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Date: Thu, 28 Jul 2011 18:09:04 +0100
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re: Lightning Story
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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
> >
> >
>
>
>
>

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<div>A ground strike is one of the most damaging mechanisms as it affects t=
hings some distance from the strike.=A0=A0=A0 A strike to ground induces la=
rge electric fields radially from the point of the strike as the current fl=
ows outwards though high resistivity ground.=A0They can be so high that som=
eone standing with legs apart sideways to the direction of the strike can g=
et a nasty shock from the voltage travelling up one leg and back down the o=
ther, even at distances of many tens of metres from the strike.=A0=A0 Someo=
ne I know saw this happen on a football pitch.=A0 Several players had to be=
 taken to hospital.</div>

<div>=A0</div>
<div>The Bell Hill microwave=A0beacons <a href=3D"http://www.scrbg.org/">ww=
w.scrbg.org</a>=A0 are on an exposed hilltop site and to ensure they would =
be minimally susceptible to all but a direct strike - we put several measue=
s in place.=A0 All metal work, mast, equipment rack, chassis, mains armouri=
ng, were all connected very firmly together via several paralleled routes.=
=A0=A0 The idea being that in the event of any induced curents, everything =
would jump together and no voltages could be imposed across anything that m=
ay be susceptible.=A0=A0 All the beacons get their power at 12V DC from a s=
ingle master battery backed PSU.=A0=A0 </div>

<div>At a distance of a few metres from the equipment cabin=A0 a ring of ea=
rth spikes were bashed in, with each one brought back via buried copper con=
ductors, consisting of bare 2.5mm diameter wire.=A0=A0=A0 All these were bo=
nded to the equipment rack.</div>

<div>=A0</div>
<div>YET, in spite of all that we did once=A0lose the master PSU once from =
a lightning strike - the 12V 10 Amp SMPSU died.=A0=A0=A0 While we had compr=
ehensive 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 s=
o robust.=A0 What I assume happened is that a ground strike induced an awfu=
l lot of volts / amps in the armouring of the cable.=A0=A0 At the supply en=
d (the bottom of the hill) =A0the armouring connects to ground, as does, pr=
esumably,the neutral supply at that point.=A0 (It is derived from a pole mo=
unted transformer also supplying a house, and surely isn&#39;t left floatin=
g - that would be dangerous).=A0=A0 At the top beacon end, live and neutral=
 go straight to the SMPSU, the armouring goes to our mass of metal.</div>

<div>=A0</div>
<div>NOW.....=A0 <br>Assume a lot of induced voltage in armouring over its =
1km length.=A0 A robust strong ground connection=A0for the=A0armouring at t=
op end, but live and neutral floating wrt. earth.=A0=A0 At the=A0bottom end=
, the neutral supply is almost certain to be connected to a ground spike, a=
nd more than likely so is the armourinf connected to the same point.=A0=A0 =
Now, assume this spike is not a particularly good ground, sufficient for tr=
ipping RCDs, but not much fault current.=A0 Neutral and armouring are now c=
onnected at the bottom end and not very well grounded.=A0=A0=A0 </div>

<div>So, the=A0induced voltage imposed on the outer armouring is sunsequent=
ly imposed on the neutral conductor, and =A0flows back up inside the cable =
using live/neutral as a transmission line.=A0=A0 The transient hits the SMP=
SU input rectifier and blows it along with one filtering capacitor to groun=
d - I can&#39;t remember if it was on live or neutral side, whichever, the =
other survived but was scrapped anyway</div>

<div>=A0</div>
<div>The PSU was easily repaired, just a new bridge rectifier needed and ne=
w filtering caps.=A0=A0 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.=A0=A0 =
Also installed a substantial mains filter.=A0=A0 </div>

<div>Touch wood, we&#39;ve had no problems since then - although there don&=
#39;t seem to have been so many bad thunderstorms in recent years.</div>
<div>=A0</div>
<div>Of course, had the cabin been supplied with the much-maligned (in am. =
rad circles !) PME system this situation would not have occurred.=A0=A0 PME=
 must, by definition, be earthed at several points along its route, and wou=
ld most defiitely be so at the top and bottome ends.=A0 Neutral and earth w=
ould be the same, and the multiple eathing would bleed off induced currents=
 in the outside of the cable.=A0=A0 There would be no wrap-round mechanism =
to induce a large live/neutral spike to damage equipment.</div>

<div>=A0</div>
<div>Andy</div>
<div><a href=3D"http://www.g4jnt.com/">www.g4jnt.com</a></div>
<div>=A0</div>
<div>=A0</div>
<div>=A0</div>
<div>=A0</div>
<div class=3D"gmail_quote">On 28 July 2011 17:03, Mike Dennison <span dir=
=3D"ltr">&lt;<a href=3D"mailto:mike.dennison@ntlworld.com">mike.dennison@nt=
lworld.com</a>&gt;</span> wrote:<br>
<blockquote style=3D"BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex=
; PADDING-LEFT: 1ex" class=3D"gmail_quote">Some years ago we had lightning =
strike a nearby lamp-post. All of the<br>video recorders in the street were=
 destroyed - except mine! I assumed<br>
that my earthed antenna formed a Faraday screen protecting my<br>property.<=
br><br>Mike, G3XDV<br>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D<br><br>&gt; &gt;On =
the bright side, TL084s are quite reasonably priced fuses, so<br>&gt; &gt;p=
erhaps an IC socket would be the best investment ;-)<br>
&gt;<br>&gt; Heh! =A0Both the original part and its replacement were pulled=
 from an<br>&gt; old pcb and cost nothing. =A0Next time I&#39;ll use someth=
ing else or<br>&gt; perhaps no preamp at all as dj1zb recommends. =A0I had =
these loops for<br>
&gt; years on LF without a preamp but found the VLF SAQ signal a bit too<br=
>&gt; weak here to feed directly into a soundcard. =A0Somehow I don&#39;t f=
ancy<br>&gt; the idea of putting protection against lightning strikes in th=
e shack.<br>
&gt;<br>&gt; Bill<br>&gt;<br>&gt;<br><br><br><br></blockquote></div><br>

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