The Importance of Surge Suppression
Most people are aware of the fact that electrical and, in particular, electronic equipment can be damaged or destroyed by lightning. This is a dramatic and
probably the most extreme example of surge damage, and equipment definitely needs to be protected from lightning damage, but surges and transients can
also be produced by equipment located inside a building.
Surges or voltage transients are abnormally high pulses of voltage that substantially exceed the normal operating voltage of a circuit. They are generally
random in nature and may last anywhere from tens of nanoseconds (a billionth of a second) to a few milliseconds (a thousandth of a second). Advances in
semiconductor processing technology, producing devices with smaller and smaller geometries, have also limited the ability of these devices to absorb transient
energy. As such devices are designed into electronic equipment, the overall susceptibility of the system increases. With billions of dollars' worth of equipment
now being lost each year to the effects of electronic overstress, transient protection is no longer a luxury but a necessary part of system design.
There is absolutely no way to protect a piece of equipment in a part of a building that gets a direct hit by lightning! Usually, however, when a building or a
utility pole gets hit, the equipment itself does not receive the direct discharge: the equipment receives a surge through the building wiring. Research has shown
that because of arc-over at the service entrance and within the building wiring, the maximum voltage that reaches a 110V outlet is 6000V. The same research
has also determined that the maximum current is 3000A. We now have an idea of what a surge protector must be able to reliably handle in order to protect
equipment from lightning damage.
All parts of the USA receive a significant frequency of lightning strikes in a year, but there are regional variations. Being on a hill increases the risk of a lightning
strike as does the geographic location of a building. A surge protector in a "hot" area such as Tampa, Florida may have to withstand more surges than a surge
protector in Alaska. Thus, it is not just the energy of a surge that has to be considered, but the frequency of surges. Also, lightning is not only hazardous to
equipment just when the strike is very close, the intense electric and magnetic fields surrounding a typical 20,000 amp strike will induce a voltage of around
2000V at a distance of 300 feet in just 3 feet of wire, and will still induce hundreds of volts at a distance of ½ mile! This is why equipment failures occur
during a storm when no evidence of a strike is apparent in the immediate area. The accompanying graph shows the typical relationship of induced voltage to distance.
When any piece of equipment that contains an inductive element, such as a motor, transformer or coil is switched off, what is called a "back-emf" is
produced. This back-emf, which is caused by the collapse of a magnetic field, is the result of one of the most basic laws of electricity, Faraday's Law of
Induction. The voltage thus produced can be several times the original voltage applied to the inductive element before it was switched off, although the
duration of these surges (or transients) is very short. Electric cattle fences are energized using this principle - a coil is repeatedly energized (often by a
12V battery) and then the current is shut off. The resulting back-emf can be several hundred volts, as anyone who has touched such a fence finds out!
Such inductively produced transients may not be as energetic as a lightning strike, but similarly damage and degrade electronic equipment when they
find their way onto a circuit board. Inductively produced transients, which travel throughout building wiring, are commonly produced by air-conditioning
equipment cycling on and off, by refrigerators, and by any other equipment containing motors or other inductive elements.
It is plainly desirable to protect equipment from lightning surges and from inductive transients produced within a building, especially if the equipment is
very costly or is in an application which absolutely must remain operational. Protecting equipment with surge protection not only prevents catastrophic failures
but also improves the reliability of electronic equipment by preventing degradation and premature failure of integrated circuits.