Megger DET2/2 Operating Manual
DET2/2
Digital Earth Tester
USER GUIDE
GUIDE DE L’UTILISATEUR
GEBRAUCHSANLEITUNG
GUÍA DEL USUARIO
M
SAFETY WARNINGS
• Special precautions are necessary when ‘live’ earths may be encountered, and isolation switches and fuses
are needed in this situation. See ‘Operation - Earth Testing Safety Precautions’.
• The earth spikes, test leads and their terminations must not be touched while the instrument is switched ‘On’.
• When working near high tension systems, rubber gloves and shoes should be worn.
• The DET2/2 must be disconnected from any external circuit while its battery cells are being charged.
• A 12 V d.c. battery must not be used as an external supply while it is still connected to the vehicle.
• Replacement fuses must be of the correct type and rating
• Before charging the DET2/2 battery ensure that the correct supply fuse is fitted and the voltage selector is
set correctly.
• Warnings and precautions must be read and understood before the instrument is used. They must be
observed during use.
THE INSTRUMENT MUST ONLY BE USED BY SUITABLY TRAINED AND COMPETENT PERSONS.
NOTE
2
Contents
Safety Warnings 2
Contents 3
General Description 4
Applications 5
Features and Controls 6
Initial Configuration 7
Setting up Test spikes 8
Earth Testing Safety Precautions 9
Operation
General Testing Procedure 11
Test condition adjustments 11
Display messages 12
Error messages 13
Battery charging 15
Measuring Techniques
Testing earth electrodes
Fall-of-Potential method 17
The 61,8% Rule 18
The Slope method 20
Method using ‘Dead’ earth 22
BS7671 (16th Edition IEE Wiring
Regulations) Requirements 23
Other methods 23
Determining ‘Touch’ potential 24
Determining ‘Step’ potential 25
Guide de l’utilisateur p43 Gebrauchsanleitung s67 Guía del usuario p91
Measuring soil resistivity -
Typical variations in soil resistivity 26
Line traverse 27
Calculation of Resistivity 27
Continuity Testing 29
Specification 30
Accessories 33
Chart for use with Slope method 35
Repair and Warranty 40
Guide de l’utilisateur 43
Gebrauchsanleitung 67
Guía del Usuario 90
Symbols used on the instrument
Caution: Refer to accompanying notes.
Equipment protected throughout by
Double Insulation (Class II)
Equipment complies with EU Directives
3
General Description
The Megger DET2/2 is a self contained compact
portable instrument designed to measure earth
electrode resistance and perform four terminal
continuity tests. It may also make earth resistance tests
which lead to the measurement of soil resistivity.
Powered by internal rechargeable battery with an
integral charger unit, the instrument design takes full
advantage of microprocessor technology and features
a large, clear liquid crystal display to provide digital
readings. Terminals on the instrument provide an
alternative power source connection to an external 12V
battery, e.g. motor vehicle battery.
Display language can be selected from English,
French, German, Portuguese or Spanish. A range of
frequencies can be selected. DET2/2 is auto ranging,
and will indicate earth resistance in the range - 0,010 Ω
to 19,99 kΩ, with a maximum resolution of 1 mΩ. The
display warns of problems with the test conditions and
also indicates low battery voltage. This enables the
earth spikes to be re-positioned or instrument settings
to be adjusted, to achieve optimum test conditions.
The red TEST push button is pressed to switch the
instrument on, and then turned clockwise to hold it in
the On position. To switch the instrument Off, the TEST
button is turned anti - clockwise and released.
To suit prevailing lighting conditions, the LCD display
4
can be adjusted by turning the contrast knob.
Four separate membrane switches (marked with ▲ or
▼) control the measurement function and are used to
set the required language and test settings.
Test leads are not supplied with an instrument but form
part of an earth testing field accessory kit which is
available as an option. This kit also includes test spikes
(electrodes) for making temporary earth spikes.
The instrument is housed in a robust and tough case
moulded in ABS plastic. All the controls, the terminals
and the LCD display are mounted on the front panel.
DET2/2 is splash proof, and suitable for outdoor use
in most weather conditions.
Terminal ‘C2’ (‘H’) is for the connection to the remote
Current test spike.
Terminal ‘P2’ (‘S’) is for the connection to the remote
Potential test spike.
Terminal ‘P1’ (‘ES’) is for the Potential connection to
the earth electrode to be tested.
Terminal ‘C1’ (‘E’) is for the Current connection to the
earth electrode to be tested.
Applications
The installation of satisfactory earthing systems is an
essential part of electricity supply, wiring safety and
installation economics. It is also of great importance in
many communications systems.
The primary application of the DET2/2 is in the testing
of earth electrodes, whether these take the form of a
single electrode, multiple electrodes, mesh systems,
earth plates or earth strips. All earthing arrangements
should be tested immediately after installation and at
periodic intervals thereafter.
Choice of electrode site
For an earth electrode system to perform satisfactorily
it must always have a low total resistance to earth. This
value will be influenced by the specific resistance of
the surrounding soil. This in turn depends on the
nature of the soil and its moisture content. Before
sinking an electrode or electrode system it is often
helpful to survey the surrounding area before choosing
the final position for the electrode. It is possible with
DET2/2 to obtain the resistivity of the soil over an area
and at different levels beneath the surface of the
ground. These resistivity surveys may show whether
any advantage is to be gained by driving electrodes to
a greater depth, rather than increasing the cost by
having to add further electrodes and associated cables,
in order to obtain a specified total earth system
resistance.
Earthing Systems Maintenance
After installation, checks may be made on an earthing
system to see if there is any significant change in the
resistance over a period of time or under different soil
moisture conditions, (e.g. brought about by changing
weather conditions or different seasons of the year).
Such checks will indicate if the earth electrode
resistance to earth has been exceeded by changing
soil conditions or ageing of the system.
Other Applications
For archaeological and geological purposes, an
investigation of soil structure and building remains can
be carried out at varying measured depths, by the
resistivity survey technique.
In all cases the accuracy of the instrument readings
may be taken to be higher than the changes caused by
natural variables in soil characteristics.
A further application is in continuity testing, for example
checking the resistance of conductors used in an
earthing circuit.
5
Features and Controls
To Current test spike
To Potential test spike
To electrode under test
(Potential connection)
To electrode under test
(Current connection)
31⁄2 digit L.C.D.
6
Charger socket
External 12 V d.c.
supply Terminals
Display contrast
adjustment
On / Off Test Button
(Rotate clockwise
to lock)
Test control
switches
Initial Configuration
Default Language Setting
Select and set the display language default as follows:
1. Press the left hand key ▲ and the TEST button
together. Rotate the TEST button clockwise to the
lock position. The language options are displayed.
2. Adjust the display contrast as necessary.
3. Using the centre ▲ key, scroll through the
language options. When the required language is
highlighted with a box surround, press the left
hand ▲ key. The test frequency options are
displayed.
Default Frequency Setting
Default test frequencies are available as follows:-
108 Hz - For use when testing with interference
frequencies in the vicinity of 16 Hz.
128 Hz - For use when testing with interference
frequencies in the vicinity of 50 Hz.
135 Hz -
150 Hz - For use when testing with interference
frequencies in the vicinity of 60 Hz.
For each default value, the test frequency range can be
incremented in 0,5 Hz steps from 105 Hz to 160 Hz;
using the ▲ ▼ keys.
Select and set the default Frequency as follows:
1. Using the centre ▲ key, scroll through the
Frequency options. When the required Frequency
is highlighted with a box surround, press the left
hand ▲ key. The Test and Calibration mode
options are displayed. The message “Please
wait...” is displayed.
Saving the Test Parameter settings
The settings made for Test current and filtering options,
and the Frequency of the Test current may be saved for
use in subsequent tests as follows:
1. After making the settings, press and hold the ▲
Scroll key during the measuring mode. The
display lists the default selection.
2. Accept the settings and press the ▲ Yes key, or
press the ▲ No key to cancel.
Once accepted, further tests may, if desired be carried
out with different settings. The instrument will default to
the saved settings if switched Off and back On again.
7
Setting up the Test spikes
For earth electrode testing and for earth resistivity
surveying, the instrument’s test leads are connected to
spikes inserted in the ground. The way the connections
are made depends on the type of test being undertaken
and details are given in ‘Measuring Techniques’.
Test spikes and long test leads are necessary for all
types of earth testing and the optional earth testing field
accessory kits contain the basic equipment. See
‘Accessories’.
1. Insert the Current test spike into the ground 30 to
50 metres away from the Earth electrode to to be
tested.
2. Connect this spike to the instrument terminal 'C2'
(‘H’).
3. Insert the Potential test spike into the ground
midway between the Current test spike and the
Earth electrode, and in direct line with them both.
4. Connect this spike to the instrument terminal 'P2'
(‘S’).
5. When running the test leads out to each remote
electrode, avoid laying the wires too close to each
other.
8
Earth Testing Safety Precautions
Electrode Isolation or Duplication
It is preferable that the earth electrode to be tested is
first isolated from the circuit it is protecting, so that only
the earth is measured and not the complete system.
When this is done, the circuits and equipment must be
de-energised. If however this is not possible, the earth
electrode should be duplicated, so that when it is
disconnected for test purposes, the other one provides
the necessary circuit protection.
‘Live’ earth safety precautions
The DET2/2 allows earth testing to be done at a
relatively safe voltage using a maximum of a 50 V RMS
square wave at a frequency of nominally 128 Hz. In use
it is normally connected only to electrodes which are at
earth potential.
A 'Live' earth is one that carries current from the mains
supply, or could do so under fault conditions.
When working around power stations or sub stations
there is a danger that large potential gradients will
occur across the ground in the event of a phase to earth
fault. A wire which is connected to ground many metres
away will then no longer be at the same potential as
local ground, and in some cases could rise to above
1 kV. The following safety precautions are essential.
1. All persons involved must be trained and
competent in isolation and safety procedures for
the system to be worked on. They must be
clearly instructed not to touch the earth
electrode; test spikes; test leads, or their
terminations if any 'Live' earths may be
encountered. It is recommended that persons
involved wear appropriate rubber gloves, rubber
soled shoes, and stand on rubber mats.
2. The 'P2' and 'C2' terminals should be connected
through a double pole isolation switch, the rating
of which will cope with the maximum fault
voltage and current. The isolation switch must
be open whilst any personal contact is made
with the remote test spikes, or the connecting
leads, e.g. when changing their position.
Fault
current
Voltage rise of
earthing system
under fault
conditions
Earth
resistance
A method of disconnection where fault conditions
may occur.
D.P. Isolation switch
Fuses
True earth
Remote
spikes
9
Earth Testing Safety Precautions
If isolation switches cannot be used, the leads should
be disconnected from the instrument before remote
spikes and leads are handled. When the remote
connections have been made, the final connections
should be made to the instrument using insulated
plugs, ensuring that the Operator takes adequate and
appropriate precautions such as insulating mats,
rubber gloves etc.
If a fault occurs while a test is being made the
instrument may be damaged. Incorporating fuses
(rated at 100 mA and able to cope with the maximum
fault voltage) at the isolation switch will provide some
protection for the instrument.
Caution: When working on live sites, do not use an
external battery to power the instrument, as this would
also become live under fault conditions.
10
Operation
It is advisable that the battery of the DET2/2 is fully
General T
charged before embarking on a test sequence. It can
be extremely inconvenient if the battery becomes too
low while a field test is in progress.
1. Firmly connect the instrument terminals to the
respective earth electrode and test spikes. See
‘Setting up the Test spikes‘ and ‘Measuring
Techniques‘.
2. Press and hold the On/Off push button, or rotate it to
the Lock position.
3. If required, carry out a Pspike test to check
continuity of the the Potential circuit.
4. The resistance value being measured is shown on
the sub display after a few moments, when the
“Please wait...” message has disappeared.
T
If the sub display message states that a true
measurement cannot be obtained, the test conditions
esting Procedure
est Condition Adjustments
can be altered to achieve optimum conditions for the
test. One or more of the following may be used:-
Test current Frequency
Using the right hand ▲ or ▼ keys, increase or decrease
the test current frequency range. See ‘Initial
Configuration and Spike set up‘.
Lo Current /Hi Current
Using the centre ▲ key, scroll through the left hand
options to select and highlight the ‘Current‘ option.
Press the left hand ▲ key to toggle between ‘Lo
Current‘ and ‘Hi Current‘. ‘Hi Current‘ assists to
overcome problems caused by high current spike
resistance. Note: Current circuit resistance is
constantly monitored during a test. If too high, a
message to this effect is displayed.
Filter
Using the centre ▲ key, scroll through the left hand
options to select and highlight the ‘Filter‘ option.
Press the left hand s key to toggle between ‘Filter off‘
and ‘Filter on‘. ‘Filter on‘ assists to reduce ‘noise‘
affecting the reading. The time taken to make a
measurement increases significantly with ‘Filter on‘.
PSpike
Using the centre ▲ key, scroll through the left hand
options to select and highlight the ‘Pspike‘ option.
Press the left hand ▲ key to automatically carry out a
11
Operation
resistance check of the of the Potential circuit. After a
short pause, the result of this check is displayed on the
sub panel. If appropriate, the ‘Pspike‘ label then
changes to ‘Retest’, giving the option to repeat the test
after any alteration to spike position etc. has been
made. Press the centre ▲ key, now labelled ‘Measure’
to repeat the measurement.
Note: If for any reason a test is made with an open
Potential circuit, the resultant test reading will be
invalid. To confirm that connections are still in place
and to check the validity of the test, a ‘P spike‘ check
should be made before each test.
Auto Ranging
If the earth resistance being measured is low, but a
high level of ’noise’ is present, coupled with a high
Current spike resistance, the instrument will
automatically make a measurement with a lower
precision. If successful, the resistance reading will be
displayed with only 3 digits, the least significant digit
being blanked out.
Greater precision can be obtained by:-
a) Reducing spike resistance (e.g. by wetting the
ground, or by inserting the spikes deeper into the
ground).
b) Toggling to ‘Hi Current‘ option.
c) Eliminating the ‘noise’ source if possible.
12
Displa
When appropriate, messages are displayed. The
following message definitions are given:
This means that the instrument is making internal
measurements and tests before displaying the
resistance reading. The ▲ and ▼ keys remain active
and measurement conditions may be adjusted before
a reading is displayed. These messages may be
repeatedly displayed if there is a high ‘noise’ level
present, close to the frequency of the measurement,
or if the Potential circuit is incorrectly connected.
“Open Circuit Current Terminals”
This means that the test current flowing is low, and
implies that a resistance of >500 kΩ is present between
the test terminals. If this message remains displayed
when terminals ‘C1‘ and ‘C2‘ are shorted together, an
internal fuse has ruptured, with the possibility of other
internal damage having been caused. In this case,
return the instrument, return the instrument to the
manufacturer or an approved repair company. See
’Repair and Warranty’.
“Check connections voltage terminals”
This message is displayed when the connections to the
‘P1‘ and ‘P2‘ connections are reversed. Check and
correct as necessary.
y Messages
“Please wait...”
“Please wait... zeroing”
“High current noise”
“High voltage noise”
These messages are displayed when the noise voltage
present is greater than the acceptable level, causing
the measurement to be invalid. Changing the test
frequency will have no effect in this instance. If
possible, eliminate the noise source, or reduce spike
resistance (e.g. by wetting the ground, or by inserting
the spikes deeper into the ground).
Further Display Messages
High level of interference or an instrument fault could
cause the display of any of the following messages:
“Invalid current”
“Invalid voltage”
“Invalid current zero”
“Invalid voltage zero”
“Current zero too big”
“Voltage zero too big”
“Noisy current zero”
“Noisy voltage zero”
Incorrect connection of the potential terminals could
cause an ‘Invalid voltage’ message.
Err
Error messages may appear on the bottom line of the
display in the event of a instrument or software fault, or
due to the existence of adverse electrical conditions. If
an error message appears, switch the DET2/2 off,
refer to ‘Repair and Warranty’ and return the
instrument to the manufacturer or approved agent,
giving details of the error message and the software
edition.
If calibration data stored in the instrument has been
incorrectly retrieved, the above message is displayed
(in English) when switching on. Switch the DET2/2
off, refer to ’Repair and Warranty’ and return the
instrument to the manufacturer or approved agent,
giving details of the error message and the software
edition.
Default language, frequency and current level are
normally retrieved when the instrument is switched on.
If unsuccessful, the above message is displayed (in
English) when switching on, with the option to “Retry”
(try reading the data again) or “Manual” (manually set
up the data again). If ‘Retry’ or ‘Manual’ is
or Messages
“Calibration data retrieval error
Refer to handbook”
“Setup data retrieval error”
13
Operation
unsuccessful, switch the DET2/2 off, refer to ’Repair
and Warranty’ and return the instrument to the
manufacturer or approved agent, giving details of the
error message and the software edition.
14
Battery Charging
Battery capacity
The capacity of the battery is continuously monitored
and displayed, adjacent to the battery symbol. The
indicator segments will show fully charged, or recede
as the battery is used, to indicate three quarters full,
half full or quarter full. A warning message is displayed
if the battery is unable to supply adequate test current.
Charging method
It is advisable that the battery is fully charged before
embarking on a test sequence. Charging is carried out
by external a.c. mains supply only. Charging
commences automatically as soon as the supply is
connected. Normal recharge time is 6 hours. Testing is
inhibited during charging.
Battery charging requires a supply voltage of 100 V to
130 V a.c., or
to a voltage from 130 V to 200 V will not cause harm,
but will not charge the battery, and the message
“Power Supply too low” will be displayed. Charging
time will be extended if either the power supply voltage
drops too low during the charge period or if the battery
has been excessively discharged. Charge the battery
as follows:
1. Switch the Test switch to Off.
2. Remove any connections to the 4 mm external
supply sockets.
200 V to 260 V, 50 - 60 Hz. Connection
3. Disconnect and remove the test leads.
4. Connect the mains supply to the IEC 320
connector on the top right of the instrument.
Confirm that the message “Charging On” is
displayed. Progressive and accumulated charging
times are displayed.
5. When fully charged, the charging current will
automatically reduce to ”Trickle Charge“.
Charging will automatically stop after a period of
24 hours.
Note: The battery will be prevented from charging if an
external battery is connected to the 4 mm sockets
during the charging process. An external connected
battery cannot be charged via the instrument.
15
Battery Charging
Battery Charging Power cord plug
If the power cord plug is not suitable for your type of
socket, do not use an adaptor. You should use a
suitable alternative power cord, or if necessary, change
the plug by cutting the disconnected cord and fitting a
suitable plug.
The colour code of the cord is:-
Earth (Ground) - Yellow/Green
Neutral - Blue
Phase(Line) - Brown
If using a fused plug, a 3 amp fuse to BS 1362 should
be fitted.
Note: A plug severed from the power cord should be
destroyed, as a plug with bare connections is
hazardous in a live socket outlet.
16
Battery Charging Notes
1) Do Not leave battery in a totally discharged state.
If the instrument is idle for long periods, recharge
the battery at least every 6 months. (More
frequently if the storage temperature is >40 °C).
2) Battery charging should be carried out in a dry
environment and at temperatures in the range 0 °C
to 40°C.
3) When charging the battery indoors, the area
should be well ventilated.
Measuring Techniques - Testing Earth Electrodes
Electrode
under test
Potential
spike
3m 3m
15m to 25m 15m to 25m
Current
spike
FALL-OF-POTENTIAL METHOD
This is the basic method for measuring the resistance
of earth electrode systems. However, it may only be
practical on small, single earth electrodes because of
limitation on the size of area available to perform the
tests.
Insert the Current test spike into the ground some 30 to
50 metres away from the earth electrode to be tested.
Firmly connect this spike to the instrument terminal 'C2'.
Insert the Potential test spike into the ground midway
between the Current test spike and the earth electrode.
Firmly connect this spike to the instrument terminal
'P2'.
Note:- It is important that the Current spike, the
Potential spike and the earth electrode are all in a
straight line. Also when running the test leads out to
each remote spike, it is preferable not to lay the wires
close to each other in order to minimise the effect of
mutual inductance.
Firmly connect the 'C1' and the 'P1' instrument
terminals to the earth electrode as shown.
Operate the instrument as explained in 'Basic Test
Procedure', and note the resistance obtained.
Move the potential spike 3 metres further away from the
earth electrode and make a second resistance
measurement. Then move the potential spike 3 metres
nearer the electrode (than the original position) and
make a third resistance measurement. If the three
resistance readings agree with each other, within the
required accuracy, then their average may be taken as
the resistance to earth of the electrode. If the readings
disagree beyond the required accuracy then an
alternative method should be used e.g. the 61,8% Rule
or the Slope Method etc.
Fall-of-Potential method connections.
17
Measuring Techniques - Testing Earth Electrodes
Fall-of-Potential Method with Short 'E' Lead
Another way of making connections to the earth
electrode is to connect to the earth electrode using only
one single connection to the ‘C1’ terminal. This should
only be done if the test lead can be kept short because
its resistance will be included in the measurement.
Note:- Earth electrode test lead resistance can be
determined separately. First remove it from the the
electrode and connect to the 'C2' and 'P2' terminals.
Press the Test push button. The lead resistance can
then be deducted from the earth resistance
measurements. This procedure is not, of course,
necessary if the 'C1' and 'P1' terminals are connected
by separate test leads.
15m to 25m 15m to 25m
Electrode
under test
Fall-of-Potential method using a single lead to the
3m 3m
Potential
spike
Current
spike
earth electrode.
18
THE 61,8% RULE
To obtain an accurate reading using the Fall-ofPotential method the current spike must be correctly
sited in relation to the earth electrode. Since both
possess ‘resistance areas’, the Current spike must be
sufficiently remote to prevent these areas overlapping.
Furthermore, the Potential spike must be between
these areas. If these requirements are not met, the Fallof-Potential method may give unsatisfactory results.
Electrode
EPC
Under Test
Resistance areas associated with an earth
electrode and current spike.
Theoretically, both the Current and Potential spikes
should be at an infinite distance from the earth
electrode. However, by graphical considerations and by
actual test it can be demonstrated that:-
The ‘true’ resistance of the earth electrode is equal to
the measured value of resistance when the Potential
spike is positioned 61,8% of the distance between the
earth electrode and the Current spike, away from the
earth electrode.
Auxiliary
Potential
Electrode
Resistance Areas
(Not Overlapping)
Auxiliary
Current
Electrode
This is the 61,8% Rule and strictly applies only when
Electrode
under test
Potential
spike
Current
spike
61,8% of EC
38,2% of EC
the earth electrode and Current and Potential spikes lie
in a straight line, when the soil is homogeneous and
when the earth electrode has a small resistance area
that can be approximated by a hemisphere. Bearing
these limitations in mind this method can be used, with
care, on small earth electrode systems consisting of a
single rod or plate etc. and on medium systems with
several rods.
Connections for the 61,8% Rule.
For most purposes the Current spike should be 30
metres to 50 metres from the centre of the earth
electrode under test. The Potential spike should be
inserted in the ground 61,8% of this distance, between
and in a straight line with, the Current spike and the
earth electrode. The distance is measured from the
earth electrode. If the earth electrode system is of
medium size containing several rods, then these
distances must be increased. The following table gives
a range of distances that agree with the rule. In the first
column ‘Maximum dimension’ is the maximum
distance across the earth electrode system to be
measured.
Maximum
dimension
in metres
5
10
20
Distance to Potential
spike in metres from
centre of earth
system
62
93
124
Distance to Current
spike in metres from
centre of earth
system
100
150
200
For greater accuracy an average reading can be
calculated by moving the current spike, say 10 metres,
towards and then away from its first position and
making further resistance measurements. (Remember
that the Potential spike must also be moved in
accordance with the 61,8% Rule). The average of the
three readings can then be calculated.
19
Measuring Techniques - Testing Earth Electrodes
P
C
Electrode
under test
EC
Position of Electrode P
measured from E
Arbitrary position
of Earth electrode
0,2 EC 0,4 EC
0,6 EC
Position of
C electrode
Earth
resistance
curve
R3
R2
R1
Resistance
20
THE SLOPE METHOD
This method is more applicable to larger earth
electrode systems or where the position of the centre of
the earthing system is not known or inaccessible (e.g.
if the system is beneath the floor of a building). The
Slope method can also be used if the area available for
siting the earth electrodes is restricted. It can be tried if
the previous methods prove unsatisfactory and
generally yields results of greater accuracy than those
methods.
Connections for the Slope method
The equipment is set up as shown. The remote Current
spike is placed 50 metres or more from the earth
electrode system to be measured and connected to the
'C2' terminal. The Potential spike is inserted at a
number of positions consecutively, between the earth
system and the Current spike, and connected to the
'P2' terminal. The test spikes and the earth system
should all be in a straight line.
The 'C1' and 'P1' terminals are connected separately
to some point on the earth electrode system.
The earth resistance is measured at each separate
position of the Potential spike and the resistance curve
is plotted from the results. At least six readings are
needed. Drawing the curve will show up any incorrect
points which may be either rechecked or ignored.
Example Resistance curve from Slope method
tests.
Suppose the distance from the earth electrode system
to the current spike is EC. From the curve equivalent
resistance readings to Potential positions 0,2EC, 0,4EC
and 0,6 EC can be found. These are called R1, R2 and
R3 respectively.
Calculate the slope coefficient μ, where
μ = (
which is a measure of the change of slope of the earth
resistance curve.
From the table commencing on page 36 obtain the
value of Pt / Ec for this value of μ.
Ptis the distance to the Potential electrode at the
position where the ‘true’ resistance would be
measured.
Multiply the value of Pt / Ec by Ec to obtain the distance
Pt.
From the curve read off the value of resistance that
corresponds to this value of Pt. The value obtained is
the earth electrode system's resistance.
Note:- (i) If the value of μ obtained is not covered in
the table then the current spike will have to be moved
further away from the earthing system.
R3-R2
(R2-R1)
)
(ii) If it is necessary, further sets of test results can be
obtained with different values of EC, or different
directions of the line of EC. From the results obtained of
the resistance for various values of the distance EC.
Example of possible results from several Slope
method tests.
This shows how the resistance is decreasing as the
distance chosen for EC is increased.
The curve indicates that the distances chosen for EC in
tests (1) and (2) were not large enough, and that those
chosen in tests (3) and (4) were preferable because
they would give the more correct value of the earth
resistance.
(iii) It is unreasonable to expect a total accuracy of
more than 5%. This will usually be adequate, bearing in
mind that this sort of variation occurs with varying soil
moisture conditions or non-homogeneous soils.
21
Measuring Techniques - Testing Earth Electrodes
E
Electrode
under test
METHOD USING A ‘DEAD’ EARTH
The techniques using test spikes explained earlier are
the preferred methods of earth testing. In congested
areas it may not be possible to find suitable sites for the
test spikes, nor sufficient space to run the test leads. In
such cases a low resistance conductive water main
may be available. This is referred to as a ‘dead’ earth.
Great care must be taken before deciding to adopt this
method and its use is not to be encouraged. Before
employing this method, the user must be quite sure that
no part of the ‘dead‘ earth installation contains plastic or
other non-metallic materials.
1) Short together terminals ‘P1’ and ‘C1’.
2) Short together terminals ‘P2’ and ‘C2’.
2) Firmly
connect a test lead to ‘C1‘ and ’P1‘ and the
other test lead to ‘P2‘ and ‘C2‘.
3) Firmly
connect the free ends of the test leads
to the ‘dead’ earth, and to the electrode under test.
4) Press the Test push, and take a reading in
the normal way.
This test will give give the combined resistance to earth
of the two earths in series. If that of the ‘dead‘ earth is
negligible then the reading may be taken as that of the
electrode under test .
22
The resistance of the two test leads can be found by
firmly joining their free ends together, pressing the Test
push and taking the reading in the usual way. Test lead
resistance can then be subtracted from the original
reading, to obtain the combined resistance of the earth
electrode and the ‘dead’ earth.
In congested urban areas, the Star-Delta method is the
preferable. This method is explained along with other
methods referred to, in ‘Getting Down to Earth’ (see
‘Accessories‘ - Publications).
‘Dead’ earth testing
BS7671(16th Edition wiring regulations)
requirements
Regulation 713-11 of BS7671 specifies that the
resistance of earth electrodes must be measured. The
accompanying Guidance Notes describe a method of
test that is very similar to the Fall-of-Potential method.
If the maximum deviation from the average of the three
readings is better than 5% then the average can be
taken as the earth electrode resistance. If the deviation
exceeds 5% then the current spike should be moved
further away from the electrodes and the tests
repeated.
Other Methods
There are other methods of earth electrode testing
among which are the Four Potential, Intersecting Curves
and Star Delta methods. Megger Limited publications
explain these test methods and give other helpful
information about earth testing. See ‘Accessories‘ -
Publications.
E
6m
P
6m
Test spike positions for BS7671 testing
C
23
Measuring Techniques - Testing Earth Electrodes
Earth
1m
Potential
spike
Current
spike
Earthed Perimeter Fence
SUBSTATION
Determining ‘Touch’ Potential
‘Touch’ potential is the potential difference a person
would experience across his body if he were, for
example, standing on the ground outside the earthed
perimeter fence of a substation and touching the fence
at the time a fault occurred.
Firmly connect the instrument as follows:-
1) Terminal 'C1' to the substation earth.
2) Terminal 'C2' to the Current spike inserted in the
ground some distance away.
3) Terminal 'P1' to the structure being tested e.g. the
perimeter fence.
4) Terminal 'P2' to the Potential spike inserted in the
ground 1 metre away from the perimeter fence
adjacent to the point where a person might stand.
5) Press the Test push, and take a reading in
the normal way. This is the effective resistance
between the point of test on the fence and the
Potential spike as seen by the test current.
The maximum value of the current that would flow in
the earth when a fault to earth occurred at the
substation must be known. The maximum fault current
has to be calculated from the parameters associated
with the substation ratings involved. From Ohms Law
(V = I x R), the Touch potential can be calculated.
24
Determining 'Touch' potential.
Determining ‘Step’ potential
Earth
Current
spike
SUBSTATION
1m
A
B
‘Step’ potential is the potential difference a person
would experience between his feet as he walked across
the ground in which a fault current was flowing.
Firmly connect the instrument as follows :-
1) Terminal 'C1' to the substation earth.
2) Terminal 'C2' to the Current spike inserted in the
ground some distance away.
3) Firmly connect the 'P1' and 'P2' terminals to test
spikes inserted in the ground 1 metre apart, (or the
length of a step) at positions A and B respectively.
A is nearest to the substation earth.
4) Press the Test push, and take a reading in
the normal way.
Record the resistance indicated. This is the effective
resistance across the positions A and B, as seen by the
test current.
The maximum value of the current that would flow in
the earth when a fault to earth occurred at the
substation must again be known. From Ohms Law the
‘Step potential’ can be calculated.
Determining ‘Step’ potential
25
Measuring Techniques - Measuring Soil Resistivity
Typical variations in soil resistivity
The resistance to earth of an earth electrode is
influenced by the resistivity of the surrounding soil. The
resistivity depends upon the nature of the soil and its
moisture content and can vary enormously as seen in
the table below:-
Material
Ashes
Coke
Peat
Garden earth - 50% moisture
Garden earth - 20% moisture
Clay soil - 40% moisture
Clay soil - 20% moisture
London clay
Very dry clay
Sand - 90% moisture
Sand - normal moisture
Chalk
Consolidated
Sedimentary rocks
Specific
resistance
in Ω-cms
20 - 800
4500 - 20000
400 - 2000
5000 - 15000
13000
300000 - 800000
5000 - 15000
1000 - 50000
350
1400
4800
770
3300
Information
source
Higgs
Ruppel
Ruppel
Ruppel
Ruppel
Broughton
Edge & Laby
26
Because it is impossible to forecast the resistivity of the
soil with any degree of accuracy it is important to
measure the resistance of an earth electrode when it is
first laid down and thereafter at periodic intervals.
Before sinking an electrode into the ground for a new
installation it is often advantageous to make a
preliminary survey of the soil resistivity of the
surrounding site. This will enable decisions to be made
on the best position for the electrode(s) and to decide
whether any advantage can be gained by driving rods
to a greater depth. Such a survey may produce
considerable savings in electrode and installation costs
incurred trying to achieve a required resistance.
Line Traverse
a
a
a
P1
a
20
The most common method of measuring soil resistivity
is often referred to as the line traverse. Four test spikes
are inserted into the ground in a straight line at equal
distances 'a' and to a depth of not more than 1/20 of 'a'.
The instrument is connected to the test spikes as
shown.
Soil resistivity measurement.
The instrument is operated and the measurement
made in the normal way. The resistivity may be
calculated from the formula given opposite or from the
nomogram overleaf. This is the average soil resistivity
to a depth 'a'.
The four test spikes are then re-positioned for further
tests along a different line. If both the spacing 'a' and
the depth a/20 are maintained, a directly comparable
reading will be obtained each time, and thus regions of
lowest resistivity can be located over a given area (at
the constant depth 'a').
Re-spacing the test spikes at separations 'b', 'c', 'd', etc
will yield results from which a profile of the resistivity at
new depthsb/20, c/20, d/20,etc.can be obtained.
If the same line for the test spikes is maintained, but the
separation of them is progressively widened, resistivity
values at various depths can be obtained. By this
means depth surveys may be made.
More details can be found in the Megger Limited
publications. See ‘Accessories‘.
Calculation of resistivity
Assuming that the tests were carried out in
homogeneous soil the resistivity is given by the
formula:-
ρ = 2πaR
where ‘R’ is the resistance measured in ohms, ‘a’ is the
test spike spacing in metres and ‘ρ’ is the resistivity in
ohm-metres.
For non-homogeneous soils the formula will give an
apparent resistivity which is very approximately the
average value to a depth equal to the test spike spacing
'a'.
27
Measuring Techniques - Measuring Soil Resistivity
Resistivity calculation Nomogram
28
Measuring Techniques - Continuity Testing
DET 2/2 can be used to measure metallic resistances
of low inductance or capacitance. To test the continuity
of conduit or other earth conductors the instrument can
be connected as shown. Ensure that the circuit is deenergised, before connecting the instrument for
measurement.
Note:- Due to the inherent high accuracy of the
instrument and the low continuity resistance to be
measured, contact resistance between the test lead
clips and the conduit becomes a factor in the measured
value. Contact resistance should therefore be kept as
low as possible.
1) Firmly short together terminals ‘P2’ and ‘C2’.
2) Firmly short together terminals ‘P1’ and ‘C1’.
3) Firmly
4) Firmly
5) Press the Test push, and take a reading in the
The resistance of the two test leads can be found by
firmly joining their free ends together, pressing the
T
est push and taking the reading in the usual way. Test
lead resistance can then be subtracted from the original
reading, to give a ‘true’ value of continuity resistance.
connect a test lead to ‘P2 and C2’,and the
other test lead to ‘P1’ and ‘C1’.
connect the free ends of the test leads
across the isolated circuit under test.
normal way.
Continuity testing.
29
Specification
Earth Resistance Ranges: 0,010 Ω to 19,99 kΩ (Auto-ranging) 1 mΩ resolution
Accuracy (23°C ±2°C): ±0,5% of reading ±2 digits. Service error ±5% of reading ±2 digits ±10 mΩ
Test Frequency: 105 Hz to 160 Hz reversing d.c. (50 Hz environments default to 128 Hz,
Test Current: 50 mA max. (selectable high and low levels)
Max Output Voltage: < 50 V r.m.s.
Interference: Typically 40 V pk to pk (50 Hz, 60 Hz, sinusoidal nature)
Max. Current spike
(Loop) Resistance: Range (R
Max. Potential Spike Resistance: Range (R
30
(meets VDE service error over 50 mΩ)
60 Hz environments default to 150 Hz). Set in steps of 0,5 Hz
) High current (Rp) Low current (Rc)
0,010 Ω - 0,499 Ω 5 kΩ 1 kΩ
E
0,500 Ω - 1,999 Ω 5 kΩ 3 kΩ
2,000 Ω - 19,99 Ω 10 kΩ 5 kΩ
20,000 Ω - 199,9 Ω 50 kΩ 20 kΩ
200, 0 Ω - upwards 50 kΩ 50 kΩ
) High current (Rp) Low current (Rp)
E
0,010 Ω - 0,499 Ω 1 kΩ 10 kΩ 1 kΩ 10 kΩ
0,500 Ω - 1,999 Ω 1 kΩ 20 kΩ 1 kΩ 10 kΩ
2,000 Ω - 19,99 Ω 1 kΩ 20 kΩ 1 kΩ 10 kΩ
20,000 Ω - 199,9 Ω 200 x R
200, 0 Ω - upwards 50 kΩ total 50 kΩ total
(Rp1) (Rp2)(R
E 20 kΩ 200 x RE 20 kΩ
) (Rp2)
p1
Loading...