Follow current (power)

Two definitions can be found:

• The current from the connected power source that flows through an arrester during and following the passage of discharge current.
• Current supplied by the electrical power system and flowing through the SPD after a discharge current impulse and significantly different from the continuous operating current (Ic).

Comment: The IEEE definition was first developed for arresters using a series combination of gap and silicon carbide varistor, where significant power frequency current flows after the surge current. Some of the durability (endurance) test concepts with varying phase angles for the surge application were derived from this situation, because the angle of application of the surge affects the duration of the follow current. In SPDs based on metal oxide varistors, the follow current is negligible (it is the standby current at the temperature resulting from the surge) unless the surge was large and presents a risk of launching a thermal runaway. The proposed IEC definition (note that is applies only to the current a er the discharge, in contrast with the IEEE definition which applies during and after the discharge) seems more appropriate to MOV based SPDs.

Source: UIE Guide to Quality of Electriical Supply for Industrial Installations, Part VI: Transient and Temporary Overvoltages and Currents