HTA medium-voltage and HTB high-voltage surge arresters

There are generally two types of surge protection devices for AC power networks: spark gaps and lightning arresters.

The spark gap is still used on electricity distribution and transmission networks, wherever the insulation of the equipment to be protected is compatible with the protection provided by spark gaps, i.e. when the insulation is in air or when it is not economically attractive to seek very low insulation levels: high-voltage HTA or medium-voltage (20 kV) equipment and certain high-voltage HVB (63 or 90 kV) equipment such as, for example, measuring reducers.

On the other hand, surge arresters are needed to reduce equipment insulation, enhance protection against overvoltages or improve service quality. These devices offer better protection characteristics, albeit at a higher cost. There are two main types of surge arresters:

• one, whose active part consists of silicon carbide (SiC) spark gaps and varistors, is referred to in the following as a silicon carbide spark gap arrester;

• the other, whose active part consists solely of zinc oxide (ZnO) varistors, is known as a zinc oxide surge arrester.

The latter, which first appeared in the early 1970s, became very attractive, first in high voltage, then fairly rapidly in medium voltage. With very few exceptions, today's surge arrester market is made up exclusively of zinc oxide arresters.

These surge protectors are more compact, simpler to design, offer superior technical features and open up new applications. Their cost is now very competitive. Last but not least, technological advances in the field of external insulation, combined with new prospects for the use of ZnO varistors, led to the development of a new generation of arresters in the early 1980s, designed to exploit the advantages of ZnO technology to the full. These are zinc oxide arresters with synthetic envelopes.