sepam 20 user manual - Schneider Electric

3
Protection functions Earth fault
ANSI code 50N/51N or 50G/51G
Description
The earth fault function comprises 4 independant
elements divided into two groups of 2 settings called
Group A and Group B respectively.
The use of the two elements may be chosen by
parameter setting:
b operation with Group A or Group B exclusively, with
switching from one group to the other dependent on the
state of logic input I13 exclusively, or by remote control
(TC3, TC4),
I13 = 0 group A
I13 = 1 group B
b operation with Group A and Group B active for 4-set
point operation
b enabling/disabling of each group of 2 elements
(A, B).
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Operation
The earth fault protection function is single-pole.
It picks up if the earth fault current reaches the
operation set point.
It is time-delayed. The time delay may be definite time
(DT) or IDMT according to the curves opposite.
The protection function includes harmonic 2 restraint
which provides greater stability when transformers are
energized (measurement of residual current by the sum
of the 3 phase CTs).
The restraint disables tripping, regardless of the
fundamental current.
The restraint can be selected by parameter setting.
The protection function can be inhibited by input I23 for
the S23 application only.
Definite time protection
Is0 is the operation set point expressed in Amps, and T
is the protection operation time delay.
Definite time protection principle.
IDMT protection
IDMT protection operates in accordance with the
IEC (60255-3), BS 142 and IEEE (C-37112) standards.
IDMT protection principle.
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The Is0 setting is the vertical asymptote of the curve, and T is the operation time
delay for 10 Is0.
The tripping time for I0/Is0 values of less than 1.2 depends on the type of curve
chosen.
Name of curve Type
Standard inverse time (SIT) 1.2
Very inverse time (VIT or LTI) 1.2
Extremely inverse time (EIT) 1.2
Ultra inverse time (UIT) 1.2
RI curve 1
IEC standard inverse time SIT / A 1
IEC very inverse time VIT or LTI / B 1
IEC extremely inverse time EIT / C 1
IEEE moderately inverse (IEC / D) 1
IEEE very inverse (IEC / E) 1
IEEE extremely inverse (IEC / F) 1
IAC inverse 1
IAC very inverse 1
IAC extremely inverse 1
The curve equations are given in the chapter entitled "IDMT protection functions".
The function takes into account current variations during the time delay interval.
For currents with a very large amplitude, the protection function has a definite time
characteristic:
b if I0 > 20 Is0, tripping time is the time that corresponds to 20 Is0
b if I0 > 15 In0, tripping time is the time that corresponds to 15 In0.
Block diagram
Restraint
I23 Input
(for S23)
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PCRED301005EN - 06/2008