GMC-I Messtechnik GmbH
27
10
Earthing Resistance Measurement (function R
E
)
Earthing resistance R
E
is important for automatic shutdown in
system sections. It must have a low value in order to assure that
high short-circuit current flows, and that the system is shut down
reliably by the RCCB in the event of a fault.
Test Setup
Earthing resistance (R
E
) is the sum of earth electrode resistance
and earth conductor resistance. Earthing resistance is measured
by applying an alternating current via the earth conductor, the
earth electrode and earth electrode resistance. This current, as
well as voltage between the earth electrode and a probe, are
measured.
The probe is connected to the probe connector socket (17) with a
4 mm contact protected plug.
Direct Measurement with Probe
Direct measurement of earthing resistance R
E
is only possible
within a measuring circuit which includes a probe. However, this
means that the probe and reference earth must be of like
potential, i.e. that they are positioned outside of the earth
electrode resistance area. The distance between the earth
electrode and the probe should be at least 20 m.
Measurement Without Probe
In many cases, especially in extremely built-up areas, it is difficult,
or even impossible, to set a measuring probe. In such cases,
earthing resistance can be measured without a probe. In this
case, however, the resistance values for the operational earth
electrode R
B
and phase conductor L are also included in the
measurement results.
Measuring Method (with probe)
The instrument measures earthing resistance R
E
by means of the
ammeter-voltmeter test (earth electrode loop resistance).
Resistance R
E
is calculated from the quotient of voltage U
E
and
current I
E
, where U
E
is between earth electrode and the probe.
The test current which is applied to earthing resistance is
controlled by the instrument and demonstrates the following
values in the various measuring ranges:
1 to 10 k
:
4 mA; 0 to 1 k
: 40 mA; 0 to 100
: 0.4 A
and 0 to 10
: > 0.65 A to approx. 3.4 A (voltage-dependent).
A voltage drop is generated which is proportional to earthing
resistance.
Note
Measurement cable and measuring adapter resistance
are compensated for automatically during measurement
and have no effect on measurement results.
If dangerous contact voltages occur during measurement
(> 50 V), the measurement is interrupted and safety
shut-down occurs.
Probe resistance does not effect measurement results
and may be as high as 50 k
.
Attention!
!
The probe is part of the measuring circuit and may carry
a current of up to 3.5 mA in accordance with VDE 0413.
Characteristic Values of Earth Resistance Measurement (Mains
Powered)
•
Measuring range 0 ... 10 k
Measurement with or without earth electrode voltage depending
upon entered parameters and the selected type of connection:
* This parameter results in automatic setting to probe connection.
Measuring Method with Suppression of RCD Tripping
PROFI
TEST
M
TECH
tester provides the opportunity of measuring
loop impedance with positive half-waves. With this method, you’ll
be able to measure loop impedances within systems which are
equipped with RCCBs.
The test instru-
ment generates a
DC current to this
end, which satu-
rates the RCCB’s
magnetic circuit.
The test instru-
ment then super-
imposes a measur-
ing current which
only demonstrates
half-waves of like
polarity. The RCCB
is no longer capa-
ble of detecting this
measuring current,
and is consequently not tripped during measurement.
A four conductor measuring cable is used between the
instrument and the test plug. Cable and measuring adapter
resistance is automatically compensated for during measurement
and does not effect measurement results.
Special Case: Manual Measuring Range Selection (test current
selection)
(R
AUTO, R = 10 k
(4 mA), 1 k
(40 mA), 100
(0.4 A),
10
(> 0.65 A), 10
/U
E
)
Note
When the measuring range is selected manually,
accuracy values are only valid as of 5% of the upper
range limit (except for the 10
range; separate display
for small values).
Evaluation of Measurement Values
The maximum allowable displayed resistance values which assure
that the required earthing resistance is not exceeded, and for
which maximum device operating error has already been taken
into consideration (at nominal conditions of use), can be
determined with the help of Table 2 on page 57. Intermediate
values can be interpolated.
Range
Connection
Measuring Functions
xx
/ xx k
No probe measurement
No U
E
measurement
10
/ U
E
*
Probe measurement activated
U
E
is measured
xx
/ xx k
Probe measurement activated
No U
E
measurement
Clamp measurement activated
No U
E
measurement
Start
t1
t3
Measure
t2
Operation
RCD disabled!
T
I
F
/mA
Suppression of RCD tripping for RCCBs which are
sensitive to pulsating current