On Wed, 10 Sep 2003 11:12:46 +1000, "Richard Freeman"
put finger to keyboard and composed:
"Franc Zabkar" wrote in message
.. .
On Mon, 8 Sep 2003 17:28:57 +1000, "Richard Freeman"
put finger to keyboard and composed:
Damn just when I thought w_tom had grasped the concept. Once again w_tom
goes on with his fantasy that Lightning comes in via AC mains and exits
via
the phone Line.
I was skeptical, too, until I saw it for myself. See this post:
http://groups.google.com/groups?selm...goblue.net. a
u&output=gplain
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.
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.
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 (???).
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?
(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?
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.
(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.
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?
Regards
Richard Freeman
I wonder if there is some forensic or metallurgical method of
determining the direction of current flow in a lightning strike.
- Franc Zabkar
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