A significant improvement of arresters to alleviate problems of utilisation was
achieved by the introduction of active gaps (also called current-limiting gaps) [2].
These are characterised by the presence of blast coils that produce a strong magnetic
field during the passage of arrester follow current. The arcs across the individual gaps
are then elongated and blown towards the edge of the chamber, thus producing an
increased voltage drop during the follow current period. The power frequency voltage
across the series non-linear resistors is therefore reduced with the consequent reduction
of the follow current. The arc voltage opposes the follow current and interrupts
it before the working voltage reaches zero. A further component of the active design
is the grading system which ensures that the voltage is distributed uniformly between
the series spark gaps