Plug-In Solar and Old Fuse Boards: Is Your Consumer Unit Safe?
This is the question most buyers do not ask. The panel, the inverter, the bracket — those are straightforward. But what the solar feeds into matters just as much. An old rewirable fuse board that predates residual current protection can create a problem the inverter itself does not cause.
What type of protection does your consumer unit have?
Modern consumer units installed since the 1990s have RCD protection — Residual Current Devices or RCBOs on individual circuits. RCDs detect earth fault currents and disconnect within milliseconds. Older boards fall into three main types with no RCD protection: rewirable fuse boards (ceramic holders with fuse wire, pre-1970), cartridge fuse boards (plug-in cartridges, 1970s to 1980s), and early MCB boards (miniature circuit breakers but no RCD, 1980s to early 1990s).
Adding a microinverter to a circuit without RCD protection does not meet BS 7671 Amendment 4 requirements. It is not immediately dangerous in normal operation — the inverter’s anti-islanding will still disconnect on a grid fault — but the circuit lacks the earth fault detection that wiring regulations require for any new electrical work including microinverter connections.
The Type B RCD issue
Even with modern RCDs, solar adds a specific requirement. Standard Type A RCDs detect AC fault currents. Solar microinverters can produce smooth DC fault currents that Type A devices miss. A Type B RCD or RCBO detects both and is the correct protection device for any circuit feeding a solar inverter on a hardwired installation. For a socket-connected certified system, the inverter design limits DC leakage so that Type A protection remains adequate — but Type B is always the safer choice and costs little extra when an electrician is already on site.
What to do
Get an electrician to assess your board before connecting solar. A rewirable or cartridge fuse board should be replaced — a consumer unit upgrade costs £600 to £1,200 and brings the whole installation up to standard, worth doing independently of solar. If you have a modern RCD board, confirm the sockets you are using are on an RCD-protected circuit with Type B protection on the specific circuit feeding the inverter.
For the full Type B RCD requirement and why it matters, see our Type B RCD article. For pre-installation safety steps, see our safety tips article.