5.4 Testing the Loop Resistance
The loop resistance of a current source is the cu-
mulation of all resistance values or impedance val-
ues within the loop of exterior conductor (L) and
neutral conductor (N). The loop resistance is com-
posed of the exterior conductor resistance, the in-
ternal resistance of the voltage generator (trans-
former) and the protective conductor resistance. .
With the loop resistance test, the short-circuit cur-
rent can be assessed. The short-circuit current
must be known to ensure that a pre-connected ex-
cessive current protection device may be safely trig-
gered between L and PE at a sufficiently high cur-
rent in the case of a body contact.
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.
Any RCDs installed ahead have to be bridged
during the loop resistance test.
-When performing tests on shockproof sock-
ets or equipments with protective conductor
connection, the orderly connection of the pro-
tective conductor must be checked. For this
purpose, briefly touch the contact electrode
‘PE’ (21) of the test instrument after having
connected the test leads and prior to starting
the test procedure. If an acoustic sound is au-
dible and the symbols ‘”
” (14) and 'PE Fail-
ure' (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.
18
Testing the Loop Resistance
PTDB90510001.qxd 02.03.2006 12:23 Uhr Seite 62
