RCD for Solar PV: Why Type B Is Required
Why does a solar PV installation need a Type B RCD? A transformerless inverter can put smooth DC fault current onto its AC output, and Type AC and Type A devices detect only sinusoidal or pulsating-DC residual current under IEC 62423 — enough DC current saturates their toroidal core so the device stops tripping, a failure called blinding. That blind spot is why most transformerless-inverter installation manuals and IEC 60364-7-712 specify a Type B RCD, or a Type A RCD backed by a residual DC-detection device (RDC-DD) on the DC bus. This article covers the blinding mechanism, when a transformer-isolated inverter changes the requirement, IΔn sizing against normal inverter leakage, TT/TN earthing on PV arrays, and the ABB, Schneider and Siemens Type B options fitted at the AC disconnect.
How a PV Inverter Produces DC Fault Current
A transformerless string or central inverter couples the DC array directly to the AC grid through power semiconductors, with no isolation transformer in between. If an insulation fault develops on the DC side — a damaged module cable, a cracked junction box, water ingress at a connector — fault current can flow through the inverter's switching devices and appear on the AC output as a smooth, unidirectional DC component superimposed on the normal AC current.
A Type AC RCD only responds to a sinusoidal residual current. A Type A RCD extends that to pulsating DC, the kind produced by single-phase rectifier loads — see the full breakdown in RCD types explained. Neither is built to detect a steady, non-zero-crossing DC current. Worse, that DC component can magnetically bias the toroidal core toward saturation, so the device's sensitivity to a simultaneous genuine AC fault drops as well. The RCD does not just miss the DC fault — it can go blind to everything.
Transformerless vs Transformer-Isolated Inverters: Does Every System Need Type B?
No. An inverter with a low-frequency or high-frequency isolation transformer between the DC array and the AC grid blocks DC fault current from reaching the AC side by design — the transformer core cannot pass DC. On those systems a Type A RCD is normally adequate, sized the same way as any other final circuit.
Transformerless topologies dominate commercial and utility-scale PV because they cost less and run cooler, which is exactly why the Type B requirement shows up so often in practice. Check the inverter's own compliance documentation before specifying: some transformerless units carry integrated residual current monitoring, sometimes marketed as RCMU under schemes like Germany's VDE-AR-N 4105, that continuously watches the DC injection and trips the inverter itself. Where that monitoring is certified equivalent to an RDC-DD, the installation can run on a standard Type A RCD instead of a full Type B — the same trade-off that drives the choice on EV chargers, covered in the EV charger RCD requirements. Where it isn't certified, or where the certificate can't be produced, specify Type B and move on — this is not a corner to guess on. For a side-by-side of the two device types across all their use cases, see Type A vs Type B RCD comparison.
Sizing IΔn: Inverter Leakage Current vs Nuisance Tripping
Transformerless inverters carry meaningful capacitive leakage current from the DC array to earth through parasitic module-to-frame capacitance, and that leakage rises with array size, cable length, and humidity. A 30 mA general-purpose RCD on a large array can trip on cumulative leakage alone, with no actual fault present. This is a sizing problem, not a device defect.
What we see in the field: installers who fit a standard 30 mA Type A on a large rooftop array often get intermittent nuisance trips at dawn and dusk, when condensation on module junction boxes briefly raises leakage. The fix is not a bigger RCD — it's the correct type and, where the installation allows it, a higher sensitivity paired with upstream discrimination, or a super-immunized (SI) variant that rejects transient leakage spikes without raising the trip threshold on a genuine fault.
Earthing and Touch Voltage on Ground-Mount and Rooftop Arrays
TT Systems: Ground-Mount and Off-Grid Arrays
Ground-mount arrays and many off-grid or islanded installations sit on a TT earthing arrangement, with the array frame earthed through a local electrode rather than a common protective conductor back to the supply transformer. On TT, an RCD is not optional — the earth-loop impedance is normally too high for an overcurrent device to clear a line-to-earth fault fast enough, and the maximum electrode resistance is set against the RCD's own trip current. See earthing system requirements for how this plays out across TT, TN and IT installations generally.
Formula: Maximum Earth Electrode Resistance for TT Systems — Source: IEC 60364-4-41
RA × IΔn ≤ 50 V
| Symbol | Description | Unit |
|---|---|---|
| RA | Resistance of the earth electrode serving the array's exposed conductive parts | Ω |
| IΔn | Rated residual operating current of the protective RCD | A |
| 50 V | Conventional touch voltage limit under normal dry conditions | V |
Lower the electrode resistance and a higher-sensitivity RCD becomes viable; raise IΔn and the electrode has to work harder. The two are not independent choices.
TN Systems: Rooftop Arrays on an Existing Building Supply
A rooftop array feeding into a building's existing TN supply inherits the building's earthing. The array's own DC-side insulation fault behavior does not change with the earthing system — a transformerless inverter still needs Type B or the equivalent RDC-DD protection regardless of whether the building is TT or TN. What changes is the AC-side overcurrent backup and whether a 30 mA RCD is already mandated for the circuit under general wiring rules.
Type A + RDC-DD vs Full Type B: Cost and Compliance Trade-offs
A full Type B RCCB costs more than a Type A of the same rating, and stocking two device families across a fleet of installations adds complexity. An RDC-DD built into the inverter, or fitted as a separate module ahead of the AC disconnect, lets the panel builder keep a standard Type A RCD upstream while the DC-injection monitoring does the job a Type B core would otherwise do.
The trade-off is documentation and commissioning discipline. A Type B RCD is a single certified device with a known IEC 62423 test result. An RDC-DD solution is two devices whose combined compliance has to be verified and recorded — inverter documentation, the RDC-DD's own certificate, and confirmation that the trip threshold and time actually match what the installation needs. On a one-off residential rooftop job, most installers default straight to Type B and skip the paperwork trail entirely.
Type AC, Type A and Type B for PV: Side-by-Side
| Criteria | Type AC | Type A | Type B |
|---|---|---|---|
| Residual waveform detected | Sinusoidal AC only | AC + pulsating DC | AC + pulsating DC + smooth DC |
| Fit for a transformerless PV inverter | Not suitable | Only with a certified RDC-DD or equivalent inverter monitoring | Suitable without added DC monitoring |
| Behavior on a smooth DC fault | Can fail to trip; core saturation risk | Can fail to trip above the monitoring threshold if unmonitored | Trips on the genuine smooth DC fault current |
| Typical PV application | Not specified for PV | Transformer-isolated inverters | Transformerless string and central inverters |
ABB, Schneider and Siemens Type B Options for the AC Disconnect
ABB's F200 B range extends the standard F200 RCCB platform (2P/4P, 25-125 A, IΔn from 10 to 500 mA) to true Type B detection, and drops into the same rail footprint as the AC/A/F variants in the same family — browse the wider range of residual current devices. Siemens covers the same requirement with the 5SM3, a Type B RCD built for DC fault currents from EV, PV and VFD sources, alongside the 5SM2 residual current relay for larger feeders using an external toroid. Schneider's Acti9 range reaches Type B through its Vigi add-on block fitted to an iC60 MCB, or the iID RCCB, both offered in Type B alongside the AC/A/SI variants — the combined MCB-plus-RCD form factor is the same one covered in the RCBOs collection and in RCCB vs RCBO vs RCD vs MCB differences.
None of the three differ on the underlying detection principle — IEC 62423 sets the Type B test. They differ on form factor: ABB and Siemens ship one-piece RCBOs for compact boards, Schneider's Vigi block lets a panel builder add earth-leakage protection to an MCB already installed rather than swap the whole device. For a PV combiner feeding more than roughly 125 A, none of the fixed RCCBs apply — specify a residual current relay with a separate toroid instead.
Frequently Asked Questions
Can I use a Type A RCD on any solar PV inverter?
Only if the inverter has transformer isolation, or carries a certified integrated DC-monitoring function equivalent to an RDC-DD. A transformerless inverter without that certification needs a Type B RCD.
What happens if a Type A RCD is fitted where Type B is required?
The device can fail to trip on a genuine DC fault because the DC component saturates its toroidal core, a failure mode called blinding. The circuit can appear normal on a test-button check while offering no real fault protection.
Does a Type B RCD cost significantly more than Type A?
Yes, a true Type B device costs more than an equivalent-rated Type A, which is why the Type A plus RDC-DD combination is used on larger installations where the price difference adds up across many circuits.
Do rooftop PV systems need a different RCD type than ground-mount systems?
The RCD type requirement comes from the inverter's topology, not the mounting method. Earthing arrangement (TT vs TN) affects whether an RCD is mandatory at all and how the electrode resistance is sized, not the AC/A/B classification.
Can I test a Type B RCD with a standard test instrument?
A general-purpose RCD tester checks AC and pulsating-DC trip performance. Confirming genuine smooth-DC trip behavior needs a tester with a Type B test function; the front test button on the device proves the mechanism only, not the trip threshold.
Is Type B required on the DC side of the PV array or the AC side?
Type B RCDs are AC-side protective devices, fitted at the inverter's AC output or the AC disconnect. The DC array itself is protected by insulation monitoring and, on transformerless inverters, the same RCD's ability to see DC fault current passing through to the AC side.
Conclusion
A transformerless PV inverter can put a genuine smooth DC fault current onto its AC output, and Type AC or Type A devices are not built to see it — the practical result is a device that looks fine on the test button and does nothing on a real fault. Type B closes that gap, or a certified Type A plus RDC-DD combination does it with more paperwork and less device cost. Check the inverter's isolation and DC-monitoring documentation first; it decides the RCD type before the array size or the earthing system does. For sensitivity, poles and type selection across the rest of the RCD range, see the RCD protection guide.
