A potential transformer is an instrument transformer that steps down high voltage from a power line to a low, safe level that meters and protective equipment can read. In most systems, it converts voltages that can reach hundreds of thousands of volts down to a standard 120V or 110V secondary output. You’ll find these devices wherever utilities and industrial facilities need to accurately measure or monitor voltage without exposing instruments or personnel to dangerous high-voltage circuits.
How a Potential Transformer Works
Like any transformer, a potential transformer operates on the principle of electromagnetic induction. It has two coils of wire, a primary winding and a secondary winding, wrapped around a shared magnetic core. When alternating current flows through the primary winding, it creates a changing magnetic field in the core. That magnetic field induces a proportional voltage in the secondary winding. Because the primary winding has far more turns of wire than the secondary, the output voltage is much lower than the input.
The ratio between the primary and secondary turns determines how much the voltage is reduced. Common ratios include 20:1 (converting 2,400V to 120V), 35:1 (4,200V to 120V), and 40:1 (4,800V to 120V). The transformer doesn’t change the frequency of the electrical supply or consume significant power itself. It simply delivers a scaled-down copy of the line voltage that meters, relays, and protective devices can safely use.
Electromagnetic vs. Capacitive Types
There are two main designs: electromagnetic potential transformers and capacitive voltage transformers (CVTs).
An electromagnetic potential transformer is the simpler of the two. It uses straightforward electromagnetic induction with no additional components altering the primary voltage. These are reliable and accurate, and they work well at lower and medium voltage levels.
Capacitive voltage transformers take a different approach. They use a capacitor voltage divider, a stack of capacitors in series inside a porcelain sleeve, to first reduce the high voltage to an intermediate level. A medium-voltage transformer then steps it down further to the standard secondary output. CVTs also include a compensating reactor and a damping device inside a sealed oil-filled tank. The damping device, made up of a resistor and reactor connected across the secondary winding, suppresses a phenomenon called ferromagnetic resonance that can occur due to the interaction between the capacitors and the transformer core. CVTs are more economical and safer for very high voltage applications, and they can also carry communication signals along transmission lines for remote measurement and protection systems.
How It Differs From a Current Transformer
Potential transformers and current transformers are both instrument transformers, but they measure different things and connect to the power system differently. A potential transformer connects in parallel across the power line so the full line voltage appears across its primary winding. It steps voltage down, typically to 100 to 220 volts on the secondary side. A current transformer connects in series with the power line so the full line current flows through it. It steps current down, typically producing 1 to 5 amperes on its secondary side.
The construction differs too. A potential transformer’s core is made of high-quality steel operating at very low magnetic flux density to maintain accuracy. A current transformer’s core uses laminated steel designed for different magnetic conditions. One critical safety difference: a current transformer’s secondary terminals must never be left open while the primary is energized, because dangerous high voltages can develop across them. A potential transformer doesn’t carry this same risk, though its secondary circuit still requires proper grounding.
Three-Phase Connections
In three-phase power systems, potential transformers can be wired in several configurations depending on the application. The two most common are wye (also called star) and open-delta connections.
In a wye configuration, one side of each transformer connects to a common neutral point, and the other side connects to a phase conductor. The line voltage in a balanced wye system equals the phase voltage multiplied by the square root of 3 (approximately 1.73). This setup works well when you need to measure each phase voltage individually relative to ground.
In a delta configuration, each transformer connects directly across a pair of line conductors. The line voltage equals the phase voltage in this arrangement. Open-delta connections use only two transformers instead of three, reducing cost while still providing the necessary voltage measurements for metering and protection.
Burden and Accuracy
The “burden” of a potential transformer is the total load placed on its secondary winding by whatever meters, relays, and instruments are connected to it. Burden is measured in volt-amperes (VA), and it directly affects how accurately the transformer reproduces the primary voltage.
Potential transformers carry accuracy class ratings that tell you how precisely they’ll perform across a range of burdens. A transformer rated 0.3 WXMY, for example, maintains 0.3% accuracy from zero load up to 75 VA. Manufacturers test accuracy at both zero burden and full rated burden, so if you’re operating somewhere in between, your actual accuracy falls along a line between those two tested points. Overloading the secondary beyond its rated burden degrades accuracy and can affect the performance of metering and protection equipment downstream.
Secondary Grounding for Safety
Grounding the secondary circuit of a potential transformer is a critical safety requirement. Without grounding, electrical charge can accumulate on the secondary circuit, creating a shock hazard for anyone who touches the connected equipment. If insulation between the primary and secondary windings were to fail, an ungrounded secondary could rise to the full primary voltage, putting both personnel and instruments at risk.
IEEE and the National Electric Safety Code specify that the secondary circuit should be grounded at a single location, using a grounding conductor no smaller than 12 AWG copper wire connected to the ground bus at the switchboard. Grounding at only one point prevents circulating currents that could introduce measurement errors. These requirements exist to protect both the people working near the equipment and the sensitive instruments connected to the transformer’s output.
Where Potential Transformers Are Used
Potential transformers serve two primary purposes in electrical systems: metering and protection. For metering, they provide the scaled-down voltage signal that revenue meters, power quality analyzers, and monitoring equipment need to calculate energy usage and system performance. For protection, they feed voltage readings to relays that detect faults, overvoltage conditions, or other abnormalities and trigger circuit breakers to isolate the problem.
You’ll find them in substations, industrial switchgear, generating stations, and anywhere high-voltage circuits need to be monitored. The design and performance requirements for these transformers are governed by international standards, primarily IEEE C57.13 in North America and the IEC 61869 series internationally. These standards define accuracy classes, insulation requirements, and testing procedures to ensure that the voltage readings downstream are trustworthy enough for billing and protection decisions.