5.3 Testing the Mains Internal Resistance
The internal resistance of a current source is the
sum of all resistance values or impedance values
within the loop of exterior conductor (L) and neu-
tral conductor (N). The internal resistance is com-
posed of the exterior conductor resistance, the in-
ternal resistance of the voltage generator
(transformer) and the neutral conductor resistance.
With the mains internal resistance test, the short-
circuit current can be assessed. The short-circuit
current must be known to ensure that a pre-con-
nected excessive current protection device may be
safely triggered between L and PE at a sufficiently
high current in case of a body contact.
According to DIN VDE, this test is not required, how-
ever, it is an important and essential task in practi-
cal applications when assessing an electrical plant
or for troubleshooting.
The test is carried out by briefly applying a test cur-
rent to the current circuit and by measuring the volt-
age prior and after the current flow. Due to the test
current, a voltage drop occurs within the current
loop under test. Therefore the voltage at the test re-
sistance briefly drops.
This test can be used in systems (TN systems)
with RC circuit breakers to detect any ex-
change of protective conductor (PE) and neu-
tral conductor (N) without opening the distri-
bution. In the event of a faulty connection the
RCD trips during the mains internal resistance
test.
This test can also be used to prove the low im-
pedance of the neutral conductor (N).
When performing tests on shockproof sockets
or equipment with protective conductor con-
nections, the orderly connection of the protec-
tive conductor must be checked. For this pur-
pose, briefly press the contact electrode ‘PE’
(21) of the test instrument after having con-
nected the test leads and prior to starting the
test procedure. If an acoustic sound is audible
and the symbols ‘”
” (14) and 'PE Failure'
(16) are displayed, the PE connection must be
checked! The test may only be started after
having corrected the protective conductor con-
nection! During protective conductor testing it
must be ensured that the measurement result
is not influenced by an insulated floor or site.
Performing the Measurement
Connect the desired test lead (either test lead with
shockproof plug or three individual test leads) to
the test instrument. Here connect the black test
lead to the black socket 'L/L1' (24), and the blue
test lead to the blue socket 'N / L2' (25), and the
green test lead to the green socket 'PE / L3' (26).
Switch on the UNITEST 0100-ELECTROcheck
using the rotary switch 'Measurement Function'
(19) and set the rotary switch to the measure-
ment function 'Mains Internal Resistance'. The
test instrument indicates the following:
- see next page-
15
Testing the Mains Internal Resistance
PTDB90510001.qxd 02.03.2006 12:23 Uhr Seite 59
