Half my house has no power: what it means

It is a strange sight. The kitchen lights work but the sockets are dead. Or upstairs is fine and downstairs is dark. When half a house has no power, there are only a few likely causes, and you can rule most of them in or out from the front of your consumer unit.

The most likely cause: one RCD has tripped

Most consumer units fitted from the 2000s onwards are what electricians call split load boards. Instead of one safety switch for the whole house, the circuits are shared between two or more RCDs.

An RCD (residual current device) watches for electricity leaking where it should not, such as to earth through a damp fitting or a faulty appliance. When it senses a leak, it cuts power to every circuit it protects. If your board has two RCDs and one trips, roughly half the house goes off.

What it looks like:

• One larger switch, usually marked RCD or with a test button, is in the down position.
• The smaller switches (MCBs, miniature circuit breakers) next to it may all still be up.
• Everything on that side of the board is dead, even circuits that have nothing wrong with them.

What to do

1. Switch off or unplug the big appliances on the dead circuits. Kettle, washing machine, heaters, outside lights.
2. Push the RCD back up once.
3. If it stays up, plug things back in one at a time. If it trips when one appliance goes in, you have found the likely culprit.
4. If it trips straight away with everything unplugged, the fault is in the fixed wiring. Leave it off.

Our guide to why electrics keep tripping goes through this in more detail.

Second cause: one circuit breaker or fuse

If only one area is off, such as the upstairs sockets or the kitchen lights, it may be a single MCB rather than an RCD. In older homes with a fuse box, it may be a blown fuse wire.

Do not repair fuse wire yourself or swap in a bigger fuse. Old rewireable fuses can be replaced with the wrong size, which removes the protection. If you still have a fuse box with pull out fuses, it is a sign the installation is due a proper look. Our consumer unit replacement page explains what upgrading involves.

When nothing has tripped

If every switch is up and half the house is still off, there are two possibilities.

A fault in your wiring. A loose or burnt connection can break a circuit without tripping anything. Sockets on a ring circuit, the loop of cable that feeds a run of sockets, can go dead because of one failed joint. We cover that case in no power to sockets but lights work.

A problem on the network. Some London homes, and many flats and larger houses, have a three phase supply. That means three separate live feeds, with circuits shared between them. If one phase is lost on the network, part of the house can go dark while the rest works. Even on a normal single phase supply, a network fault can sometimes cause dim lights or strange behaviour in several homes on a street.

Signs it may be the network:

• neighbours also report part power, dim lights or flickering
• appliances run weakly or make unusual noises
• the problem started during a storm or roadworks

In this case, call 105. It is free and connects you to UK Power Networks or, in Ealing and Hounslow, to SSEN. Switch off sensitive equipment until the supply is confirmed normal.

A quick guide

What you find	Likely cause	Who to call
One RCD down, half the house off	Earth leakage on one of its circuits	Reset; electrician if it will not hold
One MCB down, one area off	Overload or fault on that circuit	Reset; electrician if it trips again
Old fuse blown	Overload or fault	Electrician
Nothing tripped, one circuit dead	Loose or burnt connection	Electrician
Nothing tripped, neighbours affected, lights dim	Network fault	105
Burning smell or scorch marks anywhere	Overheating	Main switch off if safe; electrician

Why this is common in outer London houses

Many London suburbs were built between the wars. Figures from the VOA council tax stock of properties show that 51.8% of homes in Harrow and 40.6% in Sutton were built between 1919 and 1939. Those houses have usually been rewired at least once, and many have had extensions, loft conversions and new kitchens added since. Each change can add circuits to an RCD that was already close to its limit.

Every appliance and cable leaks a tiny amount of current. On one RCD these small amounts add up. Add a damp outside light, an old immersion heater or a tired washing machine, and the RCD trips. Half the house goes off, even though the real fault is in one place.

An electrician can measure how much leakage each circuit has and spread circuits across the board so one small fault does not take out half your home. Where the board is old, upgrading to one with an RCBO for each circuit, a combined breaker and RCD, means only the faulty circuit trips.

When it is urgent

Treat it as urgent if:

• the dead half includes a fridge or freezer with medicines, or medical equipment
• there is a smell of burning, buzzing or crackling from the consumer unit
• you see water near the consumer unit or a light fitting
• someone has had a shock. Do not touch them while they may be in contact with the supply. Switch off at the mains if you can and call 999.

Our no power in the house service covers homes across London. If you are in a 1930s semi in Harrow or anywhere else in the suburbs, the first step is the same. Look at the board, reset once, and call for help if it will not hold.