Solar Lattice

Function

Residual current monitoring

ID
func:residual-current-monitoring
IRI
https://solarlattice.org/kb/func/residual-current-monitoring
Status
active
Also known as
RCMU, residual current monitoring unit, leakage current monitoring, residual current monitoring device
Category
Protection
Function kind
protection

Residual current monitoring measures the differential current, which is the leakage current plus any residual current, during feed-in, and works out the residual current from that measurement. Fronius describes the unit as watching residual currents between the PV modules and the grid connection, and taking the inverter off the grid when it sees an improper residual current. Under IEC 62109-2, a transformerless inverter must disconnect no later than a 30 mA jump in residual current. SMA states that AC residual currents above 30 mA can be life-threatening. When leakage current is high, the residual current cannot always be calculated accurately, and the errors can shut the inverter down when it should not. If the utility or the installation calls for an extra residual-current device, Fronius specifies a type B breaker with a release current of at least 100 mA. SMA calls for a device that trips at 100 mA or higher. Grid operators may require a residual-current device in their technical connection requirements, but often they only ask for a standard-compliant installation.

Claims

trip_threshold

c1 verified · fact nominal 30 MilliA

Conditions: Residual current jump at which a transformerless inverter must disconnect from the grid at the latest, per IEC 62109-2 as described by SMA

verified by: review-workflowverified on: 2026-10-24review note: review: the compiler-flagged conflict (func:residual-current-monitoring#c2) is a scenario difference, resolved at review

c2 disputed ≥ 100 MilliA

Conditions: Trip rating of an additional external residual-current device, where one is required, for SMA Sunny Island / Sunny Boy Storage / Sunny Tripower Smart Energy backup systems

review note: support check: partial - The passage gives 100 mA or higher for an external RCD, but it doesn't name the specific SMA product families (Sunny Island, Sunny Boy Storage, Sunny Tripower Smart Energy) or backup systems.conflicts with: Residual current monitoring (c1)

c3 drafted · lead ≥ 100 MilliA

Scope: products Fronius Symo

Conditions: Release current of an additional type B residual current circuit breaker, where required by the installation or utility, for Fronius Symo inverters

Referenced by

Data