Mixed-brand MCB in a split-load consumer unit

The board looked fine. One MCB didn’t match the rest. It’s not a cosmetic issue — consumer units are designed to work as a matched system, and this one hadn’t been tested as one.

The find

Older split-load consumer unit — the kind with a main switch, one RCD protecting a bank of circuits, and individual MCBs. Common in 1990s and early 2000s housing. The board was the original installation. One MCB had been replaced at some point — different brand, slightly different profile, sat in a row of matching devices like a substitute player who never trained with the team.

What you’re looking at

Consumer units are designed as a system. The MCBs (miniature circuit breakers), the RCD, and the busbar arrangement are all tested together by the manufacturer to ensure they coordinate correctly under fault conditions. That coordination matters most when something goes wrong: the MCB on the faulty circuit should trip before the RCD does, keeping the rest of the house powered.

When you introduce a different brand, you break that tested relationship. The new MCB has different thermal and magnetic trip characteristics — it responds differently to overload and fault current. The manufacturer’s guarantee of coordination no longer applies to the whole board.

Why it matters — the analogy

The fault code

C2 — Potentially Dangerous. The board functions under normal conditions. The problem is what happens under fault conditions — the exact scenario protective devices are there to handle. With a mismatched MCB, you can’t guarantee the board will respond correctly. That uncertainty is what makes this C2.

The fix

Replace the mismatched MCB with the correct type from the board’s manufacturer — same brand, same series, correct rating for the circuit. If the original board brand is discontinued or parts are unavailable, the correct solution is a full consumer unit replacement with a matched modern board. Patching with whatever’s available is what created this fault in the first place.