A 1472, A1579 Leakage current clamp
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4.4 Influencing quantities
Unfortunately measurements with leakage current clamp are subjected to
different influencing quantities that have a big impact on their performance.
High uncertainty and non-repeatability of readings often lead to wrong
interpretations. Three most important influencing quantities are described in
this document.
4.4.1 Influence of nearby external magnetic field
Current carrying conductors, transformers and chokes in electrical
equipment are producing magnetic fields that are coupling into the sensor
and cables of the current clamp and are disturbing the results. The coupling
should be as small as possible. It strongly depends on the current sensor
quality. Protective measures are proper magnetic shielding, symmetry of
the winding, twisting of all connection cables, high permeability of the
sensor core etc.
4.4.2 Influence of load current in differential leakage
current test
This influencing quantity is similar than the influence of nearby magnetic
fields. During a differential leakage test the sum of currents flowing through
the clamp is measured. Lets assume that the load current through the line
conductor is 10 A and the returning current through the neutral conductor
is 9.999 mA. This means that a leakage current of 1mA is obtained on base
of the measured difference of: (10000 mA
– 9999 mA).
Any small anomaly in the current sensor will result in an asymmetry and
consequentially in a large measuring error. Protective measures are
symmetry of the winding, high permeability of clamp core etc)
Fig. 9: Influence of external magnetic fields
