Application Note

ManualsBrandsFluke ManualsMeasurement toolsFluke 481

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

The Selective Method

The Selective Method is a variation of the

Fall-of-Potential method, available on high-end

ground testers like the Fluke 1625. Testers with

this capability can measure the ground impedance

of a speciﬁc ground electrode without disconnect-

ing it from an array or from a structure’s distribu-

tion system. This means you don’t have to wait

for a shutdown to test or risk the safety hazards of

disconnecting the electrode from a live system.

The same rules for current stake and potential

stake placement apply as with Fall-of-Poten-

tial. If the conditions are met for the 62 % rule

(see previous page) then it can help reduce the

number of measurements. Otherwise it’s a good

idea to build a complete Fall-of-Potential plot. You

can use the Tagg Slope Technique if your curve

does not ﬂatten out.

Both the Fall-of-Potential method and the

Selective Method use stakes to inject current and

measure voltage drop. The big difference is that

selective testing can accurately measure the test

current in the electrode under test.

The utility neutral, building steel and ground

electrode are all bonded and grounded. When

you inject a current into this system of paral-

lel ground connections the current will divide.

In a traditional Fall-of-Potential test you have

no way of knowing how much current is ﬂow-

ing between any

particular electrode

and the C2 current

stake. Selective

testing uses an

integrated, high

sensitivity clamp-on

current transformer

to measure the

test current in the

electrode under

test. Figure 8 shows

how the current

transformer ﬁts into

the test circuit. The

selective ground

tester digitally

ﬁlters the current

measurement to

minimize the effects

of stray currents.

Being able to

accurately measure

the current in the

electrode under test

effectively isolates

the electrode and

allows us to test

it without discon-

necting it from the

system or from

other electrodes.

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Stakeless or Clamp-on Method

The “stakeless” or “clamp-on” method allows

you to measure the impedance of a series loop of

ground electrodes. The test is simple and it may be

performed on an electrode that is connected to a

working electric service.

To make the measurement the tester uses a

special transformer to generate a voltage on the

ground conductor at a unique test frequency. It

uses a second transformer to distinguish the test

frequency and measure the resulting current

through the circuit.

This method is available in some Fall-of-Potential

testers (like the Fluke 1625) or in a single clamp on

unit. Figure 9 shows the connection of the source

and measure clamps of the Fluke 1625.

Figure 10 (see next page) shows the equivalent

test circuit for the stakeless method. When you test

a building ground electrode using this method, you

are actually testing a loop including:

•

Electrode under test

•

Ground electrode conductor

•

The main bonding jumper

•

The service neutral

•

Utility neutral-to-ground bond

•

Utility ground conductors (between poles)

•

Utility pole grounds

Because this method uses the service as part of

the circuit, it may be used only after the service has

been completely wired, that is, it cannot be used

prior to hook-up to the utility. In this method the

clamp checks the continuity of the interconnections

of all of the components above. An abnormally high

reading or an open circuit indication on the instru-

ment points to a poor connection between two or

more of the aforementioned critical components.

Figure 7: Connections for selective ground electrode

measurement

Figure 8: Connections for selective Electrode Impedance Mesaurement

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