Wireles Networking is a practical guide to planning and building low-cost telecommunications infrastructure. See the editorial for more information....

Surge and Lightning Protection

Power is the greatest challenge for most installations in the developing world. Where there are electrical networks, they are often poorly controlled, fluctuate dramatically and are susceptible to lightning. Proper surge protection is critical to not only protect your wireless equipment, but all of the equipment connected to it.

Fuses and circuit breakers

Fuses are critical, but very often neglected. In rural areas, and even in many urban areas of developing countries, fuses are difficult to find. Despite the added cost, it is always prudent to use circuit breakers instead. These may need to be imported, but shouldn't be overlooked. Too often, replaceable fuses are removed and pocket change is used instead. In a recent case, all of the electronic equipment at at rural radio station was destroyed when a lightning strike went through the circuit, without circuit breaker or even a fuse to protect it.

How to ground

Proper grounding doesn't have to be a complicated job. When grounding, you are trying to accomplish two things: provide a short-circuit for a lightning strike, and provide a circuit for excess energy to be dissipated.

The first step is to protect equipment from a direct or near direct lightning hit, while the second provides a path to dissipate excess energy that would otherwise cause a build-up of static electricity. Static can cause significant degradation to signal quality, particularly on sensitive receivers (VSATs for example). Providing the short-circuit is simple. The installer simply needs to make the shortest path from the highest conductive surface (a lightning rod) to the ground. When a strike hits the rod, the energy will travel the shortest path and thus by-pass the equipment. This ground should be able to handle high-voltage (i.e. you need thick gauge wire, like 8 gauge braided copper).

To ground the equipment, mount a lightning rod above the equipment on a tower or other structure. Then use a thick gauge conductive wire to connect the rod to something that itself is well grounded. Underground copper pipes can be very well grounded (depending on their depth, the moisture, salinity, amount of metal and organic content of the soil). In many sites in West Africa, pipes aren't yet in the ground, and previous grounding equipment is often inadequate due to ill-conductive soil (typical of seasonally arid, tropical soils). There are three easy ways to measure the efficiency of your ground:

  1. The least accurate is to simply plug a good quality UPS or power strip into the circuit that has a ground detect indicator (a LED light). This LED is lit by energy that is being diffused to the ground circuit. An effective ground will dissipate small amounts of energy to the ground. Some people actually use this to pirate a bit of free light, as this energy does not turn an electrical counter!
  2. Take a light socket and a low-wattage bulb (30 Watts), connect one wire to the ground wire and the second to the positive current. If the ground is working, the bulb should shine slightly.
  3. The more sophisticated way is to simply measure the impedance between the positive circuit and the ground.

If your ground is not efficient you will need to bury a grounding stake deeper (where the soil is more moist, has more organic matter and metals) or you need to make the ground more conductive. A common approach where there is little soil is to dig a hole that is 1 meter in diameter and 2 meters deep. Drop in a highly conductive piece of metal that has some mass to it. This is sometimes called a plomb, which literally means lead but can be any heavy piece of metal weighing 500 kg or more, such as an iron anvil or steel wheel. Then fill the hole with charcoal and mix in salt, then top with soil. Soak the area, and the charcoal and salt will diffuse around the hole and make a conductive area surrounding your plomb, improving the efficiency of the ground.

If radio cable is being used, it too can be used to ground the tower, though a more resilient design is to separate the ground for the tower from the cable. To ground the cable, simply peel back a bit of cable at the point closest to the ground before it goes into the building, then attach a ground cable from that point, either by soldering or using a very conductive connector. This then needs to be waterproofed.

Power stabilizers & regulators

There are many brands of power stabilizers, but most are either digital or electromechanical. The latter are much cheaper and more common. Electromechanical stabilizers take power at 220V, 240V, or 110V and use that energy to turn a motor, which always produces the desired voltage (normally 220V). This is normally effective, but these units offer little protection from lightning or other heavy surges. They often burn out after just one strike. Once burnt, they can actually be fused at a certain (usually wrong) output voltage.

Digital regulators regulate the energy using resistors and other solid state components. They are more expensive, but are much less susceptible to being burnt.

Whenever possible, use a digital regulator. They are worth the added cost, and will offer better protection for the rest of your equipment. Be sure to inspect all components of your power system (including the stabilizer) after lightning activity.

Last Update: 2007-01-24