High-Voltage Lab

By Clarence Klassen 

Even though most technicians and Electrical Engineers work with extra-low voltage most of the time and occasionally with low voltage to 460V or 600V, some routinely work with hundreds of thousands or even millions of volts.

I recently visited such a High-Voltage Lab. This is used for various insulation tests for electrical components. One critical test is to simulate a lightning strike. The test is needed to certify equipment for its Basic Insulation Rating (BIL) rating. These tests may be up to 100KV for windings used in 15KV motors. Millions of volts may be required to certify distribution equipment.

A critical specification is the rise time for the voltage spike. This is measured in nanoseconds and is affected by the impedance (inductance and capacitance) of the sample being tested and the wires running to it.

We have all seen pictures or movies of spark gaps, the shiny balls used to produce arcs. It turns out that the current initiated through a spark is very controllable and has a very fast rise time and no practical upper voltage limit. Spark gaps also look spectacular.

I had to ask how millions of volts are produced. Quite simple in principle. Many capacitors are charged while connected in parallel at a modest voltage (2 to 20 times lower than the required voltage). Then the charging conductor is removed and the capacitors are discharged in series using fast switches. It turns out all the spark gaps trigger at the same time. The light from the first gap to discharge fires all the others.

And we thought high voltage was only useful for bringing monsters to life.