A current transformer (CT) is an instrument transformer that produces a small secondary current in fixed proportion to the current in a conductor. A 400/5 CT, for example, gives 5 A at its secondary terminals when 400 A flows through it, so a meter or a protection relay can measure a large current with a small input, isolated from the circuit's voltage. The conductor passes through the CT's core and acts as a one-turn primary; the secondary winding has many turns, and the ratio of turns sets the ratio of currents. CTs are found in switchboards, distribution boards and substations wherever a current is too large to pass through a meter directly.
How a current transformer works
Current in the primary conductor creates an alternating magnetic flux in the core. The flux induces a voltage in the secondary winding, and because the secondary is closed through the meter, a secondary current flows. That current creates an opposing flux. In an ideal CT the two cancel exactly, and the ampere-turns balance:
Ip × Np = Is × Ns
With a single primary turn (the cable passing once through a ring CT), the secondary current is the primary current divided by the number of secondary turns. A 400/5 CT therefore has 80 secondary turns. Passing the cable through the window twice doubles the primary ampere-turns, so the same CT behaves as 200/5. This is a common way to use a standard CT on a small load, as long as the window has room for the extra turn.
A CT is a current source. It drives whatever current the ratio requires through the secondary circuit and develops just enough voltage to do so. With a 5 A secondary and a total burden of 4.4 VA, the secondary voltage at rated current is 4.4 ÷ 5 = 0.88 V. A higher burden needs more voltage, more flux and more magnetising current, which is where errors come from.
Ratio and phase errors
A real CT needs some of the primary ampere-turns to magnetise its core. That magnetising current never reaches the secondary, so the secondary current is slightly smaller than the ratio predicts and slightly shifted in phase. The two errors are:
| Error | Definition | Effect on a meter |
|---|---|---|
| Ratio (current) error | (Kn × Is − Ip) ÷ Ip, as a percentage, where Kn is the rated ratio | Current, power and energy read low or high by about that percentage |
| Phase displacement | The angle between primary and secondary current, in minutes of arc | Power and energy errors that grow as the power factor falls |
Magnetising current is a larger share of the total at low current, which is why accuracy classes allow larger errors at 5% of rated current than at 100%. It rises with burden, which is why a CT loaded beyond its rated VA loses accuracy. An air gap, as in a split-core CT, raises it further. The CT selection guide gives the IEC 61869-2 limits for each class at each current.
Burden and the CT secondary circuit
The burden is everything connected to the secondary: the meter input, the leads and every terminal and test block in the loop. It is stated in VA at rated secondary current (VA = Is² × R for a resistive burden). The lead loop is often the largest part. A 400/5 CT feeding a meter with a 1 VA input over 10 m of 2.5 mm² copper each way has a 20 m loop of about 0.138 Ω, which adds 25 × 0.138 = 3.4 VA at 5 A:
The total of 4.4 VA fits a 5 VA CT. The same leads on a 1 A secondary add only 0.14 VA, because the loss falls with the square of the current, which is why long runs to the meter favour 1 A CTs. Low-power CTs with a 333 mV output avoid the question: the burden resistor is inside the CT, and the meter input is a high-impedance voltage input.
CT ratio as a multiplier: a worked example
A meter wired to a 400/5 CT reads 3.2 A on its secondary input. The ratio Kn is 400 ÷ 5 = 80, so the primary current is 3.2 × 80 = 256 A. If the same meter is set to read secondary values, every power and energy figure is multiplied by 80 as well: 1.5 kWh on the secondary register is 120 kWh in the circuit. Most meters let you enter the CT ratio so that they apply the multiplier themselves. A wrong ratio in the meter is one of the most common commissioning faults, because the readings look plausible. The CT meters guide explains the multiplier on CT-operated meters and how to check it against a clamp reading.
Types of current transformer
| Type | Construction | Where it is used |
|---|---|---|
| Ring or window (toroidal) | A closed core with only a secondary winding; the cable or busbar is the primary | Low-voltage switchboards and distribution boards |
| Split-core | A ring that opens to fit around a live conductor | Retrofit metering without a shutdown |
| Bar primary | A ring with a built-in busbar as the primary | Panel-built switchgear |
| Wound primary | A primary winding of several turns for low rated currents | Small rated currents, such as 5 A to 50 A |
| Bushing | A ring CT around the bushing of a transformer or breaker | Medium- and high-voltage plant |
| Summation | Adds the secondaries of several CTs into one output | One meter for several feeders |
| Low-power (mV output) | A ring or split-core CT with an internal burden and a voltage output, such as 333 mV | Submetering and monitors with voltage inputs |
A Rogowski coil also measures current around a conductor, but it has no iron core, cannot saturate and gives a voltage proportional to the rate of change of current. IEC 61869-10 treats it as a low-power passive current transformer with an air core, but it behaves unlike an iron-core CT: its output must be integrated, by the meter or a separate integrator, to give current.
Metering CTs and protection CTs
| Metering CT | Protection CT | |
|---|---|---|
| Purpose | Accurate billing and monitoring at normal load | Correct relay operation during faults |
| Accuracy range | From 1% or 5% to 120% of rated current | Up to the accuracy limit factor, for example 10 or 20 times rated current |
| Typical classes (IEC 61869-2) | 0.2S, 0.5S, 0.5, 1 | 5P10, 5P20, 10P10; PX for defined knee-point applications |
| Behaviour at fault current | Saturates early, limited by the instrument security factor (FS 5, FS 10), to protect the meter | Stays linear so the relay sees the true current |
A class 5P10 CT has a composite error of no more than 5% up to 10 times rated current at rated burden. The knee point is the point on the magnetising curve where a 10% rise in secondary voltage needs a 50% rise in magnetising current: above it, the core saturates. Use a metering CT for energy monitoring and leave protection CTs to the protection relays.
CT safety and polarity
Never open the secondary circuit of a 1 A or 5 A CT while primary current flows. With no secondary current to oppose it, the full primary ampere-turns drive the core into saturation, and the secondary produces voltage spikes that can reach hundreds or thousands of volts. Fit a shorting terminal block or a test block so the secondary can be shorted before a meter is removed, and earth one point of each secondary circuit, usually S2 or the star point.
Polarity matters for power and energy. The primary terminal P1 (K on older CTs) faces the supply and the secondary terminal S1 (k) goes to the meter's current input. A CT fitted the wrong way round gives a negative power on that phase, and a swapped CT between phases gives a wrong power factor. The negative power guide shows how to find both from the readings.
CTs with EpiSensor
The ZEM electricity monitor is supplied with its current sensors, split-core CTs up to 120 A or Rogowski coils up to 3000 A, already connected and calibrated, so the ratio is set at the factory and the meter and sensors are tested as one chain. The Current transformer selection tool finds the sensor that fits a conductor, and the conductor size guide relates cable size to aperture. For existing CTs already wired to a meter, read the meter instead, over Modbus or its pulse output.
Common questions
What does a current transformer do?
It reduces a large alternating current to a small one that a meter or relay can measure safely, in a fixed ratio such as 400 A to 5 A. It also isolates the instrument from the voltage of the primary conductor, so the meter wiring carries a few amps at a few volts instead of the full circuit current at mains voltage.
How does a current transformer work?
The primary current creates a magnetic flux in the core. The flux induces a current in the secondary winding, which flows through the meter and creates an opposing flux. In balance, primary current times primary turns equals secondary current times secondary turns, so a CT with one primary turn and 80 secondary turns gives 5 A when 400 A flows in the cable.
What is a CT ratio?
The ratio of rated primary current to rated secondary current, written for example 400/5 or 400:5. It is also the multiplier: a meter that reads 3.2 A on the secondary of a 400/5 CT is measuring 3.2 × 80 = 256 A in the primary. Energy and power read on the secondary side are multiplied by the same factor.
Why must a CT secondary never be open-circuited?
With the secondary open, no secondary current opposes the primary flux, so all the primary ampere-turns magnetise the core. The core saturates, and the secondary develops voltage peaks that can reach hundreds or thousands of volts. That is a shock and fire hazard and can damage the CT. Short the secondary with a shorting link or test block before you disconnect a meter.
What is the difference between a CT and a VT?
A current transformer is connected in series with the load and reproduces the current; its secondary works into a near short circuit. A voltage transformer (VT or PT) is connected across the supply and reproduces the voltage; its secondary works into a high impedance and must never be short-circuited. Low-voltage meters usually connect to the voltage directly and need only CTs.
What is the difference between a metering CT and a protection CT?
A metering CT, class 0.2S to 1 for example, is accurate from a few percent to 120% of rated current and saturates soon above that to protect the meter. A protection CT, class 5P10 or 10P20 for example, keeps its composite error within 5% or 10% up to 10 or 20 times rated current so that a relay sees the true fault current. One CT is rarely good at both.