The range can be increased by using which of the following instrument?

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Examoogle · Accepted Answer

Current Transformer

Explanation: A Current Transformer (CT) is an instrument transformer used to step down high currents to a standardized low value, typically 5A or 1A, which allows for the measurement of high-magnitude currents using low-range ammeters. By changing the turns ratio, the effective range of the connected measurement instrument is significantly increased.

Working Principle: The CT operates on the principle of electromagnetic induction similar to a standard transformer. The primary winding, connected in series with the high-current circuit, has few turns, while the secondary winding has a large number of turns. According to the transformer law $N_1 I_1 = N_2 I_2$, the secondary current $I_2$ is proportional to the primary current $I_1$ scaled by the turns ratio $N_1/N_2$, allowing a low-range ammeter to measure a scaled version of the primary current.

Key Formulas: $I_2 = I_1 	imes \frac{N_1}{N_2}$ — Secondary current relationship; $K = \frac{N_2}{N_1}$ — Transformation ratio of the CT

Key Points: CTs isolate measurement devices from high-voltage primary circuits.; The secondary of a CT should never be open-circuited while the primary is energized to prevent high induced voltages and core saturation.; Standard secondary current ratings are typically 1A or 5A.

Advantages: Allows measurement of very high currents using standard low-range instruments.; Provides electrical isolation between the high-voltage circuit and the secondary metering circuit.; Reduces the cost of metering infrastructure.

Disadvantages: Risk of high voltage induction if the secondary circuit is opened.; Introduces phase angle errors and ratio errors in high-precision applications.

Applications: Electrical power grid metering.; Protection relaying for overcurrent conditions.; Industrial energy monitoring systems.

Additional Info: Option B (Voltmeter) uses Potential Transformers (PT) to extend its range.. Option C (Ammeter) without a CT is limited to its own internal shunt capacity.. CT secondary terminals must be shorted before disconnecting the ammeter.

Answer: D is correct — A Current Transformer (CT) is specifically designed to step down high primary currents to a range suitable for measurement by standard ammeters, thereby increasing the effective measurement range.

Exam Tip: Always remember: CTs are connected in series with the line, and their secondaries must be kept closed to avoid high induced EMFs due to core flux saturation.

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