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