Instructions

GMC-I Messtechnik GmbH 31
10 Earthing Resistance Measurement (R
E
function)
Earthing resistance R
E
is important for automatic shutdown in
system segments. It must have a low value in order to assure that
high short-circuit current flows and the system is shut down reli-
ably by the RCCB in the event of a fault.
Test Setup
Earthing resistance (R
E
) is the sum of the earth electrode’s dissi-
pation resistance and earth conductor resistance. Earthing resis-
tance 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 (mains powered measurement)
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 poten-
tial, i.e. that they are positioned outside of the potential gradient
area. The distance between the earth electrode and the probe
should be at least 20 m.
Measurement without Probe (mains powered measurement)
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 (w. probe) (mains powered measurement)
The instrument measures earthing resistance R
E
by means of the
ammeter-voltmeter test.
Resistance R
E
is calculated from the quotient of voltage U
E
and
current I
E
where U
E
is between the earth electrode and the probe.
The test current which is applied to earthing resistance is con-
trolled by the instrument (see section 19, “Characteristic Values”,
beginning on page 86 for pertinent values).
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 kΩ.
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.
Measurement with or without earth electrode voltage depending
upon entered parameters and the selected type of connection:
* This parameter results in automatic selection of probe connection.
Measuring Method with Suppression of RCD Tripping
(mains powered earthing measurement)
The test instruments PROFITEST MTECH+, PROFITEST MXTRA and
SECULIFE IP make it possible to measure loop impedance in TN
systems with type A, F and type AC RCCBs (10, 30, 100,
300, 500 mA nominal residual current).
The test instrument
generates a direct
current to this end,
which saturates the
RCCB’s magnetic
circuit.
The test instrument
then superimposes
a measuring current
which only demon-
strates half-waves
of like polarity. The
RCCB is no longer
capable of detect-
ing this measuring
current, and is con-
sequently not tripped during measurement.
A four conductor measuring cable is used between the instru-
ment and the test plug. Cable and measuring adapter resistance
is automatically compensated for during measurement and does
not effect measurement results.
Note
Bias Magnetization
Only AC measurements can be performed with the
2-pole adapter. Suppression of RCD tripping by means of
bias magnetization with direct current is only possible via
a country-specific plug insert, e.g. SCHUKO, or the
3-pole adapter (neutral conductor necessary).
Limit Values
Earthing resistance (earth coupling resistance) is determined pri-
marily by the electrode’s contact surface and the conductivity of
the surrounding earth.
The specified limit value depends on the type of electrical system
and its shutdown conditions in consideration of maximum contact
voltage.
Evaluation of Measured 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 deter-
mined with the help of Table 2 on page 92. Intermediate values
can be interpolated.
RANGE Connection Measuring Functions
xx Ω / xx kΩ
No probe measurement
No U
E
measurement
10 Ω / U
E
*
Probe measurement activated
U
E
is measured
xx Ω / xx kΩ *
Probe measurement activated
No U
E
measurement
Clamp measurement activated
No U
E
measurement
Start
t1
t3
Measurement
t2
RCD Disabled!
t
I
F
/mA
Suppression of RCCB tripping for RCCBs which
are sensitive to pulsating current
Operation