--------snipped for brevity------------


Firstly where is this earth on the Telephone line ????? - It does not
exist.

In normal circumstances the phone line ground is at the exchange.
However, in the case of a lightning strike, the nearest earth is the
ground surrounding the cable. In any case, if lightning were to hit
the ground, then why would it not dissipate itself in the general mass
of earth rather than travelling all the way into the house and onto
its local mains earth? Granted, the earth is not homogenous, so a
lower potential _could_ exist at the dwelling, but it would seem more
likely that an earth strike would not propagate too far away from the
epicentre.

Firstly A single earth point is usually a relatively poor earth often
providing up to tens of Ohms Resistance to 'earth' when Lightning hits say
an earth stake that earth point typically rises to 50,000V or more above
nominal Earth as a result.
There have been many cases where a Lightning strike to the ground has blown
through the insulation of a cable and travelled through it to a better
ground however this usually also causes physical damage to the phone cable.

Secondly the Phone line is not earthed back at the exchange it is in fact
floating above ground potential - It needs to be as it is a Balanced
Circuit.

Thirdly the DC resistance alone of a phone line is fairly high typically
being in the order of a hundred or more Ohms especially when compared to the
mains.

In other words any earth on a typical Phone line is nowhere near as
effective as the earth provided by the MEN AC distribution System.


some (in Australia very few) phone lines have surge arrestors but these
are
not generally implemented as they have proven to be ineffective in the
case
of near/Direct strikes.

going through your list :

Firstly, my friend's neighbour lost his electrical distribution box,
so this confirms that the strike came in on the mains.

Well Maybe and I have not said it never comes in via Mains just that if
it
does it is so uncommon that I have not seen it.

I have seen it twice at the same dwelling, with similar results. On
the second occasion the datapump of an internal 33.6K ISA Rockwell
modem was damaged, but the only other problem with the entire PC was a
faulty IRQ3 signal on the ISA bus. That's two PCs, neither of which
sustained any damage other than a faulty modem DSP/DAA.

So the damage was to the modem both times! this was likely to have come via
AC mains because ?

Reading through the list of damage however suggests that your friend
suffered a direct strike in which case :

A) all bets are off
B) a Lightning strike that has blown the ****e out of a switchboard is
barely going to flinch at blowing the ****e out of a MOV
C) Equally likely and the more probable scenario is that your friends
house
was hit and the Lightning followed the path to the best Earth available -
that of the MEN Mains

Failure of PCtel DAA module (plug-in type for PCChips M585LMR
motherboard). Symptom was "NO DIALTONE". Parallel telephone was
undamaged. Both phone cables were plugged into a "protected" power
board.

Plug in module connected to the phone line - still matches the scenario
that
the strike came in via Phone and exited via Mains
The parallel phone most likely rode out the strike. In fact this sounds
like
a module in a PC if the strike came in via the Mains and exited via the
phone line why wasnt the PSU fried ?

Clearly the strike struck the mains at some point, so one would
intuitively expect that everything connected to the mains would have
suffered significant damage, as you say. The fact that the PSU
survived suggests that the major part of the potential difference
appeared across the DAA of the modem (???).

most likely it came in via the floating phone line and headed for the MEN
earth - it was possibley even sufficent to cause localised EPR (Earth
Potential Rise) however given the relatively low impedance House wiring
(against the Higher impedance phone line for eg ) and the relatively high
impedance path from the phone line to the Mains earth most of the voltage
for the strike would have appeared between the phone line and the mains
earth.

the more likely scenario is that the
strike came in via the phone line and headed for the chassis earth on the
PC.

So we have two strikes, one on the mains and another on the phone
line?

Nope one strike most likely direct to the house/Aerial/Phone line and a
****e load of current heading for the MEN earth inducing all sorts of
voltages in and around the house wiring.

(2) Portable telephone base station - shorted 15V zener diode at tip
and ring inputs.

Powered phone connected to the phone line - still matches the scenario
that
the strike came in via Phone and exited via Mains

(3) Television - shorted rectifier diode, blown fuse, and failures in
the +5V regulator, remote sensor, and microcontroller.

(4) Two VCRs - CPU and power supply failures.

Nasty sounds like a direct strike which also entered via the TV aerial
and
headed for the earth provided via the double insulated mains - note no
phone
line involved here

So now we have three strikes, one on the mains, a second on the phone
line, and a third on the aerial?

Nope still only one direct Strike and a bunch of induced current in various
bits of metal a single Lightning strike can and often does induce fairly
high currents/voltages through nearby wiring especially in a Direct (or even
near) Strike. One main strike though.
bear in mind that w_tom s 'whole house protection' would most likely not
have saved anything in this strike either.


BTW, no tuners or IF stages were damaged.

(5) Audio unit - CPU failure, P/S OK.

Dont know what this was but if double insulated equipment connected to
the
VCR/TV then Lightning was probably still travelling in via the TV aerial
and
had not dissipated completely via the VCR or TV Again no phone line
connected here

No aerial, no phone line, just mains power.

still have no idea what this is - speaker cables or signal cables ?


(6) Security light - faulty power controller, light dims but won't
turn off.

Sounds like a direct strike The Light being installed outdoors probably
copped a secondary strike not the full Strike - you would have found
that
one of w_toms surge busters would not have saved this anyway. Also no
phone
line involved here

(7) TV - blown fuse, damage to P/S

entered via the TV aerial headed for the AC mains again no phone line
involved here

As before, no tuners or IF stages were damaged.

Double insulated telly though most likely so that the highest impedance part
of the path to earth (and hence the higher voltage across components) would
have been through the PSU. The TV most likely even had some surge supression
for thhe Aerial which would have excacerbated this failure.


Well sounds like your Lightning still mostly obeyed the laws of Physics
and
entered by unearthed / poorly earthed paths and headed straight for the
best
earth available that of the AC Mains. as an addendum the mains in this
instance probably rode out the strike to a certain degree but in all the
cases a path for the lightning to have entered by other than AC mains
exsists.
Lightning aint Rocket science it obeys the laws of physics - it is just
with
high energy events such as lightning different laws of physics often come
in
to play.
Lightning seeks a path to the best earth it can find and note here that a
single earth stake buried in the ground is more often than not hardly the
best earth available the MEN system however used for 240/415V mains
distribution however frequently offers a superior earth for Lightning
strikes

I notice that the one thing that distinguishes a PC from most, if not
all the other equipment is the fact that it has an earth conductor.
What if this relatively small earth conductor allows the PC to float
to the strike potential? Wouldn't the full potential then appear
across the modem's DAA rather than the main part of the PC?

The earth conducter to a PC is going to still going to have a resistance of
a fraction of an ohm against over 1000's of ohms to earth via the Phone line



Regards
Richard Freeman

I wonder if there is some forensic or metallurgical method of
determining the direction of current flow in a lightning strike.

why ?
just think about it compare the two earths -

MEN AC Mains earth = fraction of an Ohm to Earth - Directly connected to
earth stakes at not one but Several Places with a minimum wire size of 2.5mm
square (O.k with typically 2 m of 1.5mm square cable for the IEC power
lead).

Phone Line = floating above Earth potential with a Maximum wire size of
0.28mm Square not connected to an earth anywhere.

Which sounds like the better earth to you ?
If you where Lightning where would you head ?
It is not rocket science it is just fairly basic electrical theory - very
simple physics. Dont be mislead by the wilfully ignorant especially those
who seem to have a vested interest in selling 'whole house protection'.

Regards
Richard Freeman