Operation Manual
20 GMC-I Messtechnik GmbH
9 Measuring Low-Value Resistance (to 100 )
According to the regulations, measurement of low-value resistance at
protective conductors, earth conductors and bonding conductors must
be performed with (automatic) measuring voltage polarity reversal, or with
the flow of current in one direction (+ pole to PE), or the other (– pole to
PE).
Attention!
!
Low-value resistance may only be measured at voltage-free
objects.
Attention!
!
In order to be able to start the measurement, contact must first be
established between the test probes and the device under test.
If voltage is present at the device under test, or if resistance is greater
than 100 no measurement is performed.
Automatic Polarity Reversal – AUTOFunction
With automatic polarity reversal, the instrument performs measurement
first in one direction and then in the other, after the measuring sequence
has been started.
The largest measured resistance value is always displayed. This
presupposes that AUTO is displayed underneath R
LO
. If either of
the values RLO/+ or RLO/– is displayed instead of AUTO, press
the key in the menu bar and then AUTO
Resistance values which do not demonstrate stabilized values until after
settling in, should not be measured with automatic polarity reversal.
Measurement with automatic polarity reversal may result in varying,
excessively high measured values and thus to an ambiguous read-out.
Measurement with + Pole to PE, or – Pole to PE
In order to determine whether or not test results are independent of the
direction of flow, measurement can be performed separately in both
directions. Press the key in the menu bar to this end, and then either +
or – depending upon the desired direction of flow.
Differing results indicate that voltage is present at the device under test
(e.g. thermoelectromotive force or elemental voltage).
Measurement results may be distorted by parallel connected impedances
from load current circuits and equalizing current, especially in systems
equipped with “overcurrent protective devices” (formerly neutralization)
without a separate protective conductor. A change in resistance during
measurement (e.g. inductance), or poor contact may also result in
distorted measurements.
The following are examples of resistance values which may change during
measurement :
– Resistance values for incandescent light bulbs due to warming
caused by test current
– Resistance with an excessively high inductive component
In order to assure unambiguous measurement results, it is necessary to
recognize and eliminate any cause of error.
The instrument’s batteries are depleted rapidly during resistance
measurement. Only press the START key as long as necessary during
measurement with current flow in a single direction.
START