Re: LF: Re: Lightning Story

[Andy Talbot <andy.g4jnt@gmail.com>]

A ground strike is one of the most damaging mechanisms as it affects things some distance from the strike.    A strike to ground induces large electric fields radially from the point of the strike as the current flows outwards though high resistivity ground. They can be so high that someone standing with legs apart sideways to the direction of the strike can get a nasty shock from the voltage travelling up one leg and back down the other, even at distances of many tens of metres from the strike.   Someone I know saw this happen on a football pitch.  Several players had to be taken to hospital.

 

The Bell Hill microwave beacons www.scrbg.org  are on an exposed hilltop site and to ensure they would be minimally susceptible to all but a direct strike - we put several measues in place.  All metal work, mast, equipment rack, chassis, mains armouring, were all connected very firmly together via several paralleled routes.   The idea being that in the event of any induced curents, everything would jump together and no voltages could be imposed across anything that may be susceptible.   All the beacons get their power at 12V DC from a single master battery backed PSU.  

At a distance of a few metres from the equipment cabin  a ring of earth spikes were bashed in, with each one brought back via buried copper conductors, consisting of bare 2.5mm diameter wire.    All these were bonded to the equipment rack.

 

YET, in spite of all that we did once lose the master PSU once from a lightning strike - the 12V 10 Amp SMPSU died.    While we had comprehensive protection for all the RF and control equipment, the 1km long run of armoured cable supplying the mains from the bottom of the hill was not so robust.  What I assume happened is that a ground strike induced an awful lot of volts / amps in the armouring of the cable.   At the supply end (the bottom of the hill)  the armouring connects to ground, as does, presumably,the neutral supply at that point.  (It is derived from a pole mounted transformer also supplying a house, and surely isn't left floating - that would be dangerous).   At the top beacon end, live and neutral go straight to the SMPSU, the armouring goes to our mass of metal.

 

NOW..... 
Assume a lot of induced voltage in armouring over its 1km length.  A robust strong ground connection for the armouring at top end, but live and neutral floating wrt. earth.   At the bottom end, the neutral supply is almost certain to be connected to a ground spike, and more than likely so is the armourinf connected to the same point.   Now, assume this spike is not a particularly good ground, sufficient for tripping RCDs, but not much fault current.  Neutral and armouring are now connected at the bottom end and not very well grounded.   

So, the induced voltage imposed on the outer armouring is sunsequently imposed on the neutral conductor, and  flows back up inside the cable using live/neutral as a transmission line.   The transient hits the SMPSU input rectifier and blows it along with one filtering capacitor to ground - I can't remember if it was on live or neutral side, whichever, the other survived but was scrapped anyway

 

The PSU was easily repaired, just a new bridge rectifier needed and new filtering caps.   To guard against this happening again, I installed some transsorbs at the top end where the feeder enters the cabin, one each from live and neutral to ground and one across the L-N feed as well.   Also installed a substantial mains filter.  

Touch wood, we've had no problems since then - although there don't seem to have been so many bad thunderstorms in recent years.

 

Of course, had the cabin been supplied with the much-maligned (in am. rad circles !) PME system this situation would not have occurred.   PME must, by definition, be earthed at several points along its route, and would most defiitely be so at the top and bottome ends.  Neutral and earth would be the same, and the multiple eathing would bleed off induced currents in the outside of the cable.   There would be no wrap-round mechanism to induce a large live/neutral spike to damage equipment.

 

Andy

www.g4jnt.com

 

 

 

 

On 28 July 2011 17:03, Mike Dennison <mike.dennison@ntlworld.com> wrote:

Some years ago we had lightning strike a nearby lamp-post. All of the
video recorders in the street were destroyed - except mine! I assumed
that my earthed antenna formed a Faraday screen protecting my
property.

Mike, G3XDV
===========

> >On the bright side, TL084s are quite reasonably priced fuses, so
> >perhaps an IC socket would be the best investment ;-)
>
> Heh!  Both the original part and its replacement were pulled from an
> old pcb and cost nothing.  Next time I'll use something else or
> perhaps no preamp at all as dj1zb recommends.  I had these loops for
> years on LF without a preamp but found the VLF SAQ signal a bit too
> weak here to feed directly into a soundcard.  Somehow I don't fancy
> the idea of putting protection against lightning strikes in the shack.
>
> Bill
>
>