The potential transformer symbol is two circles joined by a link — primary winding above, secondary below — and the parts that decide whether the drawing is safe are the polarity dots at one end of each winding and the earth symbol on exactly one point of the secondary. Read those two marks before you read the ratio, because a correct ratio with reversed polarity or a floating secondary will still trip a feeder for no reason the protection engineer can find.
Voltage transformers sit on the boundary between two trades. The protection engineer treats them as a source of measured voltage; the panel builder treats them as a component with terminals; the buyer sees a line in a bill of materials. The symbol is the only place where all three read the same thing, which is why it carries more information than its two circles suggest: winding arrangement, polarity, earthing point, and the fuses or links that isolate it for testing.
What Does the Potential Transformer Symbol Have to Show?
At minimum: that a voltage-transforming device exists at that point, which winding arrangement it uses, where the polarity reference sits, and where the secondary is earthed. A bare pair of circles does the first job and none of the others, and that is how drawings reach a supplier with the earthing point left to be "decided on site" — a decision nobody then makes.
The accuracy side of the device is not in the symbol at all. Ratio, accuracy class, rated burden and the voltage factor belong in the schedule, and the requirements behind them come from IEC 61869-3 for inductive voltage transformers, with marking, error limits and the general test schedule set by IEC 61869-1. A drawing that shows the symbol and stops there is a drawing that will be clarified by email four times before the panel is built.
One Circle, Two Circles, Three Circles: Potential Transformer Symbol Variants
The same device is drawn differently depending on how many windings it has and what those windings are for. Four arrangements cover most distribution work, and each has a different consequence for the secondary wiring:
| Arrangement on the drawing | What it is | Typical secondary | What the buyer must confirm |
|---|---|---|---|
| Two circles, single device | Single-phase inductive VT, one primary winding | One winding, 100 V or 100/√3 V | Connection phase-to-phase or phase-to-earth, since the ratio follows it |
| Three devices in star | Three single-phase units connected phase-to-earth | Three windings, star point at the terminal box | Where the star point is earthed, and whether a residual winding is added |
| Three devices, broken delta | Residual winding set for earth-fault detection | Open-delta residual output | Residual voltage at rated earth-fault condition, and the earthing of that winding |
| One circle with a capacitor divider | Capacitive voltage transformer | One or two windings, plus carrier accessories | Divider accuracy over temperature and the burden the carrier equipment adds |
For metal-enclosed panels the star-connected three-unit arrangement is the common case, which is why the indoor cast-resin types — the JDZ and JDZX families in the voltage transformer range — are usually ordered as a set of three rather than as single units. A drawing that shows one symbol where the panel needs three is worth querying before the bill of materials is fixed.
Polarity Marks on the Potential Transformer Symbol
The dot, or the labels P1 and P2 on the primary with S1 and S2 on the secondary, fixes the direction of the winding. It says: when P1 is instantaneously positive with respect to P2, S1 is instantaneously positive with respect to S2. That is all it means, and it is enough to make or break an earth-fault scheme, because residual voltage is built by summing three secondary voltages. One reversed unit subtracts instead of adding, and the residual that should appear on a single-phase earth fault does not.
On site the check is done with a polarity test, not by reading the drawing, and the test result belongs in the routine test record. The PT test sequence and its pass criteria set out what that record has to show. What the drawing must do is state the reference clearly enough that the tester knows which terminal the supplier calls S1 — because on a cast-resin unit the marking is a small moulded letter, and on a three-phase set it is easy to lose one unit's reference in a terminal box with twelve terminals.
Why the Secondary Is Earthed at One End Only
The secondary winding is a low-voltage circuit that is magnetically, not electrically, separated from the primary. If the primary-to-secondary insulation fails — a resin void, a tracking path, a lightning surge — the whole secondary circuit, and every relay, meter and terminal it reaches, rises to primary potential. Earthing one end of the secondary gives that fault current a defined path to earth and holds the secondary circuit near earth potential, so the insulation inside the relay is not asked to withstand 11 kV. In a metal-enclosed panel that path ends on the earth bar, which is part of the cabinet construction and its earthing arrangement, not a separate fitting added afterwards.
It also stops the secondary floating to an unpredictable potential through capacitance and leakage, which is what produces the "live" terminal box that a fitter discovers with the back of a hand. And it gives the protection scheme a stable reference: residual and open-delta circuits only produce a meaningful zero-sequence voltage if their reference point is fixed.
One end, not two. Earthing both ends of the same winding puts the earth grid and the secondary leads in parallel, and any potential difference between the two earth points — exactly what a primary earth fault produces — drives current through the secondary circuit. The result is a voltage error that appears only during faults, which is the worst kind: it is invisible in every bench test and wrong in every fault record.
What Goes Wrong When the Earthing Rule Is Broken
Three failures recur, and all three are commercial events rather than theoretical ones. The first is a secondary earthed at the panel end and again at the meter end, common when two contractors each earth "their" side. Commissioning tests pass, then a primary earth fault produces a residual voltage that does not match the calculation, and the earth-fault relay either fails to operate or operates on a healthy feeder.
The second is a secondary left unearthed because the drawing showed no earth symbol and nobody added one. That is an insulation-coordination defect that a utility inspector can reject at handover, and fixing it means opening every terminal box in the run. The third is an earth taken from the wrong point — the star point of one set bonded to the residual winding of another — which quietly mixes two reference systems. The cost is not the wire; it is the re-test of the whole protection scheme and the delay to energisation.
What the RFQ and Datasheet Must State
Phrase these as requirements so the supplier answers them, rather than as questions the drawing is expected to settle later:
- Winding arrangement and connection: phase-to-earth or phase-to-phase, single unit or set of three, residual winding required or not
- Ratio stated as primary/secondary with the secondary value written out, not implied — 100 V or 100/√3 V changes the ratio and the relay setting
- Accuracy class and rated burden per winding, with the metering and protection duties named separately
- Earthing point identified on the drawing and repeated in the datasheet, with the terminal designation used by the supplier
- Polarity marking convention and the terminal identification drawing for the actual unit
- Primary and secondary fuses or links, their ratings, and whether they are supplied with the unit
- Routine test record and type test report, with the ratio, polarity, excitation and withstand results for the units supplied
Where Nahu Group Fits
Nahu Group manufactures indoor cast-resin and outdoor voltage transformers for 10 kV to 35 kV distribution systems, single-phase and three-phase, with metering and protection windings and residual outputs where the scheme needs them. The types referred to above — JDZ10-10B, JDZ9-35 and JDZX9-35 — are catalogue models in the range. To get a configuration, send the single-line diagram with the earthing point marked, the ratio and burden per winding, the accuracy classes, and the panel arrangement. If the network is unearthed or resistance-earthed, say so: it changes both the winding choice and the ferroresonance risk the VT has to survive.
Frequently Asked Questions
Does the potential transformer symbol show the ratio?
No. The symbol shows the device and its winding arrangement; the ratio belongs in the schedule and on the nameplate. A drawing with a ratio written beside a symbol is a convenience, not a specification, and it is not used for ordering unless the same value appears in the device schedule that forms part of the contract.
Is the earthing rule for a VT the same as for a CT?
The principle is the same and the reason differs slightly. Both are earthed at one point to keep the secondary near earth potential and to fix the reference, and both misbehave when earthed twice. A CT secondary must never be open-circuited, while a VT secondary must never be short-circuited, so the terminal arrangements and the test procedures differ. The CT secondary earthing practice covers the current-transformer side, and the current transformer range shows the terminal arrangements it refers to.
Where exactly should the single earth point be?
At the winding's reference terminal, normally the star point for a star-connected secondary, taken to the panel earth bar by the shortest practical route. What matters is that it is one point for the whole circuit and that the point is documented, so a later modification does not add a second one at the far end of the multicore.
Can a VT secondary be earthed through the relay or meter instead?
No. Earthing through a device means the device carries any fault current that the earthing path is supposed to divert, and the device's own insulation becomes the weak link. Earth at the terminal, then run the circuit to the device. This is also why the earth conductor is sized as a protective conductor, not as a signal wire.
What happens if one unit of a three-phase set has reversed polarity?
The residual voltage on an earth fault no longer adds correctly, so earth-fault protection either under-reaches or misoperates, and phase-to-phase voltages stay apparently normal. It is caught by a polarity check on each unit, which is why the test record is a per-unit document rather than a per-set one.
Sources and Further Reading
- IEC 61869-3:2011 — Instrument transformers, Part 3: Additional requirements for inductive voltage transformers — the product standard for inductive VTs, covering voltage error, phase displacement, rated burden and the tests that demonstrate them
- IEC 61869-1:2023 — Instrument transformers, Part 1: General requirements — common definitions, marking and terminal designation, error limits, and the type, routine and commissioning test schedule
- IEC 61869-2:2012 — Instrument transformers, Part 2: Additional requirements for current transformers — the companion part, useful when the VT and CT secondaries share a terminal compartment and a reference point
Published Nov 9, 2026 · Nahu Group (NAHU Electric Technology)