Before Mike Craddock invented the modern lightning arrestor in 1971, surge protective devices were comprised of a spark gap to establish a turn-on voltage, also called 'spark-over voltage,' and a series resistor to limit the amount of current that would flow through the device.
Craddock's invention improved surge arrestors by eliminating the spark-gap, and the series resistor. Eliminating the resistance enabled the device to conduct large currents rapidly enough to prevent damage to the electric equipment being protected.
Craddock's device was comprised of cylindrical metal electrodes separated by granulated silicon dioxide. At normal line voltage, the silicon dioxide is a nonconductor. However when a high energy surge appears on the electrodes, the silicon dioxide compound ionizes and becomes conductive allowing the abnormal charge to be conducted out of the circuit.
The Craddock invention was first developed to fill the need for protection of oil field motors and controls from damage due to lightning and the power surges caused by lightning. In the beginning, the Craddock surge protective device was call an Oilfield Secondary Surge Arrestor.
Craddock founded a company to manufacture his inventions: Delta Lightning Arrestors, and he called the units Delta Lightning Arrestors. Originally the arrestor was placed in the oil well motor control box. When lightning caused a surge to follow power lines to the oil well's electric pump motor and control, the Delta Arrestor would divert the surge to ground, protecting the equipment.
Over time the need for surge protection grew to homes, business, and industrial applications. Delta began to offer arrestors for 120-208/220 volt home and commercial applications, as well as 480 Volt three phase motors used on pumps and other machinery.
Craddock's Delta Lightning Arrestor protects electrical equipment from damage by removing the surge from the circuit before damage occurs. Electric circuits can tolerate a dangerously high voltage for a short time. Circuits can withstand a very high voltage if the time is very short. Consider the flame of a candle. It is several hundred degrees. However you can pass your finger through the flame without being harmed, because your finger is exposed to the heat for a short time. The Delta Arrestor will clamp the surge to the lowest possible voltage for a given surge current, and remove the surge from the circuit quickly enough to prevent damage. The Delta can do this because it has no added internal resistance.
Some schools of thought endeavor to make a device that starts conducting the surge at a low voltage level. Some even pronounce this initial conduction level as the protection value. It is easy to see that this is wrong. Consider a surge that occurs on a circuit such that the surge crest will go to 5000 volts. If the SPD starts working at 200 volts, but that the surge will crest at 5000 volts, it doesn't matter at what voltage the SPD started to conduct. The protection is determined by the maximum voltage that will happen verses how quickly that it will be removed. The maximum voltage that will happen for any given surge is called the 'clamping voltage,' or sometimes, 'discharge voltage.' The voltage at which the SPD starts is called the 'spark-over voltage', or sometimes 'protection voltage.' Some producers of low spark-over units also incorporate an internal resistance so that excessive surges will not be conducted thereby possibly damaging the surge protector!
As the number of companies in the surge business grew, the number of names of the devices also grew. A surge is a sudden temporary increase in voltage on an electrical circuit. Lightning is a huge surge that happens from a cloud to earth, or another cloud. When lightning strikes an overhead conductor or even near a conductor, a big surge is induced in the line. Switching loads on or off causes surges from cemf, counter electromotive force. Devices designed to handle the big surges, like lightning and such, were called Lightning Arrestors, and Surge Arrestors. Devices for smaller surges came to be called surge protectors and transient voltage surge suppressors. Lately the term SPD, Surge Protective Device, is becoming popular.
At Delta, if there is a surge so powerful that something has to go, we would rather the protector be sacrificed than the customers equipment. If the Delta Arrestor does go out, a pressure relief will open a one inch hole in the bottom of the enclosure. The breaker feeding the arrestor should have tripped, however do not touch the arrestor or the box. Have a qualified electrician replace the damaged unit as soon as possible.
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