What is an RCD and do I need one?
An RCD (residual current device) detects current leaking out of a circuit, through a damaged cable or a person, and disconnects in milliseconds, fast enough to prevent fatal electric shock. Modern consumer units protect every circuit this way (ideally via individual RCBOs); if your board has none, that is the single most valuable upgrade available to you.
The principle is elegant: current flowing out along the live should equal current returning along the neutral. Any imbalance means electricity is escaping somewhere it should not be, through insulation damage, moisture, or a human being, and the RCD trips within milliseconds at leakage levels far below what injures.
This is categorically different from a fuse. Fuses and MCBs protect cables from overload and short circuit; they act at tens of amps. An RCD acts at thousandths of an amp, which is why it protects people. A house can have perfectly good fuses and no meaningful protection against the fault that matters most.
The modern refinement is the RCBO: one device per circuit combining both protections, so a fault on the garden socket trips the garden socket, not half the house. Older split-load boards share one RCD across many circuits, which protects people equally but makes every fault a blackout and every nuisance trip a mystery. When we replace boards, RCBO-per-circuit is our default.
How to check what you have: look for a 'test' button on the board (press it quarterly; it should trip instantly and reset cleanly), and see whether one test button covers everything (shared RCD), several do (RCBOs), or none exists (no RCD protection at all, and worth a conversation this month rather than this year).
More in rewiring & fuse boxes, or browse all 40 questions on the FAQ index.
Price your project
Two quick steps. We reply within one working day.
No account, no obligation. We only use this to prepare your quote.
Ready to price your electrical work?
Tell us the job and your postcode. We reply within one working day with an honest cost bracket, then a fixed written quote.