Earthing Resistance Measurement; Earthing Resistance Measurement (Function R ) - Gossen MetraWatt PROFITEST MBASE MTECH Operating Instructions Manual

10 Earthing Resistance Measurement (function R
Earthing resistance R is important for automatic shutdown in
E
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 ) is the sum of earth electrode resistance
E
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
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 and phase conductor L are also included in the
B
measurement results.
Measuring Method (with probe)
The instrument measures earthing resistance R
ammeter-voltmeter test (earth electrode loop resistance).
Resistance R is calculated from the quotient of voltage U
E
current I , where U is between earth electrode and the probe.
E E
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.
GMC-I Messtechnik GmbH
) Characteristic Values of Earth Resistance Measurement (Mains
E
Powered)
•
Measurement with or without earth electrode voltage depending
upon entered parameters and the selected type of connection:
Range
xx  / xx k
10  / U
xx  / xx k
* This parameter results in automatic setting to probe connection.
is only possible Measuring Method with Suppression of RCD Tripping
E
PROFITEST
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
by means of the
is no longer capa-
E
ble of detecting this
and
measuring current,
E
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
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.
Measuring range 0 ... 10 k
Connection
*
E
M tester provides the opportunity of measuring
TECH
I /mA
F
Start
Suppression of RCD tripping for RCCBs which are
sensitive to pulsating current
)
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).
Measuring Functions
No probe measurement
No U measurement
E
Probe measurement activated
U is measured
E
Probe measurement activated
No U measurement
E
Clamp measurement activated
No U measurement
E
t1 t2 t3
Measure
Operation
RCD disabled!
T
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