Electrical installation testers: CAT IV or CAT III?

This article helps to better understand dangers for the users of electrical installation testers caused by overvoltages and high current breakdowns. Protective measures against these phenomena are explained. Classification of the overvoltage protection level according to the international standard IEC/EN 61010-1 is explained and described. Dangers for the electrician Many dangers can appear on electrical installations. The most known are: Electric shock caused by a direct contact with mains voltage. Electric shock caused by leakage current if touching a conductive part under voltage. Fire hazard and burnings because of overheating. This document describes two less known, but also big dangers: Transient overvoltages (fast transients and surges). High (current) energy breakdowns. Both, breakdowns and overvoltages are more likely to occur: On installations that are delivering more power (usually near the origin). On outside conductors that are more susceptible for all kinds of electromagnetic disturbances.

How transients and surges occur?

Fast (impulsive) transients We all know how dangerous a stroke of lightning can be for electrical equipment. Transients caused by lightning are a normal phenomenon on outside conductors. Very high voltage transients can occur – even up to several 10,000 volts! Note that most lightning transients are not actually the result of direct lightning strikes. They often induce onto conductors as lightning strikes near the power line. The large electric field generated during a discharge can couple even in underground conductors. This type of transients is definitely the most dangerous one. A powerful lightning stroke can provide a peak current of 85 kA and can last several hundreds milliseconds. Surges (oscillatory transients) High overvoltages (surges) occur because of fast current changes/interruptions in the power system. In general they have lower amplitude, but are more frequent than fast transients. Common surge sources are: Fast current changes in inductive devices (switching on/off motors, transformers, large loads) during normal power system operation. In domestic installations surges can be generated while switching on/off lights. The surges spread through the installation wiring and result in flickering of TV signals, radio signals, etc. Uncontrolled switching on/off of the power system (failures, reaction of protective devices).

1