11.2.1 IDMT curves for overcurrent protection
In inverse-time modes, the operation time depends on the momentary value of the
current: the higher the current, the faster the operation time. The operation time
calculation or integration starts immediately when the current exceeds the set
Start
value
and the START output is activated.
The OPERATE output of the component is activated when the cumulative sum of the
integrator calculating the overcurrent situation exceeds the value set by the inverse-
time mode. The set value depends on the selected curve type and the setting values
used. The curve scaling is determined with the
Time multiplier
setting.
There are two methods to level out the inverse-time characteristic.
• The
Minimum operate time
setting defines the minimum operating time for the
IDMT curve, that is, the operation time is always at least the
Minimum operate
time
setting.
• Alternatively, the
IDMT Sat point
is used for giving the leveling-out point as a
multiple of the
Start value
setting. (Global setting: Configuration> System>
IDMT Sat point
). The default parameter value is 50. This setting affects only the
overcurrent and earth-fault IDMT timers.
IDMT operation time at currents over 50 x In is not guaranteed.
General function block features 1MRS758755 C
744 REC615 & RER615
Technical Manual
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