Engineering Guide
How 6mA DC residual current detection works: the fluxgate principle explained
A plain engineering explanation of fluxgate sensing: why a normal CT is blind to smooth DC, how a saturating core reveals a few milliamps of DC leakage, and what the self-test does.
Detecting a few milliamps of DC leakage on a conductor that carries 32 A of AC is a demanding measurement. It is roughly like hearing a whisper next to a running engine. Yet every AC EV charger that relies on a Type A RCD upstream has to do exactly that. This guide explains, without heavy maths, how a Type A + DC 6mA sensor does it and what that means for your charger design.
What the sensor actually measures
All the charging conductors (line and neutral, or all three phases plus neutral) pass through the sensor’s magnetic core. In a healthy circuit, the current going out equals the current coming back, so their magnetic fields cancel. The core “sees” zero.
If some current leaks to earth, the outgoing and returning currents no longer match. The difference is the residual current, and it is the only thing that produces a net magnetic field in the core. The sensor’s job is to measure that small net field and decide whether it is above a trip threshold.
For AC residual current this is old technology. For smooth DC residual current it is not.
Why a normal current transformer is blind to smooth DC
A conventional RCD or current transformer works by induction. An alternating residual current creates an alternating flux in the core, which induces a voltage in the secondary winding. No change in flux, no output.
Smooth DC residual current creates a constant flux. It induces nothing. But it is not harmless: the constant flux pushes the core’s operating point towards saturation. A partly saturated core has a much lower permeability, so it also responds less to AC. That is how DC leakage from a vehicle’s on-board charger can blind a Type A RCD. The full background is in Type A vs Type B RCD for EV chargers.
So DC detection needs a different principle, one that does not depend on the flux changing by itself.
The fluxgate principle in plain words
A fluxgate sensor makes the flux change on purpose.
- Excitation. The electronics drive an alternating excitation current through a winding on the core. The excitation is strong enough to push the core into saturation in one direction, then the other, many times per second.
- Symmetry with no fault. If there is no residual current, the core spends equal time saturated in the positive and negative directions. The excitation current waveform is symmetric.
- Asymmetry with DC leakage. A smooth DC residual current adds a constant offset to the field. Now the core reaches saturation earlier in one direction and later in the other. The waveform becomes asymmetric: the duty cycle shifts, and even harmonics of the excitation frequency appear.
- Evaluation. The electronics measure this asymmetry. Its size is proportional to the DC residual current, so the sensor can calculate the leakage in milliamps and compare it with the trip threshold.
Because the core is driven through its whole magnetic loop, the same arrangement can also measure AC residual current and mixed waveforms. One core can therefore cover the complete “Type A + DC 6mA” requirement: AC, pulsating DC and smooth DC.
From measurement to a TRIP signal
The evaluation electronics turn the measurement into a simple decision for the charger controller: a digital TRIP output. On EVsense sensors this output is low (0–0.6 V) in normal operation and goes high (4.2 V to VCC) on a fault. The controller then opens the main relay.
The published thresholds for EVsense Type A + DC 6mA sensors are:
| Waveform | Min | Typ | Max |
|---|---|---|---|
| Smooth DC | 3 mA | 4.5 mA | 6 mA |
| AC 50 Hz | 15 mA | 22 mA | 30 mA |
Trip time falls as the fault grows. For example, the 1R20 Series datasheet lists typical DC trip times of 380 ms at 6 mA, 63 ms at 60 mA and 11 ms at 300 mA. How these values relate to the standard is covered in IEC 62955 RDC-DD explained.
Why the self-test and zero-point calibration matter
Measuring a few milliamps of DC means the sensor must know very precisely what “zero” looks like. Temperature, component tolerances and the magnetic history of the core can all shift that reference.
EVsense sensors deal with this in a defined start-up sequence (from the datasheet operating instructions):
- Start-up delay: for the first 800 ms after power-on the sensor does not respond or process signals.
- Self-test: the controller triggers a self-test through the CLK&CHK (or CHK) pin once after power-up and again before each charging session. A dedicated self-test coil on the core injects a known test current, which checks the whole chain from core to TRIP output.
- Zero point: the sensor calculates its zero point during power-up and self-test.
For the charger designer this means: do not close the relay until the self-test is finished, and repeat the self-test before every session. It is also a useful diagnostic, because a failed self-test tells the charger that the protection function is not available and charging must not start.
Packaged sensor or chip + magnetic core?
There are two ways to put this technology into a charger.
Packaged RCD sensors such as the 1R20 Series contain the core and all evaluation electronics in one module with a 5 V supply and a digital TRIP output. This is the fastest route: the charger only needs to supply power, read the output and drive the self-test pin.
Chip + magnetic-core solutions split the job. A detection ring on your PCB carries the windings, and the ILM2112TQC IC on the same PCB does the fluxgate excitation and evaluation. EVsense offers two rings for this IC:
- CT1836: a compact detection ring with an 18.5 mm aperture and harness connection, with a 100-turn residual current detection coil and a 2-turn self-test coil.
- CT2728: a detection ring with an integrated 32 A current transformer (accuracy class 0.2), pre-wired for single phase and PCB-mounted. One magnetic part gives you both leakage detection and charging-current measurement.
The chip + core route gives more freedom in board layout and can reduce the total number of magnetic components, at the cost of a little more design work. The datasheets include a reference circuit and recommend reserving test points for production testing.
Practical consequences for your design
- Pass all current-carrying conductors through the core, otherwise the sensor sees a false residual current.
- Keep the sensor at least 3 cm from relays, whose magnetic fields can disturb a sensitive measurement.
- Power it from a clean 5 V LDO (the datasheets give ripple limits of 30–50 mV depending on the model).
- Place the sensor so that leakage through the switching power supply’s Y-capacitors does not flow through the core.
The RCD sensor PCB integration guide goes through these points step by step.
Summary
- Ordinary current transformers cannot see smooth DC, and DC leakage can blind them to AC faults too.
- A fluxgate sensor drives its core into saturation; DC residual current makes the saturation asymmetric, and that asymmetry is measured.
- Self-test and zero-point calibration at power-on and before each session keep the 6 mA threshold accurate.
- Choose a packaged sensor for the fastest integration, or a chip + core solution such as CT1836 or CT2728 for layout freedom and integrated current measurement.
Frequently asked questions
Why can't a normal current transformer detect smooth DC leakage?
A current transformer works by induction, which needs a changing magnetic flux. Smooth DC residual current creates a constant flux, so it induces no output. Worse, it shifts the core's operating point towards saturation, which reduces the sensitivity to AC as well.
What is a fluxgate sensor?
A fluxgate sensor actively drives its magnetic core into saturation in both directions with an excitation current. Without a DC residual current the saturation is symmetric. A DC residual current makes it asymmetric, and the electronics measure that asymmetry, for example as a change in duty cycle or even harmonics, to calculate the DC current.
Why does an RCD sensor need a self-test at power-on?
During the self-test the sensor injects a known test current through a dedicated coil to check the whole detection chain, and it calculates its zero point. EVsense datasheets require one self-test after power-up and another before each charging session.
What is the difference between a packaged RCD sensor and a chip + magnetic-core solution?
A packaged sensor such as the 1R20 Series contains the core and electronics in one module with a digital TRIP output. A chip + core solution such as CT1836 or CT2728 with the ILM2112TQC IC puts the detection ring and the IC separately on your PCB, giving more layout freedom and, with CT2728, an integrated current transformer.
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