sn
R
al alreq CT L
2
pn n
op
I
S
E E 5 I R R
I I
³ = × × + +
æ ö
×
ç ÷
è ø
EQUATION1677 V1 EN (Equation 180)
where:
I
op
The primary operate value (A)
I
pn
The rated primary CT current (A)
I
sn
The rated secondary CT current (A)
I
n
The nominal 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 grounded systems. The resistance of a
single secondary wire should be used for faults in high impedance grounded systems.
S
R
The burden of an IED current input channel (VA). S
R
=0.010 VA/channel for I
r
=1 A and S
R
=0.250 VA/
channel for I
r
=5 A
18.1.6.3 Non-directional instantaneous and definitive time, phase and residual
overcurrent protection
The CTs must have a rated equivalent secondary e.m.f. E
al
that is larger than or equal to
the required secondary e.m.f. E
alreq
below:
sn
R
al alre q CT L
2
pn n
op
I
S
E E 1.5 I R R
I I
³ = × × + +
æ ö
×
ç ÷
è ø
EQUATION1678 V1 EN
(Equation 181)
where:
I
op
The primary operate value (A)
I
pn
The rated primary CT current (A)
I
sn
The rated secondary CT current (A)
I
n
The nominal 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 grounded systems. The resistance of a
single secondary wire should be used for faults in high impedance grounded systems.
S
R
The burden of an IED current input channel (VA). S
R
=0.010 VA/channel for I
r
=1 A and S
R
=0.250 VA/
channel for I
r
=5 A
Section 18 1MRK 506 334-UUS A
Requirements
494
Application manual
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