Application Note

2 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.
should be outside of each other’s “areas of influ-
ence” to be most effective (see Figure 2). The rule
of thumb is to separate the elements by more than
their length. For example: 8-foot rods should be
spaced more than 8 feet apart to be most effective.
The NEC specifies 25 ohms as an acceptable limit
for electrode impedance. The IEEE Standard 142
Recommended Practice for Grounding of Industrial
and Commercial Power Systems (“Green Book”)
suggests a resistance between the main ground-
ing electrode and earth of 1 to 5 ohms for large
commercial or industrial systems.
Local authorities including the authority having
jurisdiction (AHJ) and plant managers are respon-
sible for determining acceptable limits for ground
electrode impedance.
Note: Power distribution systems deliver alternating
current and ground testers use alternating current
for testing. So, you’d think we would talk about
impedance, not resistance. However, at power line
frequencies, the resistive component of the earth
impedance is usually much bigger than the reactive
component, so you will see the terms impedance and
resistance used almost interchangeably.
How do ground impedance
testers work?
There are two types of ground impedance testers.
Three and four point ground testers and clamp-on
ground testers. Both types apply a voltage on the
electrode and measure the resulting current.
A three or four-pole ground tester combines
a current source and voltage measurement in a
“lunch box” or multimeter-style package. They use
multiple stakes and/or clamps.
Ground testers have the follwing characteristics:
•
AC test current. Earth does not conduct dc
very well.
•
Test frequency that is close to, but distinguish-
able from the power frequency and its harmon-
ics. This prevents stray currents from interferring
with ground impedance measurements.
•
Separate source and measure leads to compen-
sate for the long leads used in this measurement.
•
Input filtering designed to pick up its own signal
and screen out all others.
Clamp-on ground testers resemble a large clamp
meter. But they are very different because clamp-
on ground testers have both a source transformer
and a measurement transformer. The source
transformer imposes a voltage on the loop under
test and the measurement transformer measures
the resulting current. The clamp-on ground tester
uses advanced filtering to recognize its own signal
and screen out all others.
Ground Testing Safety
Always use insulated gloves, eye protection and
other appropriate personal protective equipment
when making connections. It is not safe to assume
that a ground electrode has zero voltage or zero
amps, for reasons given below.
To perform a basic ground test (called Fall-of-
Potential) on an electrode, the electrode must be
disconnected from the building. New selective
methods allow accurate testing with the electrode
still connected. See “Selective Measurements.”
A ground fault in the system might cause signifi-
cant current to flow through the ground conductor.
You should use a clamp meter to check for current
before performing any impedance testing. If you
measure above 1 amp you should investigate the
source of the current before proceeding.
If you must disconnect an electrode from an
electrical system, try to do so during a maintenance
shutdown when you can de-energize the system.
Otherwise, consider temporarily connecting a
backup electrode to the electrical system during
your test.
Never disconnect a ground electrode if there is a
chance of lightning.
A ground fault in the vicinity can cause voltage
rises in the earth. The source of the ground fault
may not even be in the facility you are testing, but
could cause voltage between the test electrodes.
This can be especially dangerous near utility
substations or transmission lines where significant
ground currents can occur. (Testing grounding
systems of transmission towers or substations
requires the use of special “Live Earth” procedures
and is not covered in this app note.)
Ground impedance testers use much higher
energy than your standard multimeter. They can
output up to 250 mA. Make sure everyone in the
area of the test is aware of this and warn them not
to touch the probes with the instrument activated.
Figure 2: Ground electrodes have “areas of influence” that
surround them