Megger VCLAMP User manual

connection/s

Megger DET4TC and DET4TCR with ICLAMP and VCLAMP
options

‘Clamp On’ electrode testing methods

To measure an individual electrode’s earth resistance
requires the disconnection of the electrode being
tested from the rest of the earth system and any
connection to a building’s earth wiring. This can also
involve downtime, or reduce the degree of
protection to the installation.

Now Megger has a solution to this problem in the
form of A.R.T., the Attached Rod Technique.

When the earth tester injects a test current into an
electrode which is still connected to the system,
current not only flows down the electrode under
test, but back into the building’s system, and down
any other electrodes connected in parallel.

Megger’s new DET4TC together with the optional
ICLAMP utilises ‘ART’ to allow the instrument to

The testing of earth systems has relied for many
years on the tried and tested “Fall of Potential” and
other related methods. These methods give reliable
results, but can be time consuming.

Megger has made an “ART” of testing

Just a little aside

The ICLAMP has the ability to measure very small
currents, so Megger used this ability to add yet
another benefit. The DET4TC is equipped with an
earth leakage range, allowing quick and easy

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Building earth

System leakage current

Ie

Ie Ie

Ie 4 leakage

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measure only the current being injected into the
electrode under test. The instrument uses this
current to calculate the resistance of the electrode
under test. No disconnection, no wasted time, no
downtime, no irritation, no scraped knuckles!

measurement of leakage current flowing into the
earth system. So if you do need to disconnect an
electrode for maintenance there are no nasty
surprises!

DET4TC set to A range

Using ICLAMP to measure electrode
leakage current

Megger Limited Archcliffe Road Dover CT17 9EN

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January 2007

Megger gets down to earth with “ART”

So how does ‘ART’ work?

The DET4TC will perform the traditional 3-pole
measurements like other earth testers. When in the
normal 3P mode the DET4TC injects a test current
at 128Hz (X to C terminals), so as not to clash with
the generation frequency and its harmonics. The
voltage measurement is then taken (X to P
terminals) at this frequency, therefore enabling the
instrument to ignore other currents flowing in the

Building
earth

I Total

Ie

e

I

Ie

Ground

Electrodes

The diagram above demonstrates the operation of ‘ART’

The ICLAMP has the ability to measure an electrode
down to 5% of the total test current generated by
the instrument. In other words the electrode
measured can have a resistance up to 20 times that

earth system. The rest is simply down to ohms law
to calculate the resistance as displayed.
The addition of the ICLAMP now enables only the
test current in the individual electrode to be
measured. As with the voltage measurement the
ICLAMP measures the current only at the 128Hz
the DET4TC generates, again allowing other
currents flowing to be ignored.

.

Ie Test > I Total

of the total system and still be measured. Values
higher than 20 times cannot be measured using
‘ART’, and so you will need to carry out a
traditional 3-pole measurement. See over leaf.

Current

20

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Don’t underestimate the power of “ART”

Even if the test current flowing through the
electrode under test is less than 5% of the total
current generated by the DET4TC you can still get
an idea of the electrode’s resistance. Should this
happen the instruments display will show the
“clamp low” indication as follows.

So if you take a reading of the complete system in
standard 3P mode you know that the electrode you
were trying to test is at least 20 times that. That is
usually enough information to make a decision. In
addition you can also measure the standing leakage
current in each individual electrode.

The DET4TC with ICLAMP is a flexible addition
to your testing ‘toolkit’.

Any

X Test

A

B

C

Let’s imagine we were trying to measure the
resistance of an earth path ‘A’ above.

Then using ICLAMP the low current symbol,
shown above, appeared on the display. In this
instance we cannot use ART.

You should still perform a standard 3-pole
measurement on the complete system.

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Parallel earth paths

If the reading is 4.5 ohms. Because we know that the
ICLAMP requires at least 1/20
we now know that resistance ‘A’ must be at least 90
ohms.

Reading 4.5 X 20 = 90 ohms

Enough information to know whether the earth
path A, be it an electrode, connection to a mesh or
whatever, needs attention.

3

Internal earth path

th

of the test current,

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The problem with Spheres of Influence

There is another factor to take into account, and
that is the sphere of influence around the earth

Consider the following diagram;

C P

Here, the spheres of influence are outside each
other, resulting in the equivalent circuit shown
underneath. ART should, subject to the 20:1 rule,
work fine.

electrode/s and the buildings earth paths, whether
it’s through water/gas pipes or metal framework.

X Test point

Note: If you are unclear as to the meaning of
“Sphere of Influence”, more information can be
found in the Megger publication “Getting down to
Earth”, part number 21500-072.

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Now look at the next diagram.

C

P

In this situation the electrode under test is very close
to the building. The result is the sphere of influence
of the electrode and that of the building are
overlapping. Effectively we now have “earth
coupling” occurring, hence the difference in
equivalent circuit.

X Test point

The overlap in the spheres of influence introduces
additional impedances that make resolving the
resistance of the electrode difficult using ART. The
result will cause either be the “clamp low” symbol
appearing or an unexpectedly high reading. If this
happens proceed with the traditional 3-pole method
with the electrode under test disconnected.

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We all like “ART” but sometimes it’s misunderstood

Like most modern and new forms of ART
sometimes it’s misunderstood, but hopefully we can
prevent that happening to you.

Current C

The error isn’t at first obvious, I want to measure
the earth resistance of the guy line with the clamp
around it. But the guy lines on this tower are all
shorted together. The current being measured by
ICLAMP is flowing not just to ground at the
anchor point, but back up the other guy lines and to

Lets look at the following diagram and see if we can
spot the error?

ground via the tower. This means the calculated
resistance will be incorrect for the anchor point.

Always consider where the test current is going to
flow, all measured test current should flow through
the required earth (soil) mass around the electrode
under test.

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Some resistance to ART

Finally, new ART forms can sometimes meet with
additional resistance. However with the DET4TC
we easily ignore such resistance.

Sometimes it can be difficult to get the instrument
close to where we need to make our ‘X’ connection.
The obvious solution is to use a long test lead, but
this is going to add additional resistance to our
reading. The solution is simple.

Select the 4P plus clamp range to use the ART
method with four poles.

Connect both the P1 and C1 terminals to the
electrode system under test.

Now the potential measurement is taken at the
electrode connection rather than the instrument X
terminal.

The result is the lead resistance is totally ignored.

C1 and P1 connections

Building earth

Building earth
connection/s

connection/s

ISystem

ISystem

C1 and P1 connections

I Total

I Total

Ie T est

Ie 1

Ie 1

Ie 2Ie 3

Ie 2Ie 3

Ie T est

Potential Probe (P) Current

Potential Probe (P) Current

Probe (C)

Probe (C)

Ground

Ground

Electrodes

Electrodes

Under

Under

Test (X)

Test (X)

The Best Application of “ART”

There are many applications where ART work
extremely well.

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These include:

• Field of earth / Earth Farms

• Pole mounted transformers

• Domestic TT (earth electrode)

• Single guy lines on towers

• Lightning protection electrodes

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systems

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“STAKELESS” earth electrode testing

Earth electrode testing can be a difficult and time­consuming business; this is why Megger is
committed to making life as easy as possible for you.
Our experience of real life situations led to the
introduction of ART, first seen with the DET3TC,
and now with the DET4TC we add the capability to
perform Stakeless measurements.

Adding a second clamp, the VCLAMP, the
DET4TC can be used to measure electrode
resistance in locations where driving stakes is not
practical.

Lets take a look at the test in practice:

Building earth

connection/s

Current is induced into the circuit by the ICLAMP
and then the resulting voltage is measured with the

To take a measurement simply select the dual clamp
range on the DET4TC, connect the ICLAMP and
the VCLAMP to the appropriate terminals. Then
clamp both of them around the electrode under test
(or the cable connected to the electrode) and press
the test button, simple. However there are a few
simple things to remember.

The first is that there must be at least 100mm gap
between the clamps. This is to ensure the field
around the clamps do not interfere with each other.
The result would be incorrect readings.

ICLAMP

Electrode

VCLAMP. From this the resistance of the compete
loop / current path can be measured.

VCLAMP

Ground

Under

Test

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The important thing to remember is that the
method measures the resistance of the total loop,
where the test current in flowing. So it is important
to understand the path the test current is taking.

25 Ohms

22 Ohms 19 Ohms

Equivalent Circuit

R Meas. = R test + 1 / (1/R1 + 1/R2 + 1/R3 + 1/R4)

The true resistance of the electrode being measured
is 45 Ohms, but as the other earth paths in parallel
total 5,6 Ohm this is measured in series, hence the
reading is 50.6 Ohms.

In this case, though, there are only four parallel
paths, the more parallel paths there are the more
accurate the reading becomes. With many parallel
earth paths, the reading will become almost identical
to the actual value of the earth electrode being
measured.

The equivalent circuit is shown below. It clearly
illustrates how the measurement is actually the
electrode under test in series with all the other earth
paths in parallel. This means the measurement will

always be pessimistic, never optimistic.

ICLAMP

R Meas.

= 50.6 Ohms

25 Ohms

VCLAMP

R tes tR1R2R3R4
45 Ohms

Since the measurement is a complete loop, another
useful feature of this measurement is that the
resistance of the cables and all connections are
measured. So any poor connections will be
highlighted too.

The readings taken are ideal for monitoring over
time the condition of installations, but without the
inconvenience associated with electrode
disconnection and the driving in of test stakes.

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T
his makes this method particularly suited to
measuring lighting protection systems on buildings.

The normal 2-pole method would be applied as follow:

Link removed for

2 pole measurement

Using the stakeless method negates the requirement
to remove these links, just clamp the ICLAMP and
VCLAMP around the tape or link. See below

Normally the time consuming practice of removing
test links would be required, and a 2-pole test
applied.

Lightning protection tape

Removable links
(Jug handles)

Lightning protection
tape

ICLAMP and
VCLAMP

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Removable
links
(Jug handles)

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Incorrect use of Stakeless testing

The stakeless method is not unfortunately, to coin a
phrase, miss-stakeless. As with the ART method,
test current must go through the earth mass to be

of lighting protection

As can be seen in the above diagram, none of the
test current flows through the earth mass. In fact the
measurement will be a short circuit.

Another example:

Testing a guy line on a telecoms tower.

As the guy line are all linked together both at the
tower metal work and at the anchor point, a loop is
formed, and no current flows through the earth

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able to measure the earth resistance. Used
incorrectly the method may give misleading results.

Lightning protection

mass. Hence the reading will not be the resistance of
the guy line to earth.

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The best application of “Stakeless” testing

There are many applications where Stakeless testing
will work extremely well.

With a good understanding ‘ART’ and ‘Stakeless’ testing will be invaluable tools, saving
both time and trouble.

Further information regarding Megger’s range of earth testers and their applications may be
found at www.megger.com/det

These include:

• Field of earth / Earth Farms

• Pole mounted transformer electrodes

• Pole mounted transformer guy line

when connected to earth system

• Earthing in Sub-station cable cellars

o It is often impossible to drive in

test stakes so this is an ideal
application for Stakeless
measurements.

• Single guy lines on towers

• Lightning protection electrodes

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Megger Limited Archcliffe Road Dover CT17 9EN

DET4TC_AP_EN_V01
January 2007