Sudden change in residual current
Maximum disconnection time from the mains
Wels, February 2nd 2016
PROTECTION BY RESIDUAL CURRENT MONITORING REGARDING
FRONIUS PRIMO
During the feed-in operation of the Fronius Primo the integrated Residual Current Monitoring Unit (RCMU) is
responsible for the detection of a positive ground fault as well as for a negative one. The RCMU measures the
total (both AC and DC components) root mean square (RMS) current to ground and comply with the
requirements in IEC 62109-2.
IEC 62109-2 states that transformerless (non-isolated) inverters shall provide residual current monitoring for
both continuous excessive residual current and excessive sudden changes in residual current, according to the
following limits:
Continuous residual current
/ If the continuous residual exceeds 300mA the inverter shall disconnect from the mains within 0.3 seconds.
That’s the limit for inverters with rated output power ≤ 30kVA (Fronius Primo)
Sudden changes in residual current
/ If a sudden change in residual current is detected the inverter shall disconnect from the mains within the limit
specified in Table 1:
30 mA 300 ms
60 mA 150 ms
150 mA 40 ms
Table 1: time limits for sudden changes in residual current for transformerless inverters
From that follows that the Fronius Primo will disconnect the neutral form the DC side of the inverter under the
first fault condition. All Fronius transformerless inverters switch off and break the neutral connection if an earth
fault occurs in accordance with VDE 0126-1-1 and IEC 62109-2.
Fronius International GmbH
Solar Energy Division
Froniusplatz 1
A-4600 Wels
DI Thomas Mühlberger
Head of Solution Management
02/2011
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