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Section 21
Requirements
21.1.6.3
Non-directional instantaneous and definitive time, phase and residual
overcurrent protection
The CTs must have a rated equivalent limiting secondary e.m.f. E
to the required rated equivalent limiting secondary e.m.f. E
³
E
E
al
EQUATION1381 V2 EN-US
where:
I
op
I
pr
I
sr
I
r
R
ct
R
L
S
R
21.1.6.4
Non-directional inverse time delayed phase and residual overcurrent
protection
The requirement according to Equation
high set instantaneous or definitive time stage is used. In this case Equation is the only
necessary requirement.
If the inverse time delayed function is the only used overcurrent protection function the CTs
must have a rated equivalent limiting secondary e.m.f. E
required rated equivalent limiting secondary e.m.f. E
³
E
E
al
EQUATION1076 V2 EN-US
where
I
op
I
pr
I
sr
I
r
Table continues on next page
454
æ
I
=
×
×
×
sr
1,5 I
R
ç
alreq
op
ct
I
è
pr
The primary operate value (A)
The rated primary CT current (A)
The rated secondary CT current (A)
The rated current of the protection IED (A)
The secondary resistance of the CT (W)
The resistance of the secondary cable and additional load (W). The loop resistance containing the
phase and neutral wires, must be used for faults in solidly earthed systems. The resistance of a single
secondary wire should be used for faults in high impedance earthed systems.
The burden of an IED current input channel (VA). S
channel for I
=5 A
r
æ
I
=
×
×
×
sr
20 I
R
ç
alreq
op
I
è
pr
The primary current set value of the inverse time function (A)
The rated primary CT current (A)
The rated secondary CT current (A)
The rated current of the protection IED (A)
ö
S
+
+
R
R
÷
L
2
I
ø
r
=0.020 VA/channel for I
R
97
and Equation
alreq
ö
S
+
+
R
R
÷
ct
L
2
I
ø
r
that is larger than or equal
al
below:
alreq
=1 A and S
r
98
does not need to be fulfilled if the
that is larger than or equal to the
al
below:
Busbar protection REB670
Application manual
1MRK 505 370-UEN D
M11622-3 v5
(Equation 96)
=0.150 VA/
R
M11339-3 v5
(Equation 97)
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