Suppressing the Surge

It's a beautiful summer day in San Francisco. Although it's the hottest it's been in years, with the breeze off the bay, the record-high temperatures don't seem so bad. Suddenly, the air-conditioning stops running, the lights go out and the day doesn't seem nearly so beautiful. Power companies have shut down the overworked grid.

Three hours later as evening falls, the power grid comes back on, sending a surge of energy through the lines. Wireless sites across the bay area brace for the rush of ac power.

Unfortunately, some of the surge suppressors fail to stop the transient voltage, and site equipment pays the price.

“When I come into a building, and I see that the rectifiers are all toast, and there's gooey substances coming out of the front of them, it's because that spike came in on the ac side, the commercial power side,” said Jay Quinlan, U.S. Cellular (www.uscc.com) director of technology and planning.

Although lightning gets more mention, ac power surges can be just as expensive to fix and usually are more common.

“Every once in a while a tower gets hit by lightning, and that's just going to happen,” Quinlan said. “Most of our problems come in on the commercial-power side.”

Jeff Moore, Emerson Network Power (www.emerson.com) surge division sales director, agreed.

“The greater quantity of surge problems are caused by grid switching rather than by lightning activity,” he said.

However, Moore said that because lightning can be damaging, carriers often forget the importance of ac protection.

Just like lightning, ac power surges can't be avoided. In fact, some ac spikes are caused when lightning hits a power line, sometimes miles from a site, and the surge travels down the line into equipment.

Ac surges come in many sizes and types, but even small surges can degrade equipment.

“Really, the whole reason to have surge protection for (an ac) event is to prevent disruption more than destruction,” Moore said. “It's the rare event that causes destruction, but it's the everyday event that can cause the disruption.”

Minor surges in ac power can lead to a few dropped calls here or there, but many minor surges and disruptions will begin to wear down equipment and lead to network degradation.

AT&T Wireless (www.attws.com) takes ac surges seriously.

“All of our offices — whether it be a central office, a 28-story building or just a little, tiny hut — are going to have ac surge protection,” said Ron Kiederer, AT&T Wireless technical services manager.

The good news for carriers is that for every ac surge out there, there's a suppressor, shunter or isolator for sale to stop it.

Suppressors and shunters literally clamp the power line and force the energy to go to ground outside a building, said Ernest Duckworth, Positron (www.positron.com) vice president, power products division. Putting an isolator in the middle of a power line breaks the paths surges follow.

All of the devices also have to work in a fraction of a second.

“When a surge is moving down a wire, we talk in terms of wavefronts that are 8ms,” Duckworth said. “So, if your device isn't fast enough to grab that energy before it goes by, what good is it?”

It's also necessary for ac surge protectors to have repeatability, so they don't have to be replaced after every hit — a maintenance nightmare for carriers.

“A lot of these towers and sites are fairly remote,” said Bill Manning, American Power (www.apc.com) director of engineering. “So, the last thing that people want to do is send someone out to fix something.”

However, carriers said the failure of one surge protector at one site often can teach them much more than the perfect operation of 100 other sites.

Kiederer said that after 30 years with AT&T, both wireless and landline, he has learned from more than a few failures.

U.S. Cellular's Quinlan agreed, saying his company recently rewrote its grounding specifications after a site took a direct lightning strike.

“It gives us a chance to pause, take a look at our specs and see what might need to be changed,” he said.

Quinlan said visiting damaged sites after failures can be interesting.

“You see all kinds of cool things,” he said. “You actually see arc marks where the current has taken energy and gone around the halo ground.”

But, because cool-looking damage means big bucks in repair, all carriers go to great lengths to avoid surges.

AT&T puts arrestors anywhere ac enters a building. U.S. Cellular does the same.

“We use surge protection on the ac coming in on the commercial power side,” Quinlan said. “You put protection on your coaxial cable coming in off the antenna and certainly on the outside of the building.”

Carolina West Wireless learned the hard way to take lightning seriously, said Greg Roark, network systems engineer (www.carolinawest.com). Because many of Carolina West's sites are up in the Blue Ridge Mountains, they experience violent thunderstorms often.

“We were having a site that was having problems due to surges,” Roark said. “We're talking once a week, sometimes once every two weeks.”

The company changed its grounding scheme from a halo ground to a single-point ground and put in some isolators. In a halo-ground system, four grounding leads go out the corners of a building and attach to a surrounding ground ring. In a single-point grounding system, all ground lines come to a single point in a building before going to ground.

Three years after making changes, Carolina West has seen major improvements.

“There's been strikes because you can see the tower,” Roark said. “But we've not had any damage whatsoever.”

Isolation systems are more expensive than traditional shunting devices, but Roark said that the extra money was worth it because the company doesn't have to pay for damage.

On the whole, though, carriers know that no matter what level of protection they have, they are at the mercy of chance and circumstance.

“Lightening and transients you can't control,” AT&T's Kiederer said. “It's impossible. You never know what God's going to send.”

here's a wide variety of surge protectors out there, but two of the most common are solid-state shunting devices and isolators.

Solid-state shunting devices heat up when there is an increase in voltage.

“The polymer goes through a melting transition, during which time it expands and its resistance increases by several layers of magnitude,” said Lisa Leo, Raychem Circuit Protection (www.circuitprotection.com) telecommunications market manager.

That cuts off the flow of current and brings the system to ground. New designs of these devices can return to normal after taking shocks and protect from several surges.

Isolators work in a completely different way.

“It doesn't need to dissipate the energy,” said Ernest Duckworth, Positron (www.positron.com) power products division vice president. “If you isolate a wire, you make one side at one potential and the other side at another potential. It's like you build a dam in the middle.”

In other words, there's no path for the surge to follow, so it can't get into equipment. A 5" isolation cable put into a wire will provide 10,000V of isolation per inch. Positron's isolator is made of fiber-optic cable.

“We take an electrical signal from a copper wire, change it into a light signal, get across that 5" gap, change it back into an electrical signal and send it on its way,” Duckworth said.