Beware the Surge

The need for surge protection and proper grounding goes back to early science lessons: Electrical currents always will take the easiest path to ground. Because you have numerous base stations in the field -- all of which are connected to ac current and many of which are connected to copper telco lines -- the threat of damage from electrical surges or improper equipment grounding is a major consideration, said John Williams, Transtector Systems vice president.

"A typical radio base station can cost around $100,000, which means you have a vested interest in ensuring that nothing happens to the equipment," he said. "Furthermore, you are faced with the reality that when equipment is down, the loss of calls across the network creates a double-edged sword."

Most wireless carriers agree that lightning is their greatest threat when it comes to electrical surges. According to Mark Ripley, GTE Wireless assistant vice president of area networks, lightning accounts for 99% of the surge-protection techniques his company installs on base stations. The remaining 1% protects against electrical surges on power lines, something the company combats by filtering lines through a battery before they reach the wireless equipment.

But vendors argue that lightning is not the only worrisome type of surge. Although the majority of electrical surge problems come from lightning strikes, the reality is that you must contend with other types of surges and grounding issues to protect your infrastructure investments.

MEET THE ENEMY HEAD ON Julie DiCarlo, Houston Cellular spokeswoman, said ac power surges have become increasingly threatening as the company has moved toward smaller pieces of equipment that are designed for low voltage.

"We have to protect against all types of potential surges, since a surge of a few hundred volts or less could easily damage the equipment," she said.

Williams said not all problems with surge protection stem from new equipment. In many cases, problems with improper grounding and poor surge protection date back to the mid-1980s, when successful cellular carriers hurried to erect towers, cut over switches and plant cell sites at every hilltop on which they could get their zone-free hands.

As a result, problems have manifested themselves in terms of inadequate surge protection. Coupled with hastily conceived grounding schemes, the results can range from poor service reliability to damaged sites.

Two other primary factors that can create surges to base stations are load-factor correction and capacitor bank switching by local utilities, Williams said.

"Demand for electrical power has grown more than 20% over the past several years, but development of new electrical infrastructure has remained flat," he said.

The electric companies must do a load-factor correction to address the needs of densely populated areas where people are doing things at the same time.

"The end result is that there are surges on the lines," Williams said.

Essentially, a surge or voltage transient is an induced rise in voltage that lasts less than half cycle or 8 milliseconds, Williams said. The day-to-day effect on wireless equipment is erosion that creates weaknesses in one or more parts of the equipment.

"When those weaknesses happen, you have a path of least resistance," he said. "So every time a surge happens, the current goes to that same component until it breaks. It's like a cancer. You can't see it, smell it or taste it, but it's eating away at your equipment nonetheless. The end result could be a breaker that blows, or it could be a blown radio."

Oscillatory decaying transient is another type of surge that can affect networks. This occurs most frequently in regions in which cell sites are on mountains or in other remote areas in which the wireless equipment often is the last piece of equipment on a line. Downed trees, ice storms, birds and other external influences can create a surge on a line that flows directly to the wireless equipment.

Although not as commonly known as other types of surges, another form of surge that you need to control is ground potential rise (GPR). According to Lucie Trepanier, Positron product manager, GPR occurs when you install base stations on electrical transmission line towers. For GPR to occur at these sites, the base station must be directly connected to copper telco lines, as well.

Essentially, GPR occurs when a surge comes up from the grounding device. This surge generally is created when a fault occurs on a transmission line. Because the equipment is connected to copper telco lines, a difference in potential occurs, creating a current of electricity that is looking for an outlet. The outlet for the current becomes the ground grid because it is the path of least resistance.

"The wires in base stations simply aren't capable of handling the kind of current that can be created when GPR occurs," she said. "We have seen instances in which telco wires -- which are tiny little wires designed to only carry a few volts -- receive 10,000V because of GPR. They simply go 'poof' and are vaporized. Either that happens, or the current damages the cellular equipment."

CONQUER THE ENEMY Ripley maintains that carriers usually address surge protection when they protect against lightning strikes. Nevertheless, he also admitted you can't plan for surge protection and grounding with a cookie-cutter mentality. Each site has different standards and faces problems from different types of surges.

"The type of grounding and surge protection we install depends a great deal upon where the site is," he said. "We have many cell sites in the Gulf Coast of Florida, which is the lightning capital of the world. The way we protect those sites is much different from the way we protect our sites in California, where lightning isn't a big deal, but surges from earthquakes are a big deal. If you try to develop cookie-cutter solutions to grounding and surge protection, it's not money well-spent."

DiCarlo said Houston Cellular has installed ac surge protectors on its commercial ac feeds. Trepanier and Williams said a proper surge protector, or shunt device, will address most potential problems.

"A shunt basically looks at the line it's protecting and monitors it for a surge," Trepanier said. "When the shunt sees a surge, it fires to ground, creating an easy pathway into the grounding device for the current to flow. This prevents the surge from going to the equipment."

Williams said such surges can occur on ac power lines, T1 lines, RS232 lines and on RF cabling that comes off the tower into the cell site. For the first three, the most common surge-protection device is known as a transient-voltage surge suppressor (TVSS). The cost of TVSSs range from as much as $3,000 for the top-of-the-line model for ac power lines to less than $100 per T1 line.

RF cabling protection generally comes in the form of quarterwave stubs, which cost between $70 and $150, or gas tubes, which range in price from $50 to $100.

The two primary types of surge protectors are metal oxide varsitors (MOVs) and silicon avalanche suppressor diodes (SASDs). MOVs are the older technology, and basically protect against insulation breakdown. Their average life span is about two years, and they are not designed to handle exposure to numerous surges, Williams said. SASDs have an average life span of about 10 years, are more costly and are designed to handle numerous surges.

To control GPR, vendors offer products that isolate wireline communications in high-voltage situations. Essentially, these products are isolation devices inserted at the copper wire on the base station. The device isolates the two ground planes from one another, rendering any current from being created between the two planes. Another option when dealing with GPR potential is for the telco to install fiber-optic lines in place of old copper lines, Trepanier said.

Although the industry finally has a better handle on surges than it did 10 years ago, Trepanier said, sites still are vulnerable and you need to ensure that they are protected appropriately.

* Lightning

* Load Factor Correction

* Capacitor Bank Switching

* Oscillatory Decaying Transient

* Ground Potential Rise