Why does the whole house go off instead of just one circuit?

In a lot of UK boards installed between the late 1990s and the late 2010s, one or two RCDs sit upstream of a whole bank of MCBs. That design is why a failing kettle in the kitchen can plunge the entire house into darkness, and it is the single most common reason people describe a trip as "everything went off".

Safety

Step by step

1. Look outside first.If the street lights and the neighbours are dark too, it is a supply outage and nothing in your board is at fault. In Great Britain, 105 reaches your distribution network operator from any phone.
2. Take a torch and look at the board before touching it.Photograph it. What you want to know is which single device is down: the main switch, an RCD with a test button, or an RCBO. That determines everything that follows.
3. If an RCD is down, understand what it covers.A split-load board typically has two RCDs, each feeding six or eight MCBs. One leaking circuit anywhere in that group drops the whole group. That is not a fault in the board, it is how the board was designed.
4. Switch off every MCB behind the tripped device.All of them, before resetting anything. This is the step people skip and it is the one that makes the rest work.
5. Reset the tripped RCD or main switch.It should hold with everything off. If it will not hold with every MCB off, the fault is upstream of the circuits and it is an electrician's job — stop here.
6. Restore circuits one at a time, slowly.Switch one MCB on, wait a few seconds, then the next. The one that drops the RCD again is your faulty circuit. Leave it off and switch the rest back on so the house is usable.
7. Narrow within the faulty circuit.With that circuit still off, unplug everything on it, then bring it back on and reintroduce loads by halves. If it will not hold with nothing connected, the fault is in the fixed wiring and the circuit stays off.
8. Consider whether the board layout is the real problem.If one circuit taking down the whole house is a recurring event, the modern answer is RCBOs so each circuit is protected individually. That is a consumer unit change: a qualified electrician, a certificate, and notification under the Building Regulations.

Tips

• Keep a torch that is not your phone somewhere you can find in the dark near the board. Diagnosing a board by phone light while the phone is at 4% is a bad evening.
• Restore the freezer and fridge circuit first once you know it is not the culprit, and check both are actually running afterwards.
• If it happens repeatedly with no identifiable cause, ask the electrician to measure standing earth leakage — cumulative leakage across a whole RCD group is a common and very confusing cause.

Common mistakes

• Resetting the main switch or RCD with all the MCBs still on — It just trips again on the same fault, and you learn nothing. Everything off first, then back on one at a time, is what turns the event into information.
• Assuming a whole-house trip means a serious wiring fault — On a split-load board it usually means one ordinary appliance fault in a group of eight circuits. The scale of the blackout says nothing about the scale of the fault.

If it doesn't work

Questions people ask