One of the more frequent questions we receive from UK installers concerns our recommendation for RCD protection on the AC side of our inverters. Specifically, when external RCD protection is required (based on local regulations or specific installation conditions), we specify a minimum 100mA RCD.
Installers often ask:
“Why not use a 30mA RCD or RCBO like we would on most circuits?”
Here’s the technical background to clarify our position — and why this isn’t unique to FoxESS.
Leakage Current in Transformerless Inverters
Like most modern solar and battery inverters, FoxESS units are transformerless. This design increases system efficiency and reduces size and cost, but it also means there is no galvanic isolation between the DC and AC sides of the system.
As a result, these inverters generate earth leakage currents during normal operation. These are typically caused by capacitive coupling between the PV array and earth, and can vary depending on system size, cable runs, and switching frequency.
These leakage currents are not indicative of a fault — they’re normal and expected. However, they can easily exceed the sensitivity of a 30mA RCD, which is why we do not recommend using 30mA devices for this application.
Why 100mA Is Recommended
A 100mA RCD is more tolerant of expected leakage currents while still offering appropriate protection for equipment and fire prevention. It also avoids the common issue of nuisance tripping, which can occur with 30mA devices on transformerless inverters even when everything is functioning correctly.
This recommendation only applies if external RCD protection is required by local regulations or site-specific conditions. FoxESS inverters do not require RCD protection for their own functionality — but if RCD protection is installed, it needs to be correctly rated to prevent operational issues.
This Is Not Unique to FoxESS
We’re not alone in making this recommendation. Most manufacturers of transformerless PV and hybrid inverters specify similar RCD requirements:
- 100mA or higher (often up to 300mA) depending on system design
- Type A or B depending on the inverter’s internal DC fault current monitoring capabilities
FoxESS inverters include built-in DC fault detection, which means a Type A RCD is sufficient in almost all cases. Always refer to the inverter’s manual for the correct specification.
If a 30mA RCD is installed on the AC side, there is a high chance of false tripping — especially in larger or more complex systems — leading to unnecessary call-outs and system downtime.
Communicating With Inspectors or Third Parties
When questioned about the use of 100mA RCDs, you can explain:
- Transformerless inverters produce normal leakage current that exceeds the 30mA threshold
- 100mA RCDs are a standard industry recommendation for this reason
- This guidance is consistent across most major inverter brands
- It ensures long-term system stability without compromising safety
Summary
When RCD protection is required on the AC side of a FoxESS inverter, we recommend:
- Minimum RCD sensitivity of 100mA
- Type A device (Type B not required unless specified for other reasons)
- Installation in accordance with local wiring regulations (e.g. BS 7671)
Using the correct RCD type and rating is essential to avoid nuisance tripping and to ensure a reliable, professional installation.
