Having just sat through the 3rd thunderstorm in as many days (this weather is getting silly now) I wondered if we really should still be regarding the whole of the UK as AQ1 (<= 25 thunderstorm days per year) - at this rate we'll have used up a years quota by the end of the month, never mind the rest of the "summer". Cue idle thoughts on installing Surge Protection Devices at the origin as a precaution.

As usual one idle thought lead to another and the memory of an incident form quite a few years ago. Dad's hearing had been declining he couldn't hear Mum calling him from the other end of the house - so Mum asked if I could rig things up so she could use the telephones to ring Dad in another room. I duly installed a little PBX by the master socket wiring each internal phone onto a separate line and the PC onto a spare on (dial-up internet in those days). They were out in the country with literally miles of overhead lines on both electricity and phone. After one thunderstorm when neighbour's telephone handset got fried, Dad got a bit jittery about his PC being connected to the phone line during a storm (he was always the type to try and unplug the aerial from the TV at the first rumble of thunder) so we put in one of those 4-ways the that had surge arrestors on both the mains and phone lines for the PC. That turned out to be a bad move - after the next storm the PBX got partially fried - the internal line that the PC was connected to went dead. Presumably the surge current had been drawn through the PBX to the PC's SPD and so to earth. Lesson learned and a SPD went in on the exchange line before the (replacement) PBX and the PC's phone line disconnected from its local SPD - and that seemed to prove satisfactory.

That got me wondering - could SPDs at the origin result in more damage to the installation if the surge originated within the installation rather than outside? The PV panels on the roof for example - or an overhead line to an outbuilding. When the SPDs operate and effectively tie the live conductors to earth, would the stress on components in between (the PV inverter for example) be increased? (as happened to Dad's PBX sat in between the surge and the SPD) Or is the mains so well tied to earth anyway that it makes little difference to the risk?

- Andy.

Cut off all the cpc's, works wonders :-)

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Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!

Maybe that's the effects of global warming. My copy of BS EN 62305 was issued in 2006 at 10-20 thunderstorm days per year in the UK; jump forward 6 years and you've got an inter-tropical convergence zone in South Yorkshire! (Though it looks from the diagram that Derbyshire's got it worse)

The complicated nature of the standard doesn't help; you need a PHD to understand it, especially when concerning LPS zoning, calcs and SPD coordination. Furse has a good guide here though. What ever happened to British simplicity? The Americans have a much more straightforward standard in NFPA 780 which basically tells you what to install where and doesn't give you much room to maneuver. For example the 'rolling sphere' is fixed at 46m and you have minimum 15m spacing between air terminals on a flat roof and 6m for those along the edge (roughly equivalent to a BS EN class III system.
I suspect that this can be overkill at times but by allowing for cost cutting like the BS EN series generates pages of calculation.

Regarding the SPDs a lot of the domestic units sold don't comply with any sort of standard and so you don't really know what protection they offer. The Masterplug types just come with a "surge warranty?" Then there is the array of types that are standardized: MOV, spark gap, type-1 common mode protection, or overvoltage diff-mode? I also read here that SPDs can destroy RCDs if installed downstream; guess I need to do some more bedtime reading.

Cheers, Tim

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Everyone loves a fireman - but hates the fire inspector.

Tim,
Very good document that: German but that is not far away so relevant.
This extract was from the above document about PV. (Lacking sensibility with respect to this topic)BS7671 will like that.


Necessity of lightning protection
For the installation of PV systems it must be generally
distinguished between an installation on a
building with or without lightning protection. For
public buildings, e.g., assembly places, schools,
hospitals, in Germany building regulations request
lightning protection systems for safety reasons. For
this purpose, buildings or structures are differentiated,
for which, according to their location, construction
type, or utilisation, a lightning strike
could easily have severe consequences. Such buildings
or structures in need of protection have to be
provided with a permanently effective lightning
protection system.
In case of privately used buildings lightning protection
is often refrained from. This happens partly
out of financial reasons, but also because of
lacking sensibility with respect to this topic.
If a building without external lightning protection
was selected as location for a PV system, the question
arises, if, with the additional installation of
the PV generator on the roof, lightning protection
should be provided for the entire structure.
According to the current scientific state of the art


In the case of a direct or
very near lightning strike into buildings with builtin
KNX, damage must be expected there

What is KNX systems in schools (Is that the data lines)

Regards
jcm

Dehn do a one day course on lightning protection and surge protection.
I'd presume it would be something like the Honeywell central heating course, where they are telling you their product is the best, while describing the regs and reasoning behind what they are selling. I think it would be worthwhile going to get a better grasp of these protections,
I was due to go in May, but they changed the date so I couldnt go, there are only 3 more this year, 2 in London, one in Newcastle, just a bit too far for me, but you are not far from London are you?
Alan.

KNX is a building automation system - http://en.wikipedia.org/wiki/KNX_%28standard%29. As you guessed, sensitive data lines & lots of electronics.
- Andy.

Just a thought - the last lightning affected installation I saw the suppliers meter had exploded and was in pieces on the floor - this suggests to me that to get full protection for a property any devices should be before the suppliers equipment, rather than being a BS7671 issue?

Thanks for the links Tim - interesting read!
- Andy.

Regarding strikes involving the BT lines, we have overhead telephone lines (yes it is rural).

Over the last 10 years lightning strikes to the BT network have taken out 4 devices on separate occasions: 3 modems, 1 router, and a network card.

I have installed a Furse ESPTN/BX with a 6mm to separate earth rod set so that the energy coming in the BT line can dissipate through that rather than through my IT equipment.

So far so good - but nothing is going to stop a close direct strike sequence on the line I fear (anyone else attend the DEHN presentation at an ELEX show? )

E.

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The meaning of life is a blank sheet; write on it wisely. ~ M.Cutler.