FANOX SIL B, User Manual, Page: 43 / 311

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4.12. Function 81O/U. Overfrequency and underfrecuency protection
There are four protection units of the variation in frequency. Each one of them has an adjustment which
determines whether the unit acts as overfrequency or underfrequency. The reaction time of the unit is
determined by the set operating time.
In case the unit acts like overfrequency, activation of the function occurs at 100% frequency level set
and is reset when the measured frequency is 50 mHz lower than set start level.
If the unit is set as underfrequency, activation of the function occurs at 100% frequency level set and is
reset when the measured frequency is 50 mHz higher than set start level.
The reset is temporized and the reset time is determined by the reset time setting.
Accuracy of operating time and reset time is adjusted time plus a maximum of 30ms.
The frequency measurement is done from the voltage of phase B. It takes a minimum of 30 volts at this
stage for 81 functions to be operational. If the measured phase voltage is less than 30 volts, it activates
a state bit indicating function blocked. When the frequency measurement is again valid, function begins
in the start state with all bits and counters reset.
This protection function is adjusted by setting five different parameters:
Function Description Minimum Maximum Step Unit Default
81_1
81_2
81_3
81_4
Overfrequency or Underfrequency
Permission - - Yes/No - No
Type - - (1*) - Underfrequency
Activation level 45.00 65.00 0.01 Hz 55
Operating time 0.06 300.00 0.01 s 100
Reset time 0.2 1200.0 0,1 s 0.2
(1*) Overfrequency or Underfrequency