Most these days have physical indicator window - showing red or green. You can pay extra for ones with auxiliary contacts too, if you wish.

  - Andy.

Sorry, I clearly hadn't read your post properly!

I suppose compared to the potential life-saving RCD a failed SPD generally only poses a risk to property. So if we can live with a broken RCD for up to (say) six months (if usual guidance is followed) I reckon in most situation a broken SPD could pose tolerable risks for a similar time.  If you want something more, then as you say, there are ones with switching contacts available so it's a simple matter to rig one up to a remote visual or audible alarm - which  of course could be positioned in a much more noticeable location than the CU. Some means of temporarily silencing the alarm might be a good idea though - otherwise it could end up like so many smoke alarms and be tampered with to remove the nuisance by those that don't have the ability to properly fix the underlying problem.

   - Andy.

That is my point.

We see plenty of "pre-populated" boards now with SPDs adjacent to the main switch, but I doubt that many customers know what they are for and will never inspect the indicators. So once their protective capacity has been used up, they may as well not be there.

There is a nice little video here for those who cannot work it out for themselves.

So far I have seen one SPD that has operated, a Fusebox SPD in a domestic installation showing a red flag.

SPDs are often (usually, always?) installed with an MCB, as SPDs tend to fail short circuit.  So if the alarm goes off, turn off the MCB and the problem goes away.

not always, with or without MCB (Wylex for example) sometimes the flag drop and alarm contact is purely mechanical, more or less a one-time fuse link that when broken by the SPD failing low resistance pops the fuse, and in turn releases a spring that operates the alarm switch. It then rather depends what circuit the alarm is on,

Mike.

mapj1 said:

It then rather depends what circuit the alarm is on,

Quite so! I have been contemplating how to connect my alarm lamp. Clearly, the circuit must be protected. So I thought connect to the lighting circuit where the DB is situated. If the lighting there has tripped, then clearly action must be taken. If it has not, then the alarm will flash. Without doubt, the alarm circuit cannot be the SPD circuit.

A further thought ...

Is there any means of counting the number of surges which have been suppressed?

The reason why I think of this is that presumably, SPDs can cope with a certain amount of surges (of different intensity) and say, a 95% confidence interval estimated.

If an SPD has one surge remaining, it doesn't help if a thunderstorm causes two surges. So if the SPD is reaching the end of its life, it should be replaced before it gets there.

Just a thought whilst I was drilling holes to fix my SPD box ...

Sort of, but it is not very practical for the home user.

Much as you can test a fuse element with a low current or a very short duration high current and measure its resistance, you can do something similar for a an SPD and test it without reducing its life.

If when new the I/V curve is swept rapidly, so as to reach the breakover voltage - but not to linger there so there is very little dissipation, then if that measurement is re-swept periodically, the degradation of the device with successive 'real' pulses can be tracked as the breakover voltage rises, as the available junction area is reduced by repeated damage and at the same time the leakage current  at voltage below breakdown also rises.

more on that here www.mdpi.com/.../4018

However, it is not a simple matter of pulse counts and stopping when one zap from failure - after all right from day one, it is always one zap from failure if the zap is large enough !

The question is how much reserve do you want over some specific test level....

Data for a real device shows a simple relation where apart from really large pulese - the one to ten pulse cases,  the no. of pulses that constitute 'life' are set by almost the square of the current and the integrated duration - almost our old 'constant energy' friend I2t actually, down to some minimum time, here about 20us, below which shortening the pulse duration does not extend the life by much more, as the current does not have time to spread laterally and be uniform in the crystal, and damage when hot spots form comes into play,

So, for this device, 100 short pulses of 7kA are comparable to 1000 pulses of ~ 2kA and so on. - and for anything much more than 20usec duration then there is an I2 T limit - which if we look at the 2kA line but now at 100us,  perhaps 2000*2000*100us* 100 pulses or about 40 thousand joules per ohm.

or take the 1million shots line at about 4 amps and 10 milliseconds - again about 40 thousand joules per ohm..

This constant let through idea is pretty a reasonable approximation for pulses in the 20us to 1msec range, and in the tens to thousands of amps - really small pulses (the infinity line) do nothing and are the sort of thing used to verify it has not been broken.

There have been brave attempts to make circuits that sniff off and integrate all the pulses to make a sort of SPD life clock based on this principle, but it is an inexact science.

The real trick in a normal design is to use an oversized one, relative to the largest surge expected, so you stay below the hundreds to thousands of shots line and expect a long and trouble free life. (not all designs get that last bit right.)

Mike

edited for clarity the morning after and to add the I2t example calcs.

Mike, thank you - as ever you bring lots of science to the discussion, but (on this occasion) I see what you mean.