3UG4625 residual current monitoring relay with 3UL23 transformer
8.3 Functions
3UG4 / 3RR2 monitoring relays
196 Manual, 05/2016, NEB927043002000/RS-AC/004
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 specifications (Page 335)" and "Potential for
optimization (Page 339)".
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.
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