A current transformer (CT) is a device that steps down high primary AC current to a safely measurable secondary current, typically 5A or 1A, for metering and protection. When you look at a diagram of current transformer circuits, you aren't just looking at a simple coil; you are looking at a galvanic isolation barrier that allows low-voltage microcontrollers or 120V panel meters to safely monitor a 400A service feeder without ever making direct metallic contact with the primary conductor.
What a Diagram of Current Transformer Shows (and What It Changes)
A standard schematic symbol for a CT consists of two overlapping circles or a single core with primary and secondary windings. The primary winding is often just a single straight line passing through the core window, while the secondary is represented by a multi-turn coil symbol. What this diagram fundamentally changes in a real circuit is the impedance and isolation boundary. It transforms a high-current, high-fault-risk node into a low-current, isolated signal, allowing you to measure power without breaking the primary conductor.
However, reading the diagram also reveals a critical operational constraint: the secondary circuit must never be left open. Unlike a voltage transformer, a CT acts as a constant current source. If the secondary path is broken while primary current flows, the core saturates, and the rapid collapse of the magnetic field induces a lethal high-voltage spike across the open terminals—often exceeding 2,000V, which can arc across terminal blocks and destroy connected equipment.
Diagrams will also specify the secondary rating. Industrial protection relays typically use 5A or 1A secondaries, while maker-bench and home energy monitors use 50mA or 1V output split-core sensors.
Worked Numeric Example: Sizing the Burden Resistor for ESP32
Let's translate a schematic into a physical build using the ubiquitous YHDC SCT-013-000 split-core CT, a favorite for Arduino and ESP32 energy monitoring projects. This sensor outputs an AC current, but our microcontroller's Analog-to-Digital Converter (ADC) reads DC voltage. We must convert the current to a voltage using a burden resistor and bias it to the ADC's mid-point.
The Math
- CT Specs: 100A primary, 50mA secondary. Turn ratio = 100 / 0.05 = 2000:1.
- Target Measurement: 100A RMS. Peak primary current = 100A × √2 (1.414) = 141.4A.
- Peak Secondary Current: 141.4A / 2000 = 0.0707A (70.7mA).
- ESP32 ADC Limits: The ESP32-WROOM-32 ADC reference is 3.3V. To measure a bipolar AC wave, we bias the signal to 1.65V (half of 3.3V). This leaves a maximum peak voltage swing of 1.65V.
- Ideal Burden Resistor: R = V_peak / I_peak = 1.65V / 0.0707A = 23.3Ω.
We select the nearest standard E12 resistor value: 22Ω.
Power Rating Check
We must ensure the resistor won't overheat. Using the RMS secondary current (50mA):
Power (P) = I² × R = (0.05A)² × 22Ω = 0.055W.
A standard 1/4W (0.25W) through-hole resistor is more than adequate, providing a comfortable safety margin.
The Biasing Circuit
Because the ESP32 ADC cannot read negative voltages, the schematic must include a DC bias. We use two 100kΩ resistors in series across the 3.3V and GND pins to create a 1.65V reference, tying the CT's secondary return to this midpoint. A 10µF electrolytic capacitor placed in parallel with the voltage divider filters out high-frequency noise, stabilizing the ESP32 ADC sampling baseline. For higher precision, bypass the internal ESP32 ADC and route the burden voltage into an external 16-bit ADS1115 I2C module.
Where You Meet This in Practice
The physical implementation of a CT diagram varies wildly depending on whether you are on a maker bench or a commercial jobsite.
| Feature | Industrial Switchgear (Protection/Metering) | Maker / Home Energy Monitor |
|---|---|---|
| Core Type | Solid window, silicon steel or nanocrystalline | Split-core ferrite (snap-on) |
| Secondary Rating | 5A or 1A | 50mA or 333mV (internal burden) |
| Wiring | 12 AWG or 10 AWG THHN in conduit | 18 AWG or 22 AWG twisted pair |
| Termination | Shorting terminal blocks (test switches) | Screw terminals or 3.5mm audio jacks |
| Standard | IEEE C57.13 / IEC 61869-2 | Consumer electronics / Maker specs |
In a 480V 3-phase panel, you will see CTs wired to a Schweitzer Engineering Laboratories (SEL) or GE Multilin protection relay. The wiring diagram will mandate that the secondary wires be routed in a dedicated conduit to prevent capacitive coupling from the 480V primary lines. On a home workbench, you are likely clamping a split-core CT over a single 240V dryer line in a subpanel, feeding an EmonPi or a custom ESP32 PCB. The OpenEnergyMonitor project provides excellent open-source schematics for these low-voltage maker implementations.
Common Confusions: CTs vs. Potential Transformers and Shunts
When reviewing single-line diagrams, beginners frequently confuse CTs with other sensing topologies.
CT vs. Potential Transformer (PT): A PT (or Voltage Transformer) steps down voltage (e.g., 4160V to 120V). PTs are wired in parallel with the line and must be fused on the primary side. CTs are wired in series with the load and must never be open-circuited on the secondary. If you see a device connected across two phases (L1 to L2), it is a PT. If a conductor passes directly through the device's window, it is a CT.
CT vs. Shunt Resistor: A shunt (like a 100A/50mV Deltec shunt) measures current by dropping a small voltage across a known resistance. Shunts provide no galvanic isolation; your microcontroller's ground becomes tied to the high-voltage DC or AC bus, which is a massive shock and fire hazard if not properly isolated via amplifiers. CTs provide inherent magnetic isolation, making them vastly safer for AC mains monitoring, though they cannot measure DC current due to the lack of a changing magnetic field (dI/dt).
FAQ: Diagram of Current Transformer Questions
Why does a diagram of current transformer wiring show a polarity dot?
The polarity dot (often marked as H1 on the primary and X1 on the secondary) indicates the instantaneous direction of current flow. If primary current enters the H1 dot, secondary current exits the X1 dot. In simple ammeter setups, polarity doesn't matter. However, if you are wiring a wattmeter, a power factor correction relay, or a directional overcurrent relay, the phase angle relationship between voltage and current is critical. Reversing the CT polarity will flip the phase angle by 180 degrees, causing the meter to read negative power or the relay to trip incorrectly during a fault.
How do I calculate burden when reading a diagram of current transformer specs?
Burden is expressed in Volt-Amperes (VA) or Ohms (Ω). To convert VA to Ohms, use the formula: R = VA / I². For example, if an industrial CT diagram specifies a 15VA burden at a 5A secondary, the maximum allowable circuit resistance (including wire resistance and the relay's internal impedance) is 15 / (5²) = 0.6Ω. If your 12 AWG wire run to the relay is 50 feet long (100 feet total round-trip), the wire resistance alone is roughly 0.198Ω. You must ensure the relay's impedance plus the wire resistance does not exceed 0.6Ω, or the CT will saturate and distort the waveform during high-fault currents.
What safety warnings appear on a diagram of current transformer secondary circuit?
Professional schematics will explicitly note: "Ground CT secondary at one point only" and "Provide shorting switch before disconnecting." Grounding the secondary (usually at the X2 terminal or the shorting block) prevents the secondary circuit from floating up to primary line voltage in the event of an internal insulation failure between the primary and secondary windings. The single-point ground rule prevents ground loops, which can introduce 60Hz hum and measurement errors into sensitive metering circuits. As detailed in instrument transformer guides, violating these grounding rules is a primary cause of erratic relay tripping in substations.
