Current transformers (CTs) and voltage transformers (VTs, also known as potential transformers, PTs) are instrument transformers widely used in power systems. They reduce high primary current or voltage to low, standard secondary values for measurement, protection and control. Although both isolate low-voltage instruments from high-voltage power circuits, they differ fundamentally in working principles, connection rules and load requirements.
A current transformer is designed to step down large load currents. Its primary winding usually has only a few turns and is connected in series with the power line. The secondary winding carries a proportional, smaller current, commonly 5 A or 1 A. The core principle is Ampere’s law: the primary magnetomotive force balances the secondary magnetomotive force under normal operation. A critical safety rule is that the secondary side must never be left open. An open secondary creates extremely high induced voltage, which can damage equipment and endanger operators. CT secondary circuits are always shorted by meters or relays, and the connected burden must stay within the rated range. Typical applications include overcurrent protection, energy metering and current monitoring in switchgears.
A voltage transformer, by contrast, steps down high system voltage to a standard low secondary voltage such as 100 V. Its primary winding has many turns and connects in parallel across the phase or phase-to-ground voltage. It works like a conventional power transformer based on Faraday’s induction law, maintaining an accurate voltage ratio. The secondary side should not be short-circuited. A short circuit generates huge secondary current, burning windings and destroying the transformer. VTs supply reference voltage for protective relays, voltmeters, power factor meters and energy meters.
The burden definition also separates the two types. CT burden refers to the impedance of the secondary loop, including cable resistance and instrument input impedance. VT burden describes the total power drawn by connected loads, measured in volt-amperes. Accuracy classes are selected for different purposes: high accuracy grades are required for revenue metering, while slightly lower grades can be used for protection.
In summary, CTs measure series current and forbid open secondary circuits, whereas VTs measure parallel voltage and forbid shorted secondary circuits. Both provide galvanic isolation between high-voltage networks and low-voltage control panels. Correct selection and wiring prevent catastrophic failure and ensure reliable power measurement and relay operation in industrial and utility electrical systems.
