Potential Transformers: Ratios, Accuracy Classes and How to Specify One

Direct answer: a potential transformer (PT), also called a voltage transformer (VT), steps a high system voltage down to a low, standardised value — typically 100 V or 110 V — so that meters and protection relays can measure it safely. You specify one by system voltage, ratio, accuracy class, burden and insulation type.

One clarification first, because it causes real confusion when buying. A potential transformer is a measuring instrument. It is not a variac, not a voltage regulator, and not a control transformer. Those devices exist to supply power at an adjusted voltage. A PT exists to represent a voltage accurately at a level an instrument can handle. Ordering the wrong family is one of the more expensive mistakes in this category, and the product names overlap enough to make it easy.

What the Ratio Actually Means

A PT ratio is written as primary over secondary: 10000/100 V, often shortened to 10 kV/100 V. That is a 100:1 ratio. Feed 10 kV into the primary and the secondary presents 100 V to your meter.

Standard secondary values are 100 V, 110 V, and 120 V depending on regional practice, with 100 V and 110 V dominant across IEC markets. Three-phase units are frequently specified as 10000/√3 over 100/√3 for phase-to-neutral connection — the √3 notation trips people up, but it simply reflects star connection rather than a different device.

Accuracy Classes

As with current transformers, metering classes and protection classes solve opposite problems. A metering PT is optimised for precision at normal operating voltage. A protection PT (3P, 6P) must stay usable during voltage collapse and fault conditions, where a metering unit's guaranteed band no longer applies. Do not substitute one for the other.

Burden — The Number Buyers Skip

A PT's accuracy class is only valid within its rated burden, expressed in VA. Burden here is the total load on the secondary: the meter, the relay coils, and the secondary wiring.

The practical failure mode is cumulative. One meter draws little. Add a second meter, a transducer and a relay to the same secondary and the total can exceed the rated VA — at which point the PT is no longer performing to its stated class even though nothing is broken. Add up the actual burden before ordering, and leave headroom for future additions.

PT vs CT — They Are Not Interchangeable

Note the last row carefully, because the hazards are inverted. Never short a PT secondary — it behaves like a shorted voltage source and draws destructive current. Never open a CT secondary under load — it develops dangerously high voltage. Applying CT habits to a PT, or the reverse, is genuinely hazardous.

For the current-side equivalent of this guide, see our CT accuracy class guide.

Epoxy-Cast vs Oil-Immersed

Real Reference Models

• JDZ10-10 — 10 kV single-phase, epoxy resin fully enclosed, Class 0.2 / 0.5. Indoor switchgear metering.
• JDZW-10R — 10 kV single-phase with fuse, coil type.
• JDZW-35/0.2 — 35 kV dry type for measurement and protection duty.
• JDJJ2-35 — 35 kV single-phase oil-immersed, outdoor.
• JLS-10 — 10 kV five-column combined metering box (PT and CT in one outdoor unit).
• Three-phase cast-insulation units with fuse protection are also produced to order.

Specification Checklist

1. System voltage — 3, 6, 10, 35 kV.
2. Ratio — and whether phase-to-phase or phase-to-neutral (√3).
3. Accuracy class — 0.2 or 0.5 for metering; 3P/6P if protection.
4. Burden in VA — total, with headroom.
5. Indoor or outdoor — drives epoxy-cast vs oil-immersed.
6. Single or three phase.
7. Fuse protection — required on many MV installations.
8. Frequency — 50 or 60 Hz.

FAQ

Is a potential transformer the same as a voltage transformer?

Yes. "PT" is common in North American and Indian practice, "VT" in IEC/European usage. Same device. Both differ entirely from a variac or voltage regulator.

Can I use a PT to power a small load?

No. It is an instrument transformer sized for a measurement burden in VA, not a supply transformer. Loading it like a power source destroys its accuracy and can damage it.

Why does my PT read slightly low?

Most often the secondary burden exceeds the rated VA. Total up every connected instrument. Also verify the secondary wiring is sized adequately for the run length.

What is ferroresonance and should I worry about it?

It is an oscillation that can occur between a PT's magnetising inductance and system capacitance in ungrounded systems, producing damaging overvoltages. It is a real design consideration on MV networks — discuss your system earthing arrangement with us when specifying.

Do you supply test certificates?

Yes. Units are tested before dispatch and documentation is available for utility submission on request.

Factory-Direct Supply

Zhejiang Yomin Electric has manufactured instrument transformers since 1996 and ships to 95+ countries. Our potential transformer range covers 3–35 kV in epoxy-cast and oil-immersed construction, single and three phase, Class 0.2 and 0.5. Browse the range or send your system voltage, ratio, class and burden for a quotation within 24 hours.