Siemens SIRIUS 3UG4 - Page 139

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If monitoring is required within limit values that are lower than those recommended, we
recommend the use of delay times, particularly if false tripping occurs exclusively during motor
start-up.
If delay times do not lead to the desired result, the use of shield sleeves may considerably lower
the minimum possible monitoring limit.
For more information see Chapters "Installation speci󹪝cations (Page 251)" and "Potential for
optimization (Page 254)".
The monitored current waveforms also have a strong in󹪞uence 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
di󹪜erence 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 󹪞ows,
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 su󹪜ers as a result of this
principle. Due to the great di󹪜erence between the peak value and the rms value, monitoring for
lower limits is useful.
3UG4625 residual current monitoring relay with 3UL23 transformer
8.3 Functions
SIRIUS 3UG4 / 3RR2 monitoring relay
Equipment Manual, 07/2021, NEB927043002000/RS-AD/005 139