General Information Why Delta


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.

The DELTA silicon oxide varistor (SOV) arrestor offers a number of improvements over the 1935 design.

The basic design of the DELTA silicon oxide varistor (SOV) arrestor is a pair of metal electrodes separated by a silicon oxide compound. Under normal conditions, the silicon oxide 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 oxide, changing 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 silicon oxide and shutting off the conduction.

The reason the DELTA silicon oxide varistor (SOV) 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, but it also impedes the lightning discharge. Since the DELTA silicon oxide varistor (SOV) arrestor needs no resistor, it can conduct more current much faster.

The DELTA silicon oxide varistor (SOV) arrestor has no spark gap. Valve arrestors have a spark gap to prevent line leakage current and to determine the sparkover voltage. Since the DELTA silicon arrestor 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 DELTA silicon oxide varistor (SOV) arrestor 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 arrestor must work as well in both directions. The DELTA silicon arrestor has uniformly shaped electrodes, and conducts equally in both directions.

So, the main improvements which cause the DELTA silicon oxide varistor (SOV) arrestor 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 Delta 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. Silicon arrestors do not conduct continuously.

As MOV units age, their clamping voltage degrades toward a short circuit failure. Silicon arrestors do not change with age, or use. MOVs deteriorate as they age and/or conduct surges. Silicon arrestors do not deteriorate with age. When MOVs do fail, they tend to fail short circuit, therefore they need to have internal fuses to clear them from the circuit. Silicon arrestors rarely fail, but when they do, they fail "open circuit" to permit continued use of the power system.

© Mike Craddock 2011 - For permissions contact