something else had to be causing the surges.

Act II The company suspected that the destructive current surges had used aerial cable along Depauville Road as their pathway into the Chaumont CO. On both occasions, customers served by pairs in that cable, including an electric power substation situated about 1.25 miles from the CO, had experienced service outages.

Because each line card interfaces the CO switch with outside plant pairs for six subscriber lines, the technicians wanted to do something that would reduce the company's losses just in case another surge occurred before they discovered its cause. Therefore, the technicians clustered the lines for customers served via the Depauville Road cable onto seven line cards. Previously, they had been scattered over all 28 of the line cards damaged in the first two incidents.

The company also contacted F.F. Tronixs. Based in Rochester, N.Y., Tronixs represents various manufacturers, distributes telecommunications equipment and does installation work. F.F. Tronixs immediately focused on the electric power substation as the probable source of the high-voltage surge currents.

High-voltage transmission lines feed power to substations that parcel it out to end users via low-voltage distribution lines. Everything in a substation that needs grounding is connected to a large metallic ground grid underlying the area. If a fault to ground occurs on one of the distribution lines, electrical current rushes through the earth back to the substation. The fault current energizes the substation and raises its ground potential much higher than that of the ground farther away, often by thousands of volts. To speed its dissipation, the high-voltage current will surge down any available conductive path to one or more remote grounds of lesser electrical potential.

Of the circuits in Township Telephone's aerial cable on Depauville Road, three served the substation. The cable pairs for these circuits were equipped with gas discharge tube protectors at the customer premises network interface and at a nearby lateral pole. Normally, any destructive voltage on the outside plant would cause the gas tube protectors to fire and, in discharging, provide the voltages with a conductive path to ground.

In this case, the current was coming from ground, so each major GPR caused the gas discharge tubes at the substation to fire in the reverse direction, providing a path for a high-voltage current to surge into telco cable pairs. Once there, it migrated to other pairs in the aerial cable and raced back to the CO. Although greatly weakened by the gas tube protectors at the CO, apparently enough of the current surge got past them to damage the line cards. Fortunately, the surges did not injure anyone or damage phones and equipment on customer premises.

F.F. Tronixs brought in the Power Products Division of Positron Inc. to recommend how high-voltage special protection devices could solve the GPR problem once and for all. Township Telephone put in a rush order for the equipment and arranged for F.F. Tronixs to install it. The telco simply had to replace the existing buried cable between the lateral pole and the substation with cable that had copper shielding.

Although all work was completed in just two weeks, it still wasn't done soon enough. By some twist of fate, one more costly GPR event occurred during that two-week period. But this time, because of Township Telephone's earlier precautionary steps, only the seven line cards with circuits serving customers along Depauville Road suffered damage.

Act III Positron's equipment, called Teleline Isolator, came in a red cabinet the size of a small suitcase that was mounted on a wall inside the substation. A shelf in the cabinet accepts up to eight individual plug-in line modules.

Each module serves one circuit and acts as a dielectric barrier that isolates facilities on the telco side of the cabinet from those on the substation side, denying passage to high-voltage surge currents. According to the vendor, the modules provide a protection level of 65 kV peak and 20 kV root mean square.

The active circuitry on modules for DC voice telephony include a short length of optical fiber used as a dielectric barrier to isolate the copper facilities on either side of the cabinet. Circuitry on the telco side of the module is line-powered, while the substation provides the power for the other side. Passive circuitry is used on modules for providing tone-type services on dedicated two- and four-wire data circuits. Passive modules are completely line-powered, and the isolating transformer on each one serves as the dielectric barrier.

Three years have gone by since Township Telephone installed the equipment. Since then, the company has not lost a single line card at the Chaumont CO. The cost was about $700 a circuit, a drop in the bucket when compared with the line card repair costs incurred for damages that any one of the GPRs caused.

Andrew G. Mahaffy Sr. is Director of Network Operations for Township Telephone Co., Chaumont, N.Y.