EV Charger, Type A RCD and DC Residual Current
A homeowner in Paphos has a 7.4 kW Mode 3 EV charger installed on a single-phase 230 V TN-C-S supply. The charger is fed from a new 40 A Type A 30 mA RCBO in the consumer unit, which also supplies the kitchen sockets, the lighting and a 4 kW PV inverter on the same busbar. The RCBO trips at random, mostly when the EV is charging and the PV is generating. An RCD tester records 1× IΔn trip time 34 ms on one polarity and no trip within 300 ms on the other; 5× IΔn trip time 19 ms. Insulation resistance, Zs and the charger's internal 6 mA DC detection test are satisfactory. The charger instructions state 'integral 6 mA DC detection, Type A RCD required'. The installer's certificate records a Type A RCD, and the customer reports the test button is pressed monthly.
1. Explain the likely cause of the random tripping and the different results on the two polarities.
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A Type A RCD detects alternating and pulsating DC residual current, but it can be 'blinded' by smooth DC above about 6 mA: the DC magnetises the core and the device may fail to trip on one half-cycle. That matches the test result - 1× IΔn trips at 34 ms on one polarity but fails to trip within 300 ms on the other, while 5× still trips in 19 ms. The good 5× time does not prove the RCD is healthy; the polarity failure is a failure of the additional protection.
Two sources can inject DC: the PV inverter and the EV charger's rectifier. The charger's integral 6 mA DC detection limits its own DC, but the PV inverter on the same busbar can still present DC to the Type A RCD. Cumulative AC leakage from kitchen sockets, lighting, PV and EV may also approach 30 mA, which explains the random tripping. The likely cause is DC saturation plus cumulative leakage, not a simply faulty RCD.
2. State the compliant arrangement and what you would change.
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Separate the EV charging circuit from the PV and from the general sockets. Give the EV charger a dedicated RCBO/RCD. If the charger has integral 6 mA DC detection and the manufacturer specifies Type A, a dedicated Type A 30 mA unit is acceptable. Where there is no DC detection, or where another DC source shares the RCD, use a Type B 30 mA RCD. In practice, fit a dedicated Type A or Type B RCBO for the EV, and move the PV to a separate RCD/RCBO or confirm with the inverter manufacturer that it does not inject DC above 6 mA.
Keep 30 mA additional protection for socket outlets. Do not rely on the test button. Test with an instrument suitable for the RCD type - a Type B device requires a DC test. Check discrimination with any upstream device and verify the charger's 6 mA DC detection function. If the PV inverter's instructions require Type B or a separate RCD, follow them.
3. What should be recorded and what should the customer be told about the tripping?
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Record the actual RCD type, rating and manufacturer, and the test results for both polarities, including the failed polarity. The existing certificate is not automatically wrong because it says Type A, but the installation is not satisfactory in service if the Type A device can be blinded or if cumulative leakage exceeds about 30% of IΔn (about 9 mA). Report the random tripping as a safety observation, at least C2, and recommend a dedicated EV supply with separation of the PV. Re-test after remedial work and issue a new certificate or schedule.
Tell the customer that random tripping is a symptom of a safety problem, not just an inconvenience. The test button is a mechanical check only and does not replace instrument testing. Advise against using the charger until the RCD arrangement is corrected, and explain that pressing the test button monthly does not prove the device will trip on a DC residual current.