MV design guide - Schneider Electric

Switchgear
definition
Current transformer
Secondary circuit's characteristics
according to IEC standards
Rated secondary current (Isr) 5 or 1 A?
c General case:
v for local use Isr = 5 A
v for remote use Isr = 1 A
c Special case:
v for local use Isr = 1 A
N.B.: Using 5 A for a remote application is not forbidden but leads to an increase in transformer
dimensions and cable section, (line loss: P = R I 2 ).
Accuracy class (cl)
c Metering: class 0.5
c Switchboard metering: class 1
c Overcurrent protection: class 10P sometimes 5P
c Differential protection: class X
c Zero-sequence protection: class 5P.
Example:
c Cable section: 2.5 mm 2
c Cable length
(feed/return):
5.8 m
Real power that the TC must provide in VA
This is the sum of the consumption of the cabling and that of each device
connected to the TC secondary circuit.
c Consumption of copper cabling (line losses of the cabling),
knowing that: P = R.I 2 and R = ρ.L/S then:
(VA)
L
= k •
S
c Consumed power
by the cabling:
1 VA
k = 0.44 : if Isr = 5 A
k = 0.0176 :
if Isr = 1 A
L : length in metres of link conductors
(feed/return)
S : cabling section in mm 2
c Consumption of metering or protection devices.
Consumption of various devices are given in the manufacturer's technical
data sheet.
Rated output
Take the standardised value immediately above
the real power that the CT must provide.
c The standardised values of rated output are:
2.5 - 5 - 10 - 15 - 30 VA.
Safety factor (SF)
c Protection of metering devices in the case of a fault is defined by the safety
factor SF. The value of SF will be chosen according to the current consumer's
short-time withstand current: 5 ≤ SF ≤ 10. SF is the ratio between the limit of
rated primary current (Ipl) and the rated primary current (Ipr).
SF =
Ipl
Ipr
c Ipl is the value of primary current for which the error in secondary
current = 10 %.
Schneider Electric
Merlin Gerin MV design guide
57