Solar Electric System: Ground Faults, Low Iso Faults, and RCD Tripping Due to Prolonged Heavy Rainfall

Addressing Electrical Faults and Safety Measures in Solar Systems During Heavy Rain

Preamble

Photovoltaic panels work in all weather conditions to different degrees of efficiency, with apparent reductions in output from low light conditions in winter and wet weather conditions, causing low light levels.

Problem

However, from feedback from our customers over the last decade, we have had occasional reports of inverters shutting down or displaying 'Ground Fault' or 'Low Iso Fault' messages, dependent upon the inverter manufacturer, during torrential rainstorms during the day and/or residual current devices (RCDs and RCBOs) tripping. These have generally been from panels that have been installed at least 7 or 8 years.

Cause

Due to the extremely wet weather conditions, the solar panels can become saturated and suffer from water ingress into the modules and back sheets of the panels, which causes a reduction of the insulation resistance of the panel between the cells and its frame, mounting system and the saturated roof. Consequently, there is DC current leakage between the panel and the building, which somehow flows, even on an unearthed mounting system, via the building structure and is picked up by the building's earthing systems, either in metal pipework, earth rods, or metal backboxes and is transferred to the inverter or the circuit/building RCD.

Solutions

If the PV circuit is connected to a distribution board with an overall RCD or dual RCDs protecting several circuits, a relatively small Solar leakage current of less than 30 mA, can be combined with an existing leakage from another circuit. Possible examples are outside lights and certain older, metal-cased appliances such as washing machines, again with leakage of less than 30 mA, but the two combined are greater than 30 mA and cause the RCD to activate and 'trip' out.

In this instance, it is relatively easy to alter the wiring of the PV circuit by adding its own RCBO, fitted in parallel with the existing RCDs in a dual RCD board, or on its distribution board and RCD if the existing board has an overall RCD.

Very rarely, however, we have seen cases where the DC leakage can trip a 30mA RCD that only protects the Solar PV. In these rare instances, the options are limited to:

• - moving the rooftop to a dryer part of the world ;-)
• - replacing the panels with newer panels ;-)
• - or, more realistically, leaving the system switched off for a day or two at worst and resetting it when the panels and roof have dried out. In real terms, you will have lost minimal generation as the wet weather reduces generation to 10-20% of optimum levels.