A Rogowski coil is a winding on a non-magnetic core. Flexible versions open, wrap around a conductor and latch closed. They fit around busbars, large cables and groups of parallel cables where a rigid current transformer (CT) will not go.
The coil does not output a current. It outputs a small voltage proportional to the rate of change of current, di/dt. An integrator turns that voltage back into a signal proportional to current. The coil has no iron core, so it does not saturate, and its output is linear over a very wide range. Its limits come from the small signal, the integrator and the installation.
How a Rogowski coil works
A uniformly wound coil around a conductor produces:
e(t) = −M × di(t)/dt, where M = μ₀ × N × A / l
M is the mutual inductance, N the number of turns, A the area of one turn and l the length of the coil. i(t) is the primary current. Coil orientation sets the sign.
By Ampère's law, the output depends only on the current enclosed by the loop. In an ideal coil, the size and position of the conductor inside the loop have no effect. Real coils are close to this ideal, and the remaining position error is discussed below.
Output level and frequency
For a sinusoidal current, the RMS output is E = 2π × f × M × I. The signal is small. LEM's ART-B22 coil has M = 71.98 nH. At 50 Hz this gives 22.6 mV per kiloamp, which matches its rated 22.5 mV/kA. LEM's application note quotes 20 mV/kA for its RT range.
The raw output also leads the current by 90°. The integrator must remove this shift exactly, because active power depends on the phase between current and voltage.
Because the output depends on di/dt, a Rogowski coil does not measure steady DC.
The integrator
The integrator divides the signal by frequency and removes the 90° shift. It can be a separate module, an analogue stage in a meter or a digital calculation on the sampled coil voltage. LEM's integrator application note describes the design problems:
- A pure integrator integrates its own input offset. Its output drifts until it saturates.
- Designers limit that drift with a static gain, an offset-compensation stage or AC coupling. Each of these makes the coil and integrator a high-pass filter with a low-frequency cut-off.
- The offset-rejection circuit adds several degrees of phase error at 50 Hz. A compensation stage corrects it, but the correction is exact at one frequency only, for example 50 Hz or 60 Hz.
- The coil's gain is never exact by construction. The integrator is calibrated against a reference for each coil, and that coil must stay with that integrator.
The PEM RCT1A shows these limits in a real product. Its coil and integrator have a -3 dB bandwidth of 0.6 Hz to 5 kHz and a rated operating range of 45 Hz to 1 kHz. It needs up to 2 minutes after switch-on to settle. Its manual says the coils and integrators must not be swapped.
Phase error and energy error
A phase error δ between the measured current and the true current changes active power by approximately δ × tan φ, where φ is the load's phase angle. The error grows quickly as power factor falls:
| Power factor | Energy error from a 1° phase error | Energy error from a 0.2° phase error |
|---|---|---|
| 1.0 | 0.02% | 0.00% |
| 0.8 | 1.3% | 0.26% |
| 0.5 | 3.0% | 0.60% |
A current reading can be correct to 0.5% while the energy reading on a PF 0.5 load is 3% out. Continental Control Systems specifies its RCSL and RCLL coils at ±0.10° typical and ±0.20° maximum from 3% to 120% of rated current. That level of phase performance is what energy measurement needs.
Rogowski coil vs conventional CT
IEC 61869-10 covers low-power passive current transformers, and it includes Rogowski coils. Its scope is newly manufactured devices with analogue output, for measuring instruments or protective devices with a rated frequency of 15 Hz to 100 Hz. LEM states that its ART-B22 coil meets the standard only in part, because a Rogowski coil differs fundamentally from a CT.
The difference that matters most at the terminals is the signal. A Rogowski input expects a raw coil voltage of tens of millivolts per kiloamp. A 1 A, 5 A or 333 mV CT input cannot use that signal, and a Rogowski input cannot use a CT secondary. Some coils come with an integrator that outputs 333 mV or 1 A, so that they can connect to a CT input.
| Design question | Rogowski coil and integrator | Conventional current transformer |
|---|---|---|
| Sensing element | Winding on a non-magnetic core | Winding on a magnetic core |
| Raw output | About 20 to 25 mV/kA at 50 Hz for the coils cited here, proportional to di/dt | Secondary current (1 A, 5 A or mA), or 333 mV from an internal burden |
| Steady DC | Not measured | Not measured by an ordinary AC CT |
| Saturation | No core to saturate; the integrator and meter input clip | Core saturates above its rated range or with DC offset in the primary |
| Open secondary | Low-voltage output; no open-circuit hazard | A current-output secondary must not be opened with primary current flowing |
| Physical form | Flexible loop; 70 mm to 125 mm across for the coils cited here | Split-core or solid-core; 10 mm to 24 mm apertures for EpiSensor's CT options |
| Typical fit | Busbars, large cables, parallel cables per phase, crowded panels, currents from a few hundred amps upwards | Single cables at a few amps to a few hundred amps, and meters with CT inputs only |
Saturation and measuring range
The coil itself does not saturate. The ART-B22 datasheet lists its linearity error as none and its short-time thermal rating as 300 kA. The integrator, the analogue front end and the analogue-to-digital converter still have a full-scale input. A fault current or inrush above that input clips, even though the coil stays linear.
The low end is the real limit. At 1 A, the ART-B22 produces 23 µV. Noise, offset and pickup from nearby conductors are then a large part of the signal. For this reason every coil and meter specifies accuracy only down to a fraction of its rating. IEC 61869-10 class 0.5, as plotted in LEM's ART-B22 datasheet, permits 1.5% ratio error at 5% of rated current, 0.75% at 20% and 0.5% from 100% to 120%. The CCS RCSL and RCLL coils are specified at ±1.0% from 3% to 120% of rated current.
Linearity and installed accuracy
A linear coil can still read wrongly in the panel. Four installed errors matter.
Position error is largest near the latch, the weakest point of the loop. The ART-B22 has a ratio error of ±0.5% with the conductor centred and ±0.75% at any position. CCS gives ±0.5% typical and ±1.0% maximum for an off-centre or tilted conductor. PEM calibrates the RCT1A with the conductor near the centre.
Current in nearby conductors couples into the loop, again most strongly near the latch. The ART-B22 specifies ±0.2% typical and ±0.4% maximum with an external conductor at the same current, in contact with the coil. PEM tells users to keep outside conductors away from the latch and to avoid positions near multi-turn inductors.
Temperature changes the coil geometry. The ART-B22's mutual inductance changes by ±30 ppm/K, or ±0.12% over a 40 K swing. CCS adds ±0.5% over the operating temperature range of its coil and integrator.
Phase error comes mostly from the integrator. It matters more than ratio error on loads with a low power factor, as the table above shows.
These errors add to the meter's own error. A coil and meter calibrated together, with a class that applies to the pair, remove the need to add them up on paper. They do not remove position and pickup errors in the panel.
Frequency response
A wide coil bandwidth does not mean the complete chain measures harmonics accurately. The ART-B22 coil alone has a -3 dB bandwidth of 420 kHz. The RCT1A coil and integrator are rated for 45 Hz to 1 kHz, which is up to the 20th harmonic at 50 Hz. The phase compensation is also tuned to the fundamental. For power quality work, check the amplitude and phase response of the coil, integrator and meter sampling together at the harmonic orders you need.
How to select a Rogowski measurement system
Current range
Size the coil to the load, not to the upstream breaker. Pick the smallest rating that covers the peak current, then check where normal load sits. Aim to keep the load that carries most of the energy above about 20% of the coil's rating, where a class 0.5 coil is allowed 0.75% or less. A 3 kA coil on a feeder that runs at 150 A is at 5% of its rating. There, a class 0.5 coil is allowed 1.5% ratio error before any installation error.
Over-range is rarely the problem with a Rogowski coil, because the coil itself does not saturate. Check the meter's input limit for peaks, inrush and fault current instead.
Physical fit
Measure the conductor or bundle with its insulation, lugs and any barrier, then check that the loop closes around it without forcing the latch or exceeding the bend radius. The RCT1A's minimum bend radius is 40 mm. Leave room to keep the latch away from other phases.
One coil can go around several parallel cables of the same phase. It measures their sum, which avoids one CT per cable. Never enclose conductors that carry current in opposite directions, such as a phase and its neutral. Their fields cancel and the reading falls to near zero.
Meter input
Record the coil model, its sensitivity, the integrator or meter input, the configured range and the calibration pairing. A coil and a "Rogowski input" from different suppliers can share a connector and still disagree on sensitivity. LEM's RT range gives 20 mV/kA and its ART-B22 gives 22.5 mV/kA. Fit one where the input expects the other and every reading is 12% out. The error is constant, so a single spot check against a clamp meter finds it.
Insulation and environment
Check the coil's voltage rating and measurement category against the bare conductor it goes around. The meter supply voltage is not the relevant figure. The ART-B22 is rated 1000 V CAT III with reinforced insulation. The RCT1A coil is rated 2 kV peak, and that rating is valid only when the free end is fully inserted in the latch. Also check operating temperature, conductor temperature (the ART-B22 allows 100 °C at the busbar), pollution degree and altitude.
Protection
Metering coils are selected for accuracy at normal load. Protection needs accuracy and response during faults. IEEE C37.235-2021 is the guide for Rogowski coils used for protective relaying. Use a coil and relay qualified for the protection scheme. A metering coil that shows a fault current on a display is not a protection input.
Installation and commissioning checklist
Only qualified personnel should install electricity-monitoring equipment. Follow the site's safe-isolation procedure and the coil manufacturer's instructions.
- Identify the conductor, or the same-phase bundle of parallel cables, to measure.
- Check that the coil, meter input and configured range match the recorded pairing.
- Inspect the coil, latch, lead and strain relief. Reject a coil with damaged insulation.
- Put the coil around the intended conductor only. Keep other phases, neutrals and return paths outside the loop.
- Close the latch fully. On coils with a locking nut, engage it where there is vibration.
- Point the arrow on the coil from source to load. Label the coil, meter channel and voltage phase.
- Centre the conductor where possible. Keep the latch away from neighbouring conductors.
- Support the coil and lead so that panel doors and vibration cannot pull the latch open.
- With a separate integrator, allow it to settle after switch-on before you take readings.
- Read back the effective meter settings: coil type or range, wiring mode, nominal frequency, phase association and import/export convention.
- Under load, compare each phase current with a clamp meter or other reference. Check that active power and power factor are plausible for the load.
- Record the reference instrument, both readings and the time window.
To check for pickup, hold the coil close to the conductor but not around it, as PEM recommends. The reading shows how much the coil picks up from currents outside the loop.
Common causes of wrong readings
| Symptom | Likely cause | How to confirm |
|---|---|---|
| Current near zero on a loaded circuit | Outgoing and return conductors both inside the loop, or wrong conductor | Check what passes through the loop |
| Negative active power on one phase | Coil reversed, or current paired with the wrong voltage phase | Check the arrow direction, then the phase labels |
| Current correct, power factor near 0 or negative | Current channel paired with another phase's voltage; the error is 120° | Swap the voltage association in configuration and recheck PF |
| Constant scale error on all loads | Coil from another meter or integrator, or wrong sensitivity setting | Compare with a clamp meter at two load levels; the ratio stays the same |
| Reading changes when the coil is moved around the conductor | Latch near the conductor or near another phase | Centre the conductor and turn the latch away from other phases |
| Reading on an unloaded circuit, or changes when nearby loads switch | Pickup from nearby conductors or lead routing | Place the coil next to, not around, the conductor and read the pickup |
| Energy correct at high PF, wrong on motor loads | Phase error in the integrator or configuration | Compare kWh at low PF with a reference; see the phase error table |
| Harmonic-rich load reads wrongly | Chain bandwidth or phase compensation too narrow | Compare with a power quality analyser of known bandwidth |
Using Rogowski coils with EpiSensor ZEM
The ZEM-65 is ordered with its current sensors:
- a 120 A split-core CT option, with a 14 mm aperture and 2 m cables; and
- 300 A, 1 kA and 3 kA flexible Rogowski-coil options. The 300 A coil is 25 cm long, closes to about 70 mm across and has 3 m cables. The 1 kA and 3 kA coils are 120 mm across with 2 m cables.
The ZEM takes the coils directly, on the same terminals as a split-core CT, and integrates the signal inside the meter. There is no separate integrator to buy, power or wire. The coils are connected and calibrated against the meter before it ships. The datasheet states system-level Class 0.5S to IEC 62053-22 for the meter and coils together, and a current measurement range of 0.1 A to 3 kA. Keep each supplied coil with its meter and channel.
That pairing matters for the class claim. IEC 62053-22:2020 does not apply to meters intended for low-power instrument transformers, which include Rogowski coils, when they are tested without them. A class stated for a Rogowski input alone does not tell you the accuracy with a coil.
Current transformer selection builds a selection brief for current range, conductor envelope and meter input. A 30 mm conductor with a chosen 2 mm allowance on each side needs a 34 mm round opening. Selecting a raw Rogowski input also prompts checks of coil sensitivity, frequency response and integration requirements:
The optional EpiSensor matching section can compare these requirements with published ZEM and ZDR options. A dimensional match is not proof that an arbitrary Rogowski coil works with a meter. Confirm the complete coil and integrator or meter combination.
The CT ratio and burden calculator applies to conventional current-output CT secondaries only. A Rogowski input has no burden in that sense.
Common questions
What is a Rogowski coil?
A Rogowski coil is an air-core current sensor. Its winding produces a voltage proportional to the rate of change of current in the conductor that passes through the loop. An integrator, either a separate module or part of the meter, converts that voltage into a signal proportional to current.
Can a Rogowski coil measure DC current?
No. The coil output depends on changing current, so steady DC produces no output. Practical integrators also block very low frequencies to stop offset drift. One commercial coil and integrator, the PEM RCT1A, has a lower -3 dB limit of 0.6 Hz.
Does every Rogowski coil need a separate integrator?
The signal must be integrated, but the integrator does not have to be a separate box. Some meters, including the ZEM-65, integrate the coil signal internally. Other coils come with an integrator that outputs 333 mV or 1 A so that they can replace a conventional CT.
Is a Rogowski coil more accurate than a current transformer?
Not inherently. A good flexible coil with a matched integrator reaches about 0.5% to 1% of reading with the conductor centred, and position, nearby conductors and phase error add to that. Compare the error of the complete coil and meter at the currents the site runs at, and the phase error if the load has a low power factor.
Is a Rogowski coil safe to install on an energised conductor?
Only if the product instructions and the site's live-working rules allow it. Some manufacturers forbid fitting their coils around uninsulated live conductors. Check the coil's voltage rating and measurement category against the bare conductor it goes around, and follow the site's safe-isolation procedure.