No lightning strike or Network issue action following a massive lightning storm this morning.

You'll never know  

It is possible to combine a CT, (over)current relay and pulse counter on the SPD earth connection* to count how many times it's operated; this is sometimes done on HV networks to plan maintenance/replacement ahead of failure. So you could do that if you felt the need to see whether it is doing anything. Whether this has actually successfully clamped the voltage at the device would need to be checked another way of course.

Someone might correct me but I'd suggest it's possible that the SPD operating swapped a voltage spike for a dip, because it's shorting to earth, which devices' power supplies may or may not ride through but at least won't be damaged by the spike. And/or the supply within your house was wobbly because the local network was wobbling with sections of the network dropping in and out for the same reason, automatic recloses etc. An SPD doesn't pretend to be a UPS.

*I'm sure actual products are cleverer than this, for example to avoid counting repeat strikes, but I think this is the working principle

Simple pulse counting of currents over some level is probably enough, I do not know of any domestic product for this but we can count pulse events on other systems. Actually for this you do not want a 'good' current transformer that will add significant inductance and may create embarrassing voltages on the counter, and the spikes are always short, at least relative to 50Hz so a small core is fine - if the peak currents lasted for a whole cycle not, then the SPD would soon be a melted mass.

IF I was asked to design something, then on a domestic SPD I'd put no more than a small ferrite core (no larger than a snug fit over the 10mm earth wire and 5 -10 turns of thin pick-up wire)  with anti-parallel diodes on the secondary to limit the voltage to +//- 0.6 or so, and op-amp to square the pulses. For those of a software bent it is then almost an Arduino type project to record counts of pulses each day or whatever.

Mike.

It's likely that an operating SPD  doesn't cause a dip. Due to the fast rise time of a typical spike (type 2 SPDs are tested with an 8uS rising edge), inductance dominates - especially on the leads (which is why we're told to keep them below 0.5m) So what is more likely to happen is that a 4kV spike gets reduced to a 1.5kV spike, rather than dipping below 230V.

It's likely that an operating SPD  doesn't cause a dip. Due to the fast rise time of a typical spike (type 2 SPDs are tested with an 8uS rising edge), inductance dominates - especially on the leads (which is why we're told to keep them below 0.5m) So what is more likely to happen is that a 4kV spike gets reduced to a 1.5kV spike, rather than dipping below 230V.