Measuring Earth Resistance - Velleman AVM52ERT Mode D'emploi

4.3 Checking Battery Voltage and Cable Connection
Both battery strength and cable connections can be tested when the BATT.CHECK button is placed in the
pressed-down position.
Battery Voltage
The battery voltage is sufficient if the needle stays in the GOOD zone of the scale. If not, replace the batteries
(see 5. Battery Replacement).
Cable Connection
Press the MEAS. button. The OK lamp lights up if the cables of terminals P and C are connected properly and if
the earth resistance of the auxiliary earth bars falls within the limits that you have determined. If the lamp does
not light up: check the connection of the cables with terminals P and C, or reduce the earth resistance of the
auxiliary earth bars to an acceptable level by moistening the ground with water or moving the earth bars.
Short the alligator clips at the end of the yellow and red wires to check them for damage.
Note that you can check the battery voltage without connecting any cables. Simply press the BATT.CHECK
button once. The OK lamp does not light up.
4.4

Measuring Earth Resistance

Press one of the range buttons x1Ω, x10Ω, or x100Ω. Press the MEAS. button next. Multiply the reading
by 10 for the x10Ω range, or by 100 for the x100Ω range.
The OK lamp lights up if the instrument is working normally. If the lamp is not lit, this indicates that normal
operation is made impossible by an excessive earth resistance across terminals C and E. Verify again that there
is no contact between the cables and recheck the earth resistance of the auxiliary earth bars according to the
principles outlined in 4.3 Checking Battery Voltage and Cable Connection above. Despite all previous
checks, it is still possible for the OK lamp to stay off and for the needle to exceed the upper limit value. This
can be due to several things: the tested device may not be functioning properly, its cables may be damaged, or
the green connection cable may be damaged.
4.5 Simplified Earth Resistance Measurement
This method is recommended when measuring an earth resistance > 10 Ω, or when it is impossible to drive the
auxiliary earth bars into the ground. An approximate value can be obtained through the two-wire system, which
uses previously earthed devices (see fig. 3 below).
Note: Make sure that the earth is connected to terminal P when measuring earth resistance through mains
power (A).
Press the AC V button as outlined in 4.2 Measuring the Earth Voltage of the Earthed Equipment under
Test to measure the earth voltage of the equipment under test, and make certain that earth voltage is
below 2 V.
Press the x10Ω button and then the MEAS. button. Read the earth resistance from the scale. Press the x100Ω
button if the needle tilts to the far right.
The obtained reading (RE) renders the approximate earth resistance value. There is no need for external
shorting as terminals P and C are shorted internally.
The earth leakage breaker will not trip because the measuring current is as low as 2 mA.
The OK lamp lights up in the normal operating mode regardless whether the simplified or the ordinary
measurement method is used (this indicates that continuity exists between the various terminals E, P and C).
In case of the simplified measurement method, only two terminals are used. The earth resistance "re" of an
earthed electrode connected to terminal P shall be added to a true earth resistance value REX. Therefore, the
earth resistance reading is the result of the following formula:
RE = REX + re
Suppose that "re" is a known value.
Let us also assume that RE is 100 Ω and that the earth resistance to be measured is, for example, 100 Ω.
Consequently, the true earth resistance is expressed as:
REX = (100 Ω) – re
Since re is greater than 0, a true earth resistance may be expressed as follows:
REX ≤ 100 Ω
When measuring an earth resistance of several dozen ohms, we may assume that the indicated earth
resistance is a true value.
V. 02 – 27/05/2014
AVM52ERT
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