Unidirectional RCBO — Solar PV circuit
The RCBO protecting this solar circuit could only detect a fault coming from the grid. Solar panels also generate electricity — so it was facing the wrong way for half the risk.
The find
Routine EICR on a property with Solar PV installed by a registered MCS contractor. The system was generating, the paperwork was in order, and the board looked fine from the outside. When I checked the RCBO specification on the solar circuit, the label said it all: supply-side protection only.
The photo below shows the device fitted. You can see “Solid Neutral” and the supply-in-only diagram on the label — that’s the tell.
[Photo: RCBO showing “Solid Neutral” label — upload as blog/what-i-found/images/001-rcbo-solid-neutral.jpg]
What you’re looking at
An RCBO (Residual Current Breaker with Overcurrent) is the device that protects a circuit from both overloads and earth faults. The one fitted here is a standard domestic RCBO designed for a normal supply circuit — electricity flows in one direction, from the grid into your home.
Solar PV changes that. The inverter generates AC electricity and feeds it back into the circuits. Fault current can now flow in either direction. A unidirectional RCBO — one designed for supply-only — will not detect a fault originating from the inverter side.
Why it matters — the analogy
Think of the RCBO as a security guard watching a door. A standard RCBO is trained to spot trouble coming from one direction only — from outside, through the door, into the building. Solar panels mean there’s now another entrance at the back. The guard is still watching the front door. Whatever comes in the back goes undetected.
A bi-directional RCBO monitors current in both directions. It costs a few pounds more and takes the same space in the board. There is no justification for fitting the wrong device on a generating circuit.
How the two devices compare
The fault code
C2 — Potentially Dangerous. The installation functions day-to-day. But a fault on the inverter side of the circuit has no protective device to detect it. Under the right conditions — an insulation failure, a loose connection, a component fault inside the inverter housing — that undetected fault current has nowhere safe to go.
The fix
Replace the standard RCBO with a bi-directional RCBO of the correct rating for the solar circuit. This is a like-for-like swap in the consumer unit — same form factor, same installation process, correct specification. The remedial work is straightforward; the original installation decision was not.
Have solar panels? Get them independently checked.
An MCS certificate confirms the system was registered. An EICR confirms the electrical connection is safe.
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