17th edition design given 18th edition certifcate

Digressing slightly from the topic, but are any studies/monitoring exercises being carried out as to the effectiveness of spds in the domestic environment? Otherwise, how will we know how good they are ( or rather I suspect, not)?

 If they truly work, then where is the hard evidence? Surely any manufacturer worth his salt would be only too glad to provide such evidence in order that he sells more of the same, or is it the IET doing the selling now by mandating the things?

I don't think it unreasonable to ask how effective something is if I am being forced to request that a customer pays up for it.

whjohnson: 
 

Digressing slightly from the topic, but are any studies/monitoring exercises being carried out as to the effectiveness of spds in the domestic environment? Otherwise, how will we know how good they are ( or rather I suspect, not)?

 If they truly work, then where is the hard evidence? Surely any manufacturer worth his salt would be only too glad to provide such evidence in order that he sells more of the same, or is it the IET doing the selling now by mandating the things?

I don't think it unreasonable to ask how effective something is if I am being forced to request that a customer pays up for it.

There are standards for them, which describe their operation.

In terms of effectiveness … well, people have said “products should contain these”.

However, we now classify EMC and surge performance of equipment, and, to turn the argument on its head, the manufacturer of electronic products for use in European installations now has a right to assume the installation has some “performance” in this respect, whereas that was never the case (back in the day when we didn't really care because we had relatively few “modern” electronic products).

whjohnson: 
If they truly work, then where is the hard evidence?

I guess that it is easy to know if your electronics get fried - it didn't work, but that may have been because the surge exceeded the device's capabilities, or a previous surge went unnoticed.

If an SPD works, the tell-tale will show that it has done so (and the cartridge needs to be replaced).

The problem is that if there has been no surge, you just don't know.

There are standards for them, which describe their operation.

Only tested as a component in isolation of course - knowing that the voltage at the SPD is clamped to a kV or so tells you surprisingly little about what voltage will appear across the terminals of a socket even just a few metres away in any typical  installation that includes all sorts of  open ended branches, propagation delays and random changes of transmission line impedance at every joint. The sure fire way to protect an expensive appliance is to clamp the voltage at the point that appliance is connected or inside it.

Mike.

Or be really old school and unplug everything before going to bed, going out or when there’s the slightest hint of a thunderstorm and actually turning the main switch off when you go on holiday.

I am still somewhat unhappy with the SPD thinking, because the test specification is far too vague, and assumes a number of parameters that are in reality undefined. The place for protection is ideally next to or within the device to be protected. The fundamental problem is that people keep saying “clamped to some voltage”. This is fundamentally untrue unless the SPD has zero impedance when operating, and also there are no “transmission line” effects mentioned by Mike above. If the pulse under consideration strikes a short circuit (the SPD) in RF terms it is reflected not removed, and thus may damage something else!

As real VDRs have a significant series resistance when conducting, the remaining pulse amplitude depends on the original pulse energy, some of which is dissipated and some reflected. A pulse still continues on the original circuit, although the amplitude may be reduced. There are so many unknowns in this description that what actually happens is undefined, although in the ultimate case is probably some improvement in damage levels. To work better it is necessary that the pulse source impedance is defined, and for in-product protection, this is usually some series inductance in the supply lead. The VDR can then be chosen to have a suitable clamping voltage and current (operating series resistance) to reduce the pulse of some width to a known potential, even if the supply impedance is zero, and even less if it is significant.

From the above one can see that the standard SPDs in BS7671 may help in some circumstances, but appliances also have specifications for resistance (sic) to spike damage, although cheap stuff usually ignores these. It is not worth spending £1 to protect a led light bulb whose manufacturing cost is 50p.

There are ways to protect or remove spikes at a known voltage but these also have some problems, related to the possible pulse energy to be clamped. Again controlling the supply impedance is essential, and excess pulse energy may cause thermal failure. They are also much more expensive than the simple VDR (voltage dependant resistor) types, using power electronics.