The optimum series connection system cannot be used under all system conditions.
Nominal currents carried via the double terminals on the SPD as part of the series
wiring are limited by the thermal load ability of the double terminals. For this reason,
the manufacturer of the SPD prescribes a certain maximum permissible value of the
backup fuse, which, in turn, means that series wiring can sometimes not be used for
systems with higher nominal operating currents.