  Eruptive events of solar activity often trigger abrupt variations of the
geomagnetic field. Through the induction of electric currents, human
infrastructures are also affected, namely the equipment of electric power
transmission networks. It was shown in past studies that the rate of power-grid
anomalies may increase after an exposure to strong geomagnetically induced
currents. We search for a rapid response of devices in the Czech electric
distribution grid to disturbed days of high geomagnetic activity. Such
disturbed days are described either by the cumulative storm-time $Dst$ or
$d(\textit{SYM-H})/dt$ low-latitude indices mainly influenced by ring current
variations, by the cumulative $AE$ high-latitude index measuring
substorm-related auroral current variations, or by the cumulative $ap$
mid-latitude index measuring both ring and auroral current variations. We use
superposed epoch analysis to identify possible increases of anomaly rates
during and after such disturbed days. We show that in the case of abundant
series of anomalies on power lines, the anomaly rate increases significantly
immediately (within 1 day) after the onset of geomagnetic storms. In the case
of transformers, the increase of the anomaly rate is generally delayed by 2--3
days. We also find that transformers and some electric substations seem to be
sensitive to a prolonged exposure to substorms, with a delayed increase of
anomalies. Overall, we show that in the 5-day period following the commencement
of geomagnetic activity there is an approximately 5--10\% increase in the
recorded anomalies in the Czech power grid and thus this fraction of anomalies
is probably related to an exposure to GICs.
