"Sal M. Onella" wrote in
:


"Jimmie D" wrote in message
...



One way to determine the low frequency effectiveness of a ground is
to connect the hot side of the power mains to the ground through a
fuse and
see
how big of a fuse you can blow. There are special devices built for
doing this but it can easily and safely be done with less than
professional equipment. Just get all the pets and kids out of the way
first.

Well, yeah! :-)

Well, dangerous and imprecise.


But there is a standard test for ground conductivity. Some pros told
me how they do it.

Drive three ground rods, A, B and C along a line. Pass a known
current between A and C. Measure the voltage drop between A and B.
Ohm's Law will give you the resistance at that place. I don't

You are measuring the resistance of the A electrode in this case. A could
be a ground system (ie more than one electrode).

There are some rules to follow wrt spacing of B and C from A. Worth a
read of how it is done before making dodgy measurements.

Commercial instruments using this principle may use an internal AC
generator somewhere around 1kHz with a synchronous detector, and chosen
to be in between the harmonics of 50Hz and 60Hz, so they can make
measurements that are less disturbed than measurements that are
susceptible to 50Hz or 60Hz ground currents from other things. Yes, you
can make measurements of a ground electrode connected to an working AC
installation... so long as the electrode voltage due to the 50Hz or 60Hz
current is too great.

personally know what a "good" number is for that test -- however,
http://mrtmag.com/techspeak/radio_we...ed_principles/ suggests 10
ohms. These guys http://www.rainmaster.com/pdf/TB025C.pdf say the NEC
wants no more than 25 ohms.

In this part of the world, the relevant standard requires that the entire
lightning protection earth system has a resistance of less than 10 ohms
before equipotential bonding to other services.

Owen