PV inverter — residual current monitoring (RCMU) and earth-fault detection per IEC 62109-2

The guide on anti-islanding covers how a PV inverter responds to the loss of the public grid. This article covers a different, internal protection mechanism of the inverter that operates independently of that: the residual current monitoring unit (RCMU), as specified in IEC 62109-2, which in a transformerless inverter partly takes over the function of an AC-side RCD.

Why a transformerless inverter needs an internal earth-fault mechanism

A transformerless PV inverter, unlike an inverter with an isolation transformer, has no galvanic isolation between the DC side (panels) and the AC side (grid). This means a DC-side earth fault — for example damaged panel insulation — can cause a leakage current that flows back via the earthing without behaving like a classic phase fault on the AC side. IEC 62109-2 therefore requires a transformerless inverter to carry out its own, continuous residual current monitoring that detects both AC and DC components of the leakage current, regardless of whether an external RCD is present on the AC side.

What the RCMU measures and when it disconnects

The RCMU continuously monitors the residual current and distinguishes between two situations:

A gradually building, continuous residual current: the inverter must disconnect from the grid once this exceeds a fixed threshold (on the order of 300 mA, within a time of several hundred milliseconds), comparable to the function of a type-B main RCD.
A sudden jump in residual current: for an abrupt increase in residual current (characteristic of a newly occurring fault), the inverter must disconnect faster and at a lower absolute threshold — typically on the order of 30 to 150 mA, within only tens to a few hundred milliseconds, depending on the size of the jump.

This distinction between a slowly building and a sudden residual current is functionally comparable to the distinction made by a type-B RCD, but with an RCMU it is handled entirely internally in the inverter software rather than by a separate, external device.

Why the RCMU is not a full replacement for an external RCD

Note: the RCMU primarily protects the inverter and the installation against an internally detected earth fault, and cannot simply be equated with the regular RCD protection that NEN 1010 requires for touch safety of persons. Whether, and in what form, an external RCD is still required on the AC circuit to the inverter follows from the installation requirements of NEN 1010 §712 and the inverter manufacturer's specification — the RCMU is an additional internal protection, not an automatic exemption from the external RCD requirement.

In addition, the RCMU of a transformerless inverter typically does not fully cover every conceivable DC residual current: a small, sustained DC leakage current below the trigger threshold can, under certain conditions, go undetected, which is one of the reasons why the installation requirements of §712 require an assessment tailored to the inverter topology rather than a generic assumption.

Practical relevance

When assessing a PV installation with a transformerless inverter, it is important to verify that the RCMU function per IEC 62109-2 is demonstrably present and active (this is typically stated on the inverter nameplate or declaration of conformity), and that the installation requirements for any external AC-side RCD have been correctly applied based on the specific inverter topology — not solely on a general assumption that "the inverter handles it by itself".

Common mistakes

Assuming a transformerless inverter with an RCMU no longer needs an external RCD — the installation requirement for the AC circuit follows from §712 and the inverter specification, not automatically from the presence of an internal RCMU.
Confusing the RCMU function with a full type-B RCD replacement for touch safety of persons — the RCMU is primarily intended to protect the inverter itself against an internally detected earth fault.
Not verifying that the RCMU is actually active and certified per IEC 62109-2 when assessing a transformerless inverter — not every inverter on the market automatically meets the same trigger thresholds.
Overlooking a small, sustained DC leakage current below the trigger threshold as a possible, creeping cause of an underlying insulation problem, simply because the RCMU has not disconnected.