The responses re the use of multiple ground stakes daisy-chained back to
a single point on the house are fine.  If high current flows, it will
only be between the ground stakes, not between pieces of your
equipment.  As another bit of information, on USA systems, there is a
ground from the neutral on the transformer at the power pole.  So there
could be large voltages and currents in the neutral leg and thereby
superimposed on the AC line.  The protection here is the circuit breaker
panel at the entrance to the house and the ground rod at the house.

I have not looked at the current NEC codes, but the use of multiple
ground stakes in a daisy-chain makes sense.  A big problem with ground
stakes is keeping their resistance low.  Techniques have included
pouring salt solutions such as copper sulphate (bad for the
environment), sodium or calcium chloride (still not much better for the
environment in the concentrations needed) in the ground with them or
even making special rods which weep salt solutions.  The use of the
plumbing as a ground was a two edged sword.  The connection was good
because it typically has many feet buried in the ground, but the
galvanic corrosion can eat up the pipes and if the connection was bad
and there was a ground fault, the plumbing goes =93hot=94 with possible
consequences of electrocution.  Like so many things, at first glance it
seems simple, but there can be many ramifications as you dig into it.

One way to kind of look at it is the Faraday cage.  If the ground system
you make is a grid or cage like thing, then even it the ground rod goes
up in voltage, you are enclosed in a zero volt environment.  This falls
to pieces when a wire extends through that cage to the outside world.
Then you bring in whatever is outside.  Hence my discussion of carbon
blocks or gas protector on sensor lines to your remote seismometer
equipment.  Complete isolation is becoming more feasible today.  There
are low current op-amps and serial output, high accuracy A/D=92s that can=

be easily connected to fiber optic transmission links.  So the power
requirements on the sensor side can be made quite low and supplied by a
fiber optic link.  Combine that with a fiber optic link information
return, and your sensor combination can be made immune to millions of
volts -- pretty safe, even from lightning strikes.
Charles R. Patton


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