Conventional valve type arrestors are based on a 1935 patent. The basic design is a spark gap for the lightning to arc over, and a series resistor which is intended to limit the flow of follow current from the line. For many years this was the best design on the market, but now there is something better.
New Silicon Compound designs offer a number of improvements over the 1935 design. The new silicon design is a pair of metal electrodes separated by a very high tech special silicon compound. Under normal conditions, the silicon compound is a good insulator so that no line current flows between the electrodes to ground. When an excessively high voltage occurs on the electrodes, the high energy electrical field ionizes the silicon compound. This changes it from a compound to silicon separate from oxygen. Since the silicon ion is a good conductor, the high energy current is conducted to ground. When the voltage falls toward normal, the silicon and oxygen recombine, forming the silicon compound and shutting off the conduction.
The Advantages of Silicon
The reason the silicon compound design works so much more effectively is that there is no series resistor to limit the flow of current. In valve arrestors, the resistor is there to limit the flow of the line current. However, it also impedes the lightning discharge. Since the silicon compound needs no resistor, it can conduct more current much faster.
The silicon compound has no spark gap. Valve arrestors have a spark gap to prevent line leakage current and to determine the spark-over voltage. Since the silicon compound conducts at the ionization potential, no spark gap is needed. This is more effective because spark gaps cause high-intensity voltage spikes which act as surges and damage electrical equipment. The silicon compound works equally well in both directions. Valve arrestors do not. Valve arrestors customarily have a pointed electrode on one side of the spark gap and a flat electrode on the other. Therefore, they conduct less in one direction. Since lightning is positive 90% of the time and negative 10% of the time, an arrester must work as well in both directions. The silicon arrester has uniformly shaped electrodes, and conducts equally in both directions.
So, the main improvements which cause the silicon compound to be more effective are:
- Elimination of the current limiting resistor to speed conduction.
- Elimination of the spark gap to avoid voltage spikes.
- Use of bi-directional electrodes to conduct positive and negative lightning and surges. MOV devices are much like silicon arrestors EXCEPT that the metal electrodes are separated by zinc oxide rather than silicon.
The functional differences include:
• MOV units draw current from the circuit continuously. This is an energy drain, and causes the MOV to gradually age. Our silicon compound does not conduct continuously.
• As MOV units age, their clamping voltage degrades toward a short circuit failure. Our silicon compound does not change with age, or use.
• MOVs deteriorate as they age and/or conduct surges. Our silicon compound does not deteriorate with age.
• When MOVs do fail, they tend to short circuit. Therefore, they need to have internal fuses to clear them from the circuit. Silicon arrestors never fail, but if they somehow did, they would fail “open circuit” to permit uninterrupted use of the power system.