RCD vs RCBO vs MCB: what's the difference?

An MCB protects against overload and short circuit. An RCDprotects against earth leakage — the shock risk. An RCBO is both in one device, protecting a single circuit against overcurrent and earth leakage. The difference matters because missing RCD protection is one of the most common reasons an EICR comes back unsatisfactory.

MCB — Miniature Circuit Breaker

Trips when a circuit draws too much current — an overload or a short circuit. It protects the cable from overheating. What it does not do is protect a person from an electric shock: an MCB won’t trip fast enough on the small leakage current that flows through a body to earth.

RCD — Residual Current Device

Monitors the balance between line and neutral. If current is leaking to earth — through a fault, or through a person — the balance breaks and the RCD trips, fast. A 30 mA RCD is the level used for “additional protection” against shock. A single RCD often protects several circuits at once, which is why one earth fault can trip half the house.

RCBO — the best of both

An RCBO combines the MCB and the RCD into one module, per circuit. Two advantages: every circuit gets its own shock protection, and a fault on one circuit trips only that circuit — the freezer stays on when the bathroom light develops a fault. Modern boards increasingly use RCBOs throughout for exactly this reason.

Why this shows up on EICRs

BS 7671 requires 30 mA additional protection for most socket circuits and many others in domestic installations. Older boards — rewireable fuses, or MCBs with no RCD — frequently don’t provide it, and that’s coded C2 where the protection is needed and absent. “It was compliant when it was fitted” doesn’t change the in-service risk today, which is what the code reflects.