Isolation Transformer Symbol
Definition: The Isolation Transformer symbol represents a transformer with a 1:1 turns ratio used in circuit diagrams to indicate galvanic isolation between two circuits, drawn as two adjacent coils separated by a gap (IEC 60617-11 / ANSI Y32.2 / IEEE 315), with designator T, eliminating direct electrical connection between primary and secondary windings while allowing AC power or signal transfer.
Also known as: 1:1 transformer, galvanic isolator, safety isolator transformer, medical-grade isolator, isolation barrier.
What the Isolation Transformer symbol means
The Isolation Transformer symbol denotes a transformer whose primary and secondary windings have no galvanic (direct electrical) connection, relying solely on electromagnetic induction to transfer energy. The 1:1 turns ratio means the secondary output voltage equals the primary input voltage, but the two circuits share no common ground path, breaking any DC or fault current path between them.
In electrical diagrams the isolation transformer symbol signals a safety or noise-isolation boundary. Medical-grade isolating transformers (IEC 61558-2-15) power operating theatre equipment to prevent leakage current from passing through a patient. In industrial electronics, isolation transformers separate sensitive measurement circuits from noisy power circuits, and in audio they eliminate ground-loop hum.
How to identify the Isolation Transformer symbol
The Isolation Transformer symbol consists of two sets of curved arcs (representing coils of wire) drawn side by side within a rectangle or between two parallel lines. The primary winding is on the left (pins L and N) and the secondary winding is on the right (pins L2 and N2). A clear gap or space between the two coil groups — with no lines connecting them — visually represents the galvanic isolation. Some versions include a shield symbol or dotted line between the coils to indicate an electrostatic screen.
Function in a circuit
An isolation transformer transfers AC electrical power from its primary winding to its secondary winding through magnetic coupling only, with no conductive connection between the two circuits. This breaks the ground reference, so a person contacting the secondary L2 terminal while standing on true earth ground does not complete a shock hazard circuit. It also blocks DC offsets and attenuates common-mode noise, making it valuable in medical, audio, and instrumentation applications.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-11 (transformers and reactors): the isolation transformer uses the standard two-winding transformer symbol — two coil groups face to face — without a core line, or with a single dashed core line if laminated iron is present. IEC 61558-2-15 specifically governs medical isolating transformers. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975: uses the same two-winding transformer symbol with curved arcs for each winding; a '1:1' ratio annotation or the label 'ISO' indicates isolation type. Both IEC and ANSI use the same basic transformer glyph. |
| Key difference | IEC and ANSI glyphs for a two-winding transformer are effectively identical. IEC schematics may show a dashed horizontal core line between the coils for iron-core types; ANSI may omit it. A Faraday shield between windings is shown as a dotted vertical line in both conventions. |
Terminals / pins
| Pin | Name |
|---|---|
| pri_h | L |
| pri_l | N |
| sec_h | L2 |
| sec_l | N2 |
Typical values
Turns ratio: 1:1. Primary voltage: 120 V or 230 V AC (50/60 Hz). Power ratings: 100 VA to 10 kVA typical. Leakage inductance: <1% of magnetising inductance for high-quality units. Interwinding capacitance: <50 pF for medical-grade units. Dielectric isolation: 1.5 kV–4 kV AC between windings.
Where the Isolation Transformer symbol is used
- Medical equipment power supplies in IEC 61558-2-15 IT systems to protect patients from leakage current
- Audio and Hi-Fi signal chains to eliminate ground-loop hum between equipment with different ground potentials
- Test bench setups to safely power line-connected equipment under measurement without exposing the engineer to mains shock
- Industrial instrumentation to isolate 4–20 mA loop power from sensitive analog measurement circuits
- Data center power conditioning to block common-mode noise from upstream UPS systems
- Telecommunications line interfaces to provide galvanic isolation between subscriber equipment and exchange equipment (per ITU-T K.44)
Example
In a hospital patient-monitoring device schematic, an Isolation Transformer symbol shows the 230 V mains input on the primary side (pins L and N) transferring power to the secondary side (L2 and N2) at 230 V with no shared ground. The secondary feeds the device's internal power supply, ensuring that a fault current path through the patient back to mains earth is broken, complying with IEC 61558-2-15 and IEC 60601-1 safety requirements.
Key facts
- The Isolation Transformer symbol represents a 1:1 turns-ratio transformer that transfers AC power by electromagnetic induction while providing galvanic isolation — no direct electrical path — between primary and secondary circuits.
- The symbol shows two coil groups (primary on left: pins L, N; secondary on right: pins L2, N2) drawn face-to-face with a visible gap between them indicating no conductive connection.
- IEC 60617-11 governs the transformer symbol; medical isolating transformers additionally comply with IEC 61558-2-15 and IEC 60601-1 for patient safety.
- Isolation voltage (dielectric strength between windings) is typically 1.5 kV–4 kV AC, verified by a hi-pot test at 1.5× rated isolation voltage.
- Interwinding capacitance in medical-grade isolation transformers is kept below 50 pF to minimise high-frequency leakage current through the capacitive coupling path.
- The designator for an isolation transformer is T per IEC 60617 and ANSI Y32.2 / IEEE 315.
- Unlike an autotransformer, an isolation transformer's primary and secondary windings are physically and electrically separate, making it essential for safety-critical applications.
- A Faraday electrostatic shield between the primary and secondary windings (shown as a dotted vertical line on the symbol) further reduces interwinding capacitance and common-mode noise coupling.
Frequently asked questions
What does the isolation transformer symbol mean in a circuit diagram?
The isolation transformer symbol means that AC power is transferred from the primary winding (L, N pins) to the secondary winding (L2, N2 pins) by electromagnetic induction only, with no direct electrical connection between the two sides. It signals a galvanic isolation barrier that breaks fault current paths and eliminates shared ground references.
What does the isolation transformer symbol look like?
The isolation transformer symbol shows two sets of curved arcs (coils) drawn side by side facing each other, enclosed between two vertical lines or within a rectangle. A visible gap between the two coil groups represents the absence of electrical connection. Primary pins (L and N) connect on the left; secondary pins (L2 and N2) connect on the right.
What is the difference between an isolation transformer and a regular transformer symbol?
The glyph is structurally the same: two coil groups facing each other. The isolation transformer is distinguished by a 1:1 turns-ratio annotation, the absence of a continuous core line (or a dashed core for low-leakage iron-core types), and often by the label 'ISO' or a note indicating galvanic isolation. A step-up or step-down transformer would show a turns ratio other than 1:1.
What standard defines the isolation transformer symbol?
IEC 60617-11 (transformers and reactors) defines the two-winding transformer symbol used for isolation transformers. Medical-grade isolation transformers additionally fall under IEC 61558-2-15. ANSI Y32.2-1975 / IEEE 315-1975 uses an equivalent coil-pair symbol in North American schematics.
What is the designator letter for an isolation transformer?
The reference designator for any transformer, including an isolation transformer, is T per IEC 60617 and ANSI Y32.2 / IEEE 315. In a schematic with multiple transformers they are numbered T1, T2, etc.
Why is an isolation transformer used in medical equipment?
Medical isolation transformers (IEC 61558-2-15) break the earth ground reference for equipment connected to patients, eliminating the shock hazard of fault current passing through a patient's body back to mains earth. They limit leakage current to under 0.5 mA in normal operation, meeting IEC 60601-1 patient safety requirements.
What is the IEC vs ANSI difference for the isolation transformer symbol?
The IEC 60617-11 and ANSI Y32.2 / IEEE 315 symbols for a two-winding transformer are visually identical: two coil-arc groups facing each other. IEC schematics may include a horizontal dashed line between the coils to represent an iron core; ANSI practice often omits this line for air-core types. Both conventions use the same basic glyph.
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