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A Bulletin of Circuit Protection Technology


High voltage surges on building wiring.

The ideal electrical system in a building is pure sixty-hertz ac sine waves. In reality, they’re not always so pure.

External and internal influences can cause voltage transient “surges” or “spikes” as high as several thousand volts on 120V circuits. Unless those surges are provided with a harmless path to ground, they can damage the building system and utilization equipment connected to the system. In most severe cases, they can start fires. There are two main sources of power systems surges – lightning and switching.

Lightning is the biggest cause of surges from outside a building. We’re excluding “direct hit” lightning damage from this discussion. Lightning surges are the result of induced voltages on the power lines caused by lighting strikes in the vicinity of the lines. The surges ride the entrance conductors.

Switching is the major cause of surges from inside a building. The inductive “kick” of a transformer, solenoid, motor, or switching of an appliance are typical examples.

The electric clock has given us an insight into just how high these surges can be. Millions of clocks are exposed day and night, year in and year out to whatever surges are on the lines. A General Electric Company study of clock motor performance over a period of several years showed that electrical failures were practically eliminated when the withstand voltage level of the clock motor insulation was increased from 2000V to 6000V.

This provides evidence that many surges do exist over 2000V, and is an example of a manufactures’ action to improve product performance. Normally products would be expected to withstand high voltage surges of two times their rating plus 1000V, or approximately 1250V, for most home appliances. For mobile homes, ART. 650-10(a) requires a dielectric strength test of 900V for 1 minute or 1080V for 1 second. Protection from voltage surges can pay for itself in short order in appliance repairs alone.


General Electric