Which of the following reduces high voltages and currents to save values for measurements?

• A. Auto transformer
• B. Megohmmeter
• C. Instrument transformer
• D. Variable transformer

Answer: (C)

Instrument transformers are the way to measure high voltages and currents safely by scaling them down to values your meters can handle while providing electrical isolation from the live circuit. A current transformer converts a large current in a conductor into a smaller, proportional current on its secondary winding (often 1 A or 5 A) so meters and relays can read it without being in the high-current path. A potential (voltage) transformer does the same for voltage, stepping a high line voltage down to a safe, standard level for measurement. This scaling preserves the relationship between magnitude and phase, which is essential for accurate reading and proper protection coordination, and it protects both people and equipment from dangerous voltages.

The other options serve different purposes: an auto transformer changes voltage levels without providing isolation, not primarily for safe measurement; a megohmmeter tests insulation resistance by applying DC voltage and isn’t used for normal measurement of energized systems; a variable transformer can adjust voltage but also lacks the necessary isolation and standard scaling for metering.