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Examoogle • User • info@examoogle.com • EE-2024-8821
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Most used rated secondary current in CT is
3A
5A
1A
2A
5A
Quick Summary: In Current Transformers (CTs), the rated secondary current is the value at which the CT is designed to operate to supply connected metering or protection devices. 5A is the traditional and most widely used standard rated secondary current globally for power system instrumentation.
In Current Transformers (CTs), the rated secondary current is the value at which the CT is designed to operate to supply connected metering or protection devices. 5A is the traditional and most widely used standard rated secondary current globally for power system instrumentation.
Kn=IsIp — where Kn is the turns ratio, Ip is primary current, and Is is rated secondary current.
Ploss=Is2×Rw — representing power loss in secondary leads, where Rw is lead resistance.
The CT operates on the principle of magnetic induction, where the primary current creates a magnetic flux in the core, inducing a proportional current in the secondary winding based on the turns ratio. The secondary current rating is standardized to ensure compatibility with standard ammeters, wattmeters, and relay coils. While 1A is becoming more common in long-distance substation wiring to reduce I2R power losses, 5A remains the most prevalent standard due to historical legacy and lower burden requirements for shorter lead lengths.
Standardization of 5A facilitates the use of universal protective relays and measuring instruments.
Lower secondary current (e.g., 1A) is preferred in high-voltage switchyards to minimize cable copper cross-section and losses over long runs.
The secondary of a CT should never be left open-circuited while primary current flows, as it can cause high flux saturation and dangerous secondary voltages.
Typical rated secondary currents are 1A, 2A, and 5A according to IEC/IS standards.
High compatibility with legacy protection and metering equipment.
Lower internal impedance of standard 5A relay coils compared to 1A coils.
Higher secondary currents lead to higher I2R heat dissipation in long control cables.
Requires heavier cable cross-sections compared to 1A systems for long-distance remote metering.
Power system protection relays (overcurrent/earth fault).
Energy and current metering in substations and distribution panels.
Standard secondary currents are defined by IEC 60044 or IS 2705.
Option C (1A) is increasingly used in modern installations to reduce copper costs, but 5A remains the most widely installed legacy standard.
3A and 2A are not standard rated values for secondary output current in conventional CTs.
B is correct — 5A is the most standard and widely used rated secondary current for Current Transformers in power systems.
Always remember that while 1A is technically superior for long cable runs, 5A remains the most frequently encountered answer in competitive exams due to legacy standardization.