A chain of series-connected spark gap piles have two grading chains in parallel therewith with connections therebetween. An arrangement is provided for triggering one of the spark gap piles. When this pile is triggered, all the spark gap piles in the chain spark over if the voltage over the spark gap piles between two connections from a grading chain is not less than the spark-over voltage for each of these piles. One grading chain is formed of a number of resistors and capacitors in parallel connected across each end gap pile and each pair of other gap piles, and the other grading chain is formed of resistors in the chain between the pairs of gap piles and circuits formed of a capacitor and a resistor in parallel with each pair of gap piles.