A potential transformer, also called a voltage transformer, steps a high system voltage down to a safe, standardized value that meters and relays can read. Just as you cannot feed thousands of volts directly into a voltmeter, you use a VT to produce a proportional low-voltage copy of the line voltage - commonly around 120 volts at full scale. It is the voltage-sensing counterpart to the current transformer, and together they let instruments measure high-voltage power safely. This guide explains what a VT does, how it differs from a power transformer, and where it fits in a facility.
Potential Transformer (VT) in one line: A potential transformer (VT), also called a voltage transformer or PT, is an instrument transformer that steps a high primary voltage down to a low, standardized secondary voltage - typically about 120 volts - so meters and protective relays can measure system voltage safely and accurately. Unlike a power transformer, it carries almost no load current; its job is faithful voltage measurement, not energy transfer, so it is wound and rated for accuracy rather than power.
A potential transformer is connected across the voltage being measured, in parallel with the line, unlike a current transformer which sits in series with the current. Its primary winding has many turns and its secondary fewer, so the voltage is stepped down by the turns ratio. A VT rated 7200:120, for example, produces 120 volts on its secondary when 7200 volts are applied to the primary, and a relay or meter reading the secondary multiplies by the ratio to know the true line voltage.
The secondary feeds voltmeters, watt and watt-hour meters, power factor meters, synchronizing equipment, and the voltage inputs of protective relays. Because the connected instruments draw very little current, the VT is a lightly loaded transformer sized for precision. Its accuracy class defines how faithfully the secondary voltage tracks the primary in both magnitude and phase, which matters because power and energy calculations combine this voltage with CT current.
VTs are used wherever the system voltage is too high to measure directly, which in most facilities means medium-voltage switchgear and the incoming service. At low voltage, instruments can often be connected directly, so VTs are mainly a medium-voltage instrument.
It is easy to confuse a potential transformer with a power transformer because both change voltage, but their purposes are completely different. A power transformer, sometimes called an energy transformer, exists to transfer real power - it steps voltage up or down to move energy efficiently and carries the full load current of whatever it feeds. A VT transfers essentially no power; it exists only to produce an accurate voltage signal for instruments, so it is small and rated in the tens of volt-amperes rather than the kilovolt-amperes or megavolt-amperes of a power transformer.
The VT also pairs with, but differs from, the current transformer. A CT senses current and is connected in series; a VT senses voltage and is connected in parallel. A CT must never have its secondary opened under load, whereas a VT must never have its secondary shorted, because a short would draw heavy current and could damage it. The two are opposite in almost every respect, and metering and protection systems use both together - CT for amps, VT for volts - to compute power, energy, and directional protection.
Understanding these distinctions prevents real mistakes: sizing a VT like a power transformer wastes it, and treating a VT secondary like a CT secondary during maintenance risks the wrong short-or-open procedure. Each instrument transformer has its own handling rule.
The output of a VT is what lets a facility know its system voltage, and that measurement is valuable well beyond the local meter. When VT-fed meters report voltage, along with CT-fed current, to an RTU or PLC and on to a cloud SCADA platform such as Merobix, an operator sees the actual line voltage at a site trended continuously, correlated with the loads it serves.
That remote visibility catches power-quality problems that would otherwise go unnoticed at an unmanned site. A sagging voltage that trips motors, a phase reading low, or a supply that drops entirely can be seen the moment it happens rather than inferred from stalled production. Because power is computed from VT voltage and CT current together, monitoring both gives a full electrical picture - voltage, current, real power, and power factor - at the site level.
For a distributed oil and gas operation, this means the health of the incoming power feeding pumps, compressors, and heaters is watched the same way the process is, so an electrical issue reaches an operator as a clear alarm instead of surfacing as a mysterious production loss.
A power transformer transfers real energy, stepping voltage up or down to move power efficiently, and carries the full load current. A potential transformer transfers almost no power; it exists only to produce an accurate low-voltage copy of the line voltage for meters and relays. One is sized for energy in kVA or MVA, the other for measurement accuracy at just a few VA.
A VT measures voltage, which is a difference across two points, so it is connected across the line in parallel just like a voltmeter. A CT measures current, which flows through a conductor, so it must be in the current's path in series. This is also why a VT secondary must never be shorted while a CT secondary must never be opened.
A common standard secondary voltage is around 120 volts at full rated primary voltage, so meters and relays everywhere can use the same low-voltage inputs regardless of the actual system voltage. The VT ratio, such as 7200:120, tells the metering system how to scale the reading back up to the true line voltage.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.