Setting
Reduction factor
considers the fact that only part of the earth-
fault current (I
F
) will flow back through “remote” earth and introduces
earth potential rise and touch voltages.
When
Operating curve type
= “Inverse time IEEE80” is selected, then
setting
UTp multiplier
is not effective.
Consider next the following example with Operating curve type = “Inverse time
IEEE80”:
Example 6. Inverse time operation of IFPTOC, when
Operation principle
= “Touch
voltage based”,
Operating curve type
= “Inverse time IEEE80” and setting
IEEE
multiplier
= 500 is illustrated in
Figure 354
. Start value for operation is set to be 20 V
(setting
Touch Vol Str Val
). IFPTOC is set to have minimum operate time 100 ms and
maximum operate time of 5000 ms (settings
IDMT stage Min Op Tm, IDMT stage
Max Op Tm
). Maximum earthing resistance value encountered in the protected
feeder is set to be 10 ohms (setting
Maximum earthing Ris
) and
Reduction factor
= 1. For illustrating the operating speed requirement of protection, limits of earth
potential rise durations defined in Finnish national Electrical Safety Regulations 89
are shown for reference.
1MRS759142 F
Protection functions
REX640
Technical Manual
623
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