7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices
Chapter 7 Page 21 of 31
Section 3: Current Transformer (CT) Requirements
3.1 CT ratio
The CT primary rating is usually chosen to be equal to or greater than the motor full load current.
Amps
86.3
3
.
3
3
493
kV
3
KVA
current
load
Full
KVA
493
0.955
0.85
400
efficiency
P.F.
Output
power
Input
=
×
=
=
=
×
=
×
=
The C.T. ratio should be chosen as equal to or the next standard above the motor full load rating here a 100 Amp
primary is chosen. The secondary could be either 1 or 5A, to reduce CT burden a 1A secondary is used, so a
current ratio of 100/1 is chosen.
The motor full load CT secondary current is:
)
86
.
0
(
86
.
0
100
1
3
.
86
In
A
Ratio
CT
MotorFLC
×
=
=
×
=
3.2 Thermal and Overcurrent Protection CTs
Motors are typically located in industrial environments with relatively low fault current. The motor is the end load of
the system and therefore has no onerous grading requirements. The CT can typically be a class 10P10 with VA
rating to match the load.
Definite Time and Instantaneous Overcurrent
For industrial systems with requirements as for i.d.m.t.l. relays item (a) above, a class 10P10 (or 20).
Note that overcurrent factors do not need to be high for definite time protection because once the setting is
exceeded magnitude accuracy is not important.
3.3 Earth Fault Protection CTs
Considerations and requirements for earth fault protection are the same as for Phase fault. Usually the relay
employs the same CT's e.g. three phase CTs star connected to derive the residual earth fault current.
The accuracy class and overcurrent accuracy limit factors are therefore already determined and for both these
factors the earth fault protection requirements are normally less onerous than for overcurrent.
3.4 Sensitive Power (32S)
For sensitive reverse power applications e.g. Ps < 5%Pn class 1 metering CTs are recommended with a rated
burden to match the secondary load connected.
For higher settings a 5P10 CT can be used.
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