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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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C.T. and P.T. are used for
Measuring low current and voltages
Measuring very low current and voltages
Measuring high current and voltages
Measuring intermediate currents and voltages
Measuring high current and voltages
Quick Summary: C.T. (Current Transformer) and P.T. (Potential Transformer), collectively known as Instrument Transformers, are used to step down high currents and voltages to standard low values. This allows the use of standard, safe, and cost-effective low-range measuring instruments for monitoring high-power electrical circuits.
C.T. (Current Transformer) and P.T. (Potential Transformer), collectively known as Instrument Transformers, are used to step down high currents and voltages to standard low values. This allows the use of standard, safe, and cost-effective low-range measuring instruments for monitoring high-power electrical circuits.
Is=Ip×NsNp — Transformation ratio of C.T.
Vs=Vp×NpNs — Transformation ratio of P.T.
A C.T. works on the principle of electromagnetic induction by stepping down current proportional to the primary load, keeping secondary current at standard ratings like 1A or 5A. A P.T. acts like a step-down transformer to reduce high transmission voltage to a standard secondary voltage, typically 110V, providing galvanic isolation and safety for measuring equipment.
C.T. and P.T. provide electrical isolation between high-power circuits and measuring instruments.
The secondary of a C.T. should never be left open-circuited while primary is energized due to high flux density causing saturation and lethal voltage spikes.
Instrument transformers enable the measurement of high energy parameters without direct contact with hazardous voltage levels.
Electrical isolation enhances operator safety.
Standardization of secondary ratings (5A/1A and 110V) allows mass production of uniform meters.
Introduces small transformation error (ratio and phase angle error).
Requires proper calibration and maintenance.
Energy metering and billing systems.
Protection relaying in substations.
Power quality monitoring systems.
C.T. is a series device (primary in series with line), while P.T. is a parallel device (primary across the line).
Option A and B are incorrect because these transformers are designed specifically to handle high magnitudes, not low magnitudes.
C is correct — C.T.s and P.T.s are utilized to step down high currents and high voltages to safe, manageable levels for measurement and protection.
Always remember that a C.T. secondary must be shorted if not connected to a load to prevent high voltage induction due to lack of flux cancellation.