Siemens SIMOCODE pro Operating Manual page 126

Parameters
4.3 Monitoring functions
If monitoring is required within limit values that are lower than those recommended, we
recommend the use of delay times, particularly if spurious tripping occurs exclusively during
motor startup. If delay times do not lead to the desired result, the use of shield sleeves may
considerably lower the minimum possible monitoring limit.
You will find more information in Chapters "2.5.2 Installation specifications" and
"2.5.3 Optimization options" in the "3UG4/3RR2 monitoring relay" manual.
The monitored current waveforms also have a strong influence on the measuring accuracy.
In the case of loads with generalized phase control, deviations from the measuring accuracy
can occur when monitoring for high residual current limits. The cause of this is the extreme
difference between the monitored rms values and the peak values of the residual current.
The more extreme the generalized phase control, the shorter the time during which current
flows, and the lower the resulting rms value. To achieve and monitor a high rms value in
such a case, an extremely high peak value of the residual current is necessary. In the case
of high currents, current transformers tend towards saturation in which a further increase in
current on the primary side does not result in an equivalent increase on the secondary side.
In the case of extreme peak values of the residual current, the measuring accuracy suffers
as a result of this principle. Due to the great difference between the peak value and the rms
value, monitoring for lower limits is useful.
See also
Manual "3UG4/3RR2 Monitoring Relay"
(http://support.automation.siemens.com/WW/view/en/50426183/133300)
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SIMOCODE pro - Parameterize
Operating Manual, 04/2017, A5E40507630002A/RS-AA/001