[sdiy] Isolation transformer question..

[J. Larry Hendry]

Steve,  This was most likely caused by multiple location earth ground
connections.  I have seen this many times.  Since the communications
equipment has the failure, I suggest that the communications equipment
probably has an "unauthorized ground" installed somewhere.  The Ma Bell was
famous for this years ago.  They would connect their grounds to anything
that looked remotely appropriate.  Cold water pipes were their favorites
back in the days of mostly metal pipe.  Electrical codes spell out that a
residence must have one and only one ground connection.  That does allow
multiple grounds provided they are all bonded together with sufficient
conductor to maintain the same potential at each.  That ground rod is
supposed to be near your electrical service entrance and tied to the utility
neutral at the meter base or main panel (varies).

But, the point is that lightning dissipation creates large step potentials
across the ground surrounding the strike. Multiple, unbonded ground
connections in the path of dissipation through the ground can develop
thousands of volts difference between ground # 1 and # 2 (Good old Ohm's law
stuff)  At the places where these grounds meet (communication and electrical
ground meet at the computer communications interface), these potentials
equalize through anything not designed to withstand that potential
difference (the solid state devices on your communications card).

This is the most common cause of lightning damage in my opinion.  And, it's
the most preventable.
Larry Hendry




----- Original Message -----
From: Steve Begin <Steve.Begin at pwgsc.gc.ca>
To: 'J. Larry Hendry' <jlarryh at iquest.net>
Sent: Tuesday, July 23, 2002 8:05 AM
Subject: RE: [sdiy] Isolation transformer question..


This is not _quite_ related but a few months ago lightning struck near my
girlfriends house and it ruined their network hub and 3 ethernet cards (and
a printer?) but the computers seemed to survive the ordeal fortunately.  I
just wanted to mention that, since it hadn't occured to me to protect
against power surges coming from my internet connection.

> Steve Begin


-----Original Message-----
From: J. Larry Hendry [mailto:jlarryh at iquest.net]
Sent: Sunday, July 21, 2002 10:15 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Isolation transformer question..


Amen.  The ONLY perfect lightning protection is disconnection.  I'll take it
Harry's 2 pole switch is for 120 only.  I have a 3 pole switch on my studio
feed since I have a 240 volt circuit (common 3 phase device about $35).
But, the common point here is don't forget to switch off the neutral.  In my
opinion, more lightning damage is caused by step potential induced neutral
voltages that go high in reference to actual ground than spikes that
actually come in the hot line.  That transformer that feeds your house
should have a surge arrestor on the primary that clamps at about 110% and
itself a fairly decent filter to high frequencies.  So, hot leg only
switches are useless IMO.  Of course, the isolation transformer makes a huge
difference.  But, neutral disconnection is often overlooked.

Larry (who could go on forever about bad residential grounding techniques
that allow most lightning damage).

----- Original Message -----
From: harry <harrybissell at prodigy.net>
Get a huge double pole toggle switch (mine is a 30A / 120VAC) and switch
both sides of the incomming line to off when you leave the studio... it
should protect the gear from most everything (when off) including small
lightning strikes.
H^) harry