Components and equipment for protection against surge are always based on a fundamental principle – to keep the isolation state up to acceptable voltage level. Shortcircuit happens after exceeding this level and thereby very high difference of potentials between conducting parts of one equipment or appliance is restricted to an acceptable value. Electronic switches used for this purpose are called surge arresters or devices for protection against surge. Nowadays open spark gaps, closed spark gaps, gas discharge tubes, varistors, limiting diodes or their combinations are used for protection against lightning and overvoltage.
The arresters class I are the applications of the SPD most oftenly used on principle of a spark gap. They are designed for 1st stage of surge protection. It is possible to divide them on so-called „open“ or „closed“ spark gaps according to constructional implementation. The shape of electrodes, their material and the distance between electrodes determine the protection level, discharge ability and features, which characterize behaviour of the spark gap when follow current extinguish. Open spark gaps excel in very high discharge abilities (up to Iimp = 50 kA (10/350) during high levels of self-extinguishing follow current (up to Ifi = 50 kArms). Their fundamental inadequacy is burning plasma bursting from SPD housing during their activation by lightning current. This fact significantly complicates projective preparation (switchboard construction) regarding fire safety. The construction of the closed spark gaps has this inadequacy solved, although at the expense of parameters of self-extinguishing follow current (Ifi = max 25 kArms) being decreased. Some constructions of the closed spark gaps have very high discharge abilities (Iimp > 100 kA (10/350), on the other hand the level of self-extinguishing follow current is low (Ifi = cca 100 Arms), so their application possibilities are comparable with gas discharge tubes.
Gas discharge tubes
In the quiescent condition gas filled arresters and spark gaps (gas discharge tubes) behave as high resistance isolators thanks to application of corundum ceramic. They are usually constructed in the shape of cylindric ceramic housing, closed by metal electrodes on both sides. They are filled by inert gas mixture under low pressure generally. They excel in their short response time and high-level discharge ability up to Iimp = 100 kA (10/350). They have small self-capacity (few of pF units) and high isolation resistance (> 1000 MΩ). Gas discharge tube´s application posibilities are restricted by their generally low values of self-extinguishing follow current (Ifi = c. 100 Arms). Quality guarantee for a particular application is carried out with by specific choice of used material, gas filling and electrode geometry. The special dilatation composition is used for a production of modern gas discharge tubes, because they ensure their high resistance against high temperatures up to 2000°C and extreme pressure during discharge in gas, during currents up to 100kA in the waveform of 10/350μs. The electrical parameters can be predicted in a large range. E.g. direct switching voltage can be set up from 100V to 2000V with typical tolerance +/-20%. Gas discharge tubes have long lifetime and parameter stability. So they fulfil basic presumptions for their usage at the constructions of maintenance-free arresterswith long lifetime.
Varistors are voltage dependent resistances with symetrical voltampere characteristic. They consist of 90% ZnO as a ceramic basis and 10% additives. High possible load of this type of arresters during their loading by impulse discharge currents is achieved by application of its great mass varistor capacity for energy absorbtion. Almost universal possibilities of varistors are limited only in the field of high frequency, where relatively high capacity (few of nF units) has a negative effect.
Limiting diodes are basically Zenner diodes dimensioned for high peak current values and extremely short access time (a few ps units). These diodes are highly suitable for protection of sensitive electronic circuits thanks to their small size, short access time and low protection levels in data and telecommunication systems.