Application Note

ManualsBrandsFluke ManualsMeasurement toolsFluke 481

7 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.

This method requires a low-impedance path in

parallel with the electrode under test. The ground

electrode of most facilities is in parallel with numer-

ous utility ground electrodes. These electrodes can

be pole electrodes, pole butt plates or un-insulated

neutral conductors. The impedance of the utility

ground electrodes usually combines into a very

low impedance.

Let’s take an example. Say you have 40 pole

electrodes of roughly 20

Ω each, and these elec-

trodes are connected together by a low-impedance

ground wire from pole to pole. The equivalent

resistance of the 40 electrodes in parallel is:

Since half an ohm is small compared to the resis-

tance we expect for our electrode under test, we can

assume that most of the measured resistance is due

to the earth resistance of the electrode under test.

There are some potential pitfalls for this method:

• If you measure in the wrong place in the system,

you might get a hard-wired loop

resistance,

for example on a ground ring or on a bonded

lightning protection system. If you were intend-

ing to read earth resistance, measuring the

conductive loop would give unexpectedly low

resistance readings.

• You may get low readings due to the interaction

of two very close, bonded electrodes, like buried

conduit, water pipes, etc.

•

The quality of the measurement depends on

the availability of parallel paths. If a building

is solely supplied by a generator or transformer

that has only a single electrode, the assumption

of multiple paths won’t work and the measure-

ment will indicate the earth resistance of

both electrodes.

This method will not measure

earth resistance.

•

A problem with the utility grounding system

might interfere with readings.

40 x

20Ω

In general, if you get readings below 1 ohm,

double-check to make sure you are not measuring

a hard-wired conductive loop instead of the

earth resistance.

Two-pole Method

The two-pole method uses an “auxiliary electrode”

such as a water pipe. Figure 11 shows the connec-

tions. The tester measures the combined earth

resistance of

the electrode

under test, the

earth resistance

of the auxiliary

electrode, and

the resistance

of the measure-

ment leads.

The assump-

tion is that the

earth resistance

of the auxil-

iary electrode

is very low,

which would

probably be

true for metal

pipe with-

out plastic

segments or

insulated joints.

The effect of the measurement leads may be

removed by measuring with the leads shorted

together and subtracting this reading from the ﬁnal

measurement.

Although it’s convenient, be very careful using

the two-pole method:

•

A water pipe may have PVC components, which

could greatly increase its earth resistance. In

this case the two-point method would give an

excessively high reading.

•

The auxiliary electrode may not be outside the

inﬂuence of the electrode under test. In this case

the reading might be lower than

reality.

Because of the unknowns

involved in this technique, it is

recommended only when the

grounding system and auxiliary

electrode are well known.

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Figure 10: Test current paths in the stakeless method

Electrode

Under test

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