The standard symbol for a current transformer (CT) in North American IEEE schematics is two overlapping circles representing the primary and secondary windings, while the international IEC standard uses a single circle with a straight line passing through it. Identifying the correct symbol, understanding its polarity marks, and interpreting regional variants is critical for proper metering and protective relay coordination.
The Complete Current Transformer Symbol Reference Table
Before wiring any CT to a digital multimeter, power analyzer, or protective relay (like an SEL or GE Multilin), you must correctly interpret the single-line diagram. The table below maps the most common schematic symbols to their physical counterparts and governing standards.
| Symbol Description | Governing Standard | Primary / Secondary Representation | Common Application |
|---|---|---|---|
| Two overlapping circles | IEEE C57.13 (US) | Both windings shown as distinct loops | North American switchgear, ANSI device 50/51 diagrams |
| Single circle with straight line through center | IEC 61869-2 (Intl) | Line = primary conductor, Circle = secondary | European/Asian MV/HV substations, IEC 61850 networks |
| Circle with a looped line through it | IEEE / IEC | Looped line = primary passed through window twice | Low-current applications requiring double the turns ratio |
| Circle with a gap or square bracket | Proprietary / General | Split-core or clamp-on secondary | Retrofit energy monitoring, temporary power logging |
| Three overlapping circles | IEEE C57.13 | Primary, secondary, and tertiary/auxiliary winding | Specialized differential protection or core-balance schemes |
Regional and Standard Variants Explained
When reading schematics from international equipment manufacturers, you will encounter conflicting representations of the same physical device. Knowing which standard applies to your region prevents catastrophic wiring errors.
IEEE C57.13 (North America)
In the US and Canada, the Electrical Engineering Portal and standard ANSI/IEEE single-line diagrams rely heavily on the overlapping circles. The primary winding is typically drawn on the left or top, and the secondary on the right or bottom. Terminal designations strictly follow the H and X convention: H1/H2 for the primary line and load, and X1/X2 (up to X5 for multi-ratio CTs) for the secondary.
IEC 61869-2 (International)
The IEC standard simplifies the drawing to a single circle intersected by a straight line. This visually reinforces the physical reality of a toroidal CT where the primary is just a straight busbar passing through the secondary's magnetic core. IEC schematics abandon the H/X nomenclature in favor of P (Primary) and S (Secondary), marking terminals as P1/P2 and S1/S2.
Legacy BS 3938 (Old UK)
On older British schematics, you may find a CT represented by a simple rectangle with a diagonal line, or a circle with the letters 'CT' inside. While largely superseded by IEC standards, maintenance electricians working on legacy 11kV/33kV switchgear in the UK and Commonwealth nations still encounter these. Treat them functionally identical to IEC symbols, but verify terminal markings physically on the nameplate.
Markings People Get Wrong and Faded Label Recovery
Symbols on paper are clean; physical CTs in a damp, 40-year-old switchgear cabinet are not. Here are the most common interpretation failures and how to resolve them on the bench.
The Multi-Ratio Tap Trap
A symbol showing a secondary winding with multiple tap points (e.g., X1, X2, X3, X4, X5) indicates a multi-ratio CT (such as a 100/200/300:5A model). The mistake: Electricians often short all unused taps to ground 'for safety'. The reality: Shorting an unused tap section (like X3 to X4) creates a shorted turn on the transformer core. This induces massive circulating currents in that specific winding section, leading to localized overheating and core failure. Only short the active terminals (e.g., X1 to X2) if the circuit must be disconnected.
Recovering Faded Polarity Marks (The 9V Kick Test)
If the physical nameplate is faded and you cannot confirm which terminal is X1 (the polarity mark corresponding to the dot on the schematic), do not guess. Wrong polarity will cause an ANSI 87 differential relay to trip instantly on load, or cause a wattmeter to read negative power. Use this safe, de-energized bench test:
- Disconnect the CT entirely from the circuit. Ensure it is de-energized.
- Connect an analog multimeter (set to low DC millivolts) or a digital meter with a min/max capture feature across X1 and X2. Meter positive to suspected X1.
- Momentarily tap a 9V battery across the primary terminals: Battery positive to H1, negative to H2.
- Observe the meter kick. If the needle kicks upscale (positive), your H1 and X1 are correctly identified. If it kicks downscale (negative), the X1/X2 labels are swapped relative to H1.
Current Transformer Symbol FAQ
What does the dot on a current transformer symbol mean?
The dot (or small square) adjacent to the winding terminals on a schematic indicates the polarity mark. It designates the instantaneous current direction. When primary current flows into the H1 dot, secondary current flows out of the X1 dot. This is critical for directional relays (ANSI 67) and differential protection (ANSI 87), where the phase angle relationship between voltage and current determines the trip logic.
How do I read a split-core CT symbol on a single-line diagram?
Split-core CTs, used for retrofitting energy monitors like the Schneider PowerLogic or Accuenergy meters, are typically drawn as a circle with a distinct gap, hinge line, or a square bracket enclosing the primary conductor. This tells the installer that the primary busbar does not need to be disconnected to install the device. Electrically, they function identically to solid-core toroidal CTs, though they typically have lower accuracy classes (e.g., Class 1.0 instead of 0.3) due to the air gap at the hinge.
Why does my CT symbol have three overlapping circles?
A three-circle symbol represents a CT with three distinct windings on a single core. This is rare in standard distribution but common in high-voltage transmission or specialized generator protection. The three windings might be configured for metering (high accuracy, low saturation), fast-acting overcurrent protection (high saturation threshold), and a tertiary winding used for core-balance or zero-sequence ground fault detection. Always refer to the manufacturer's technical textbook guidelines to ensure you do not cross-wire the metering and protection cores.
What is the difference between a CT and a PT (Potential Transformer) symbol?
While a CT symbol uses overlapping circles or a line-through-circle to denote current induction via a series conductor, a Potential Transformer (PT or VT) symbol uses two parallel lines or two non-overlapping circles separated by a space. This visually represents a parallel-connected voltage transformer with distinct primary and secondary coils, rather than a single primary pass-through. Confusing the two on a schematic usually results in attempting to wire a PT in series with a load, which will immediately blow the PT's primary fuses.
