Approximately. The meter is measuring the sampled leg against the
parallel resistance of the other two legs (if there is no other
connection). So if the reference parallel resistance is low then the
error for the measured leg is low. (but always positive)
Measuring Against
r1 r2||r3 ie r2*r3/(r2+r3)
r2 r1||r3 etc
r3 r1||r2
where || means parallel. In my ground field, none of the leg
resistances are the same, so there isn't a simplification of the
equations for me.
If a more accurate result is desired, then if you want to mess with the
algebra substitutions, the 3 simultaneous equations have three variables
so are solvable. I've searched for the handy web app to do these calcs,
but not found one.
As if this is not hard enough, the algebra is very messy with more than
3 legs. I had a 630 sq ft Ufer + service entrance connection to my
tower so think my measurement error for 6 radials is quite small. So if
your tower also has a similar connection, the error should be low when
that connection is made.
Note that if the full field is installed, connected, and the rod is
accessible below the #2 wire exothermic weld then a clamp meter
measurement rod by rod will be against the N other rods and there should
be little error.
Any ground to your shack - coax shield, rotator ground, ground lead,
surge protection, etc should be a fairly low ground resistance. Which
is one of the reasons thought needs to be given to the actual circuit
being measured, it is really easy to be measuring a copper loop. eg my
tower ground wire to the shack against the coax shields bonded to the
tower. Any resistance measurement less than 10 ohms is a reason to draw
out the circuit, IMO.
Grant KZ1W
On 10/5/2019 1:51 PM, John Simmons wrote:
Can I infer that a 3-legged tower with 3 ground wires, each with
multiple rods would have a total ground resistance of each wire's
resistance in parallel? I.E. 1/R wire1, etc?
-de John NI0K
Grant Saviers wrote on 10/2/2019 3:46 PM:
The loop you want to measure is the single rod through the earth to
multiple rods or a very low R ground such as a building Ufer.
Start with two rods connected with a wire, put the meter on that wire,
measure R. The assumption is that each rod has a resistance of R/2.
When you install the 10th rod the assumption is that the nine already
placed, connected together, and connected by a single wire to rod #10
have a much lower parallel resistance than the new rod. Of course
there is some residual error, but if you go back and measure them one
at a time, one against nine, you will get close approximate
resistances of each rod.?? Then these values can be computed as
parallel resistances for a close approximation of the net ground field
resistance.
This is why a star layout of rods and connecting wires that looks like
a vertical antenna radial field is easy to measure, one rod at a time.
OTOH, if you have a perimeter wire, the meter will measure the wire
resistance of that loop.
Plus such a layout is the way to minimize the paralleled inductance.
Grant KZ1W
On 10/2/2019 11:59 AM, Wes wrote:
Hmmm.. Every other reference I see about using one of these says that
a loop is required.
On 10/2/2019 11:00 AM, Bryan Fields wrote:
On 9/29/19 2:51 PM, Julio Peralta via TowerTalk wrote:
Does anyone have any experience using a clamp on ground resistance
meter?
Yes, they work well.
I bought one to use to help with my grounding project but don't
have any
experience using this type of meter. Any advice would be appreciated.
Ensure it's calibrated every time you use it.?? You should ensure
it's not
measuring a loop, that will give errors.?? Single point or off a
master ground
bar feed is the right place to measure it.
I have this model:https://www.amazon.com/gp/product/B01N7FSF6N
Was a steal for 300 dollars.?? I've used it at my home and numerous
sites to
ensure the building ground is sufficient.
73's
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