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[Weaver]

The house is earthed by PME.

As for a sub-one-ohm site, I wonder  about using the stream immediately below the house. What about burying a large copper plate in the stream, or in the damp soil in the bank by the stream?

Below the house there is an aquifer into which we with the help of my neighbour Niall have drilled a deep borehole. Water comes down from Beinn nan Càrn and we exploit the limestone layer deep below the house which by the way emerges on the surface on the far side of the public road to the north east of the cattle grid (the one on the public road, not the one on our drive). I don’t know anything about it but I wonder if it would be effective to just drop a serious conductor down the borehole?

[burakkucat]

As for a sub-one-ohm site, I wonder  about using the stream immediately below the house. What about burying a large copper plate in the stream, or in the damp soil in the bank by the stream?

Hmm . . . I'm not qualified in the field but "tingles in the whiskers" are hinting that would not be appropriate.

One other thought: I wonder if it would be possible to engage the services of the relevant division of the BT Group to install, at the Weaving Shed, identical protective devices as that installed within an exchange building?

Perhaps licquorice, 4candles or Black Sheep may have an idea as to what would be required?

[sevenlayermuddle]

I know I said I’d forgotten most of this stuff, but if a common-mode spike appears on the phone lines, a surge protection impedance of many hundreds of ohms might be beneficial compared to being presented to a completely ungrounded modem.

[Weaver]

Sorry 7LM, could you clarify? I have a Theoretical Physics degree so I am semi-literate in things electrical. (The "theoretical" does not help here, 

[sevenlayermuddle]

I’m not trying to portray myself as an expert in this and in fact I never have been.  But my take is this....

When a nearby lightning strike causes a nasty spike to appear in the phone lines feeding my modem, I reason it will be common-mode, ie both conductors will see the same spike with same polarity.  Since my modem appears to be ungrounded, that spike will simply pass through the modem (possibly destroying it) before heading out into the ethernet cables, repeating the process at my router etc, trying to seek a path to ground.

My possibly flawed understanding is that whilst lightning is a complex energy phenomenon, at the manifestation point of contact above, it behaves as a current source.   A fixed current will flow in passing the spike to ground, regardless of the resistance to ground.  In order to satisfy Ohm’s law, the greater the resistance, the greater the voltage across the path.  If the current can find no convenient metallic path, even if it is a relatively weak current, it will create a very high voltage in the form of a spark, and that may be what causes damage.

I’m not convinced these currents are necessarily very high (we’re not talking of a direct strike), so
I would assume that a path to ground comprising a few hundred ohms would still be preferable, compared to the ‘fresh air’ between my modem input and ground.

Repeat again I am no expert.   Would be fascinated to be corrected on above thinking.

[sevenlayermuddle]

Interesting discussion of phone wire lightning problems here...

https://www.britishtelephones.com/lightng.htm

It’s quite an old article and he describes NTE5A gas discharge tube 11B as a ‘waste of time’ in the context of lightning protection, suggesting it be replaced in problematic areas with tube 21A, which is 3 pole and can be earthed.

I found this tech spec for said 21A...

https://assets.mayflex.com/downloads/EXCEL/E0206-S-Exc-21A.pdf

Unfortunately I vaguely recall (confirm/deny, anybody?) that even the ‘waste of time’ 11B is no longer fitted as it impeded broadband signals.   If true then I would imagine same would apply to 21A, though my hunch is that the stated capacitance (max) of 5pF wouldn’t be massively detrimental.

[aesmith]

What do they have at the exchange end?

[burakkucat]

What do they have at the exchange end?

That's what I hope either licquorice, 4candles or Black Sheep may be able to explain.

[sevenlayermuddle]

What do they have at the exchange end?

My hunch would be that to protect a telephone exchange, a bottomless budget would be spent on the maximum possible lightning protection, with exotic earthing arrangements starting with the fabric of the building itself.   I’m not sure it would be economically viable, or even possible, to replicate that at subscriber end hence alternative, compromised solution,  might be more appropriate.

That's what I hope either licquorice, 4candles or Black Sheep may be able to explain.

Indeed, such advice would be welcomed.

[Weaver]

I emailed AA and asked if BT could be employed to fit lightning protection; not surprisingly the sales manager replied that it isn’t something BT will sell. BT just fixes it when it gets wrecked again and again.

There’s nothing stopping me spending a fortune of my money on protection at a point on my side of the border, beyond BT-Land.

[burakkucat]

I emailed AA and asked if BT could be employed to fit lightning protection; not surprisingly the sales manager replied that it isn’t something BT will sell.

[grumpy mode]
A&A are not the BT Group. What an employee of A&A decides to state is just her/his opinion.
[/grumpy mode]

[benji09]

  You don't need a lightning STRIKE to knock out your equipment. A near miss will do. I would think any earth is worth trying. If you place a wire , or better, a rod down a bore hole may give an earth, provided that the water at the bottom is not pure. But I would have thought lime in the water may solved the conduction problem perhaps.
  One place my firm installed some equipment at, had a radio station on the site.  Lightning struck the mast, and this we thought knocked our system off. But after a time it was found that the computer type equipment was perfectly safe, but a number of mains operated VDUs were knocked out. When the VDUs were checked out, they were found to have the interfaces blown up. Seems that mains earth and station earth were not bonded sufficiently well to protect against the problem that occurred. Incidentally, all the landline speech band modems were all totally unaffected............

[Alex Atkin UK]

Our chimney got hit years ago and it took out the answerphone, modem, blew a hole in the IO chip on the motherboard the modem was plugged into (yes it was back in dialup days and I shared dialup across the LAN) and blew the diode of every network card (10BASE2 coax).  It also blew the neon in the immersion heater switch.

Interestingly, I was able to reuse those network cards when I moved to a hub as the RJ45 ports were fine, as was all the PCs other the router PC where the spike came in.

[Weaver]

I agree with Benji09 that you don’t need a lightning strike to cause damage, a near miss will do. And don’t forget GPR/EPR, which cooks your hardware when there is no strike, because there hasn’t been a strike.

[sevenlayermuddle]

Worth mentioning that even if some devices appear to survive an event that damaged connected devices, the surviving devices may still be degraded from factory spec.   They may be more vulnerable to the next storm, and/or their performance may be impacted.

Verging off topic a little...  A few years ago I had the privilege of witnessing a direct strike on a small passenger aircraft on its landing approach at a regional airport.   Really amazing, the zig-zag from the clouds touched the nose while a zig-zag from the tail continued to ground.  I momentarily wondered if I was about to witness some awful catastrophe but no, the plane just continued its path and made a normal landing.   The plane was immune simply because the lightning currents found no resistance within the metal bodied aircraft.   Being presented with no resistance, the lightning simply passed the length of the plane, causing no damage.