Transient Suppressors prevent noise pulses travelling past them, thus preventing damage to equipment further down the line.
The suppressor clamps each signal line within a permitted range (relative to the earth connection). Surges travel along the cable just like the network traffic. If a surge reaches a machine before a suppressor, it may cause damage to that machine. Suppressors should be installed between likely sources of interference or where surges may occur. Usually, one supressor is places at each end of an external cable at the point where it enters the building.
Inside a suppressor (taking, for example the SJ transient suppressor (pictured above)), there are four suppressor diodes. Under normal conditions, there is very little current through the diodes. As soon as a surge hits the diode, it conducts thus clamping the surge and dissipating it safely.
The diodes are similar to regular Zener diodes, but can suppress larger amounts of energy. The diodes in the SJ suppressor are rated at being able to keep the line voltage below 13V when 130A is flowing! They can apparently suppress 1kW of power for up to five minutes, and larger amounts of power for shorter durations.
The diodes are designed to fail safe, so if the surge fries them, it won't fry the rest of the network. A normal diode is likely to fail open-circuit, thus losing protection entirely, but the suppressors are designed to fail short-circuit.
One of the most common causes for transients in the home or workplace come from switching inductive loads. Vacuum cleaners, PCB solder tanks, heating systems... They can all put spikes on the mains line which could find their way into the network. Another source of problems is a large building where different buildings could be wired on different phases.
But the most damaging of all problems is the thunderstorm. When lightning occurs, the electromagnetic pulse is sufficient to cause magnetic energy to occur in loops of wire (such as a ring main). This magnetism collapses rapidly and could cause damage to the wiring, and certainly will cause enormous electrical spikes. It has even been shown that wires literally boiled themselves out of the walls as a result of a nearby lighning strike (note: not a direct hit).
As for direct hits - Tomorrows World has shown how it is perfectly possible for lighning to pass through a modern brick building with little impedence. There is nothing that can protect from a direct hit, short of physically unplugging the computers and moving them at least a metre away from anything metal. If you have a direct hit, there are three simple steps to follow: