Fault finding is the one piece of electrical work where you are not buying a part. You are buying a diagnosis. The component that finally gets replaced might cost a few pounds; the value is in knowing which of the several hundred possible components it was, and getting there without pulling a flat apart. That is an uncomfortable idea for some clients, because there is nothing to look at when it is done — the lights just work again. It is also why fault finding is where the biggest gap sits between an electrician who knows the stock and one who does not.
Central London makes it harder than most places. The flats we work in have usually been altered by four or five generations of owners, each of whom added a circuit, buried a junction box behind a plasterboard overlay, or extended a lighting run into a room that did not exist when the building was wired. Nobody left a drawing. So the fault is rarely where the symptom is.
What people mean by "a fault"
Most calls come in as one of a handful of symptoms, and each points at a different family of causes.
- Something trips as soon as you reset it. A live fault is present right now. Usually easier to find, because it is there while you look at it.
- Something trips occasionally, sometimes at 3am, sometimes when it rains. An intermittent. These are the expensive ones, because the fault refuses to appear on demand.
- Half the flat has no power. One circuit, or one protective device, or one connection somewhere in a chain.
- Lights flicker or dim when something else switches on. Frequently a connection problem rather than a component problem — a loose terminal heating and cooling under load.
- A socket is dead but the board says everything is on. Usually a break in the circuit itself, often at a point where somebody made a connection badly years ago.
- A tingle from a tap or an appliance. Stop using it and get it looked at now. This is not a "book it for next week" symptom.
The point of listing them is not to let you self-diagnose. It is that the symptom shapes the method, and the method shapes the time — which is the thing you are actually paying for.
What you can safely do before calling anyone
There is a clear line here and it is worth being precise about it. Resetting a tripped RCD or circuit breaker at the board is a normal householder action — that is what the switch is for. Working out which appliance causes the trip is also fine, and genuinely useful: unplug everything on the affected circuit, reset, then plug things back in one at a time, leaving a gap between each. If the trip returns the moment the tumble dryer goes back on, you have narrowed the job enormously and you may not need an electrician at all — you may need an appliance engineer.
Note which circuit it is, and whether the trip is the main switch, an RCD covering several circuits, or one individual breaker. Note what was happening at the time. If it is intermittent, keep a note of when it happens and what was running. That information is worth real money to whoever attends, because it substitutes for hours of testing.
What you must not do: remove the cover from the consumer unit, work on any fixed wiring, open sockets or switches, or investigate anything that has not been isolated and proven dead by someone competent to do it. The board's cover is not a service panel. Electrical work in dwellings falls under Building Regulations Part P and installations are tested to BS 7671 for a reason — the failure modes here are not "it does not work", they are fire and electric shock. If you find yourself reaching for a screwdriver, that is the point to stop. Our guide to no power and a tripping fuse box goes through the safe steps in more detail.
How a fault is actually found
The professional method is unglamorous and systematic. Broadly, an electrician isolates and proves dead, then splits the installation into halves and tests each half to see which one carries the fault. Then the faulty half gets split again. Each split roughly halves the search space, which is why a methodical approach beats intuition on anything non-obvious. Insulation resistance testing across a dead circuit will show up degraded cable or a wet connection. Continuity testing will find a break. Measurements at the board will often say, before anyone lifts a single floorboard, whether the problem lies in the fixed wiring or in something plugged into it.
What takes the time is not the testing. It is the access. Splitting a circuit means getting to the point where it can be split, and in a Central London flat that point may be behind a fitted wardrobe, under an engineered floor that was laid over the top of everything, or inside a junction box that a previous contractor plastered over. This is the practical reason fault finding cannot be quoted blind: the electrical work is predictable, and the building is not.
Earth leakage: the most common and most misunderstood
A large share of nuisance tripping in older flats is cumulative earth leakage rather than a single dramatic fault. Every appliance leaks a tiny amount to earth by design. Add enough of them to one RCD — a dishwasher, a washing machine, a kettle, an old immersion heater, a set of LED drivers — and the total can sit just under the trip threshold on a good day and just over it on a damp one. Nothing is broken. The installation is simply arranged in a way that stacks all the leakage onto one device.
The fix for that is usually not a repair but a rearrangement: splitting the load across more devices, which frequently leads to a consumer unit discussion. That is a legitimate outcome of fault finding, not an upsell — but you are entitled to have it explained, with the measurements that support it, before you agree to anything. Our page on what drives fuse board replacement cost covers the next step if it comes to that.
Why period conversions are the hard case
Victorian and Edwardian conversion stock across Bayswater and W2 generally, and the ex-commercial buildings of Fitzrovia, throw up problems that a purpose-built modern flat never will. Circuits cross between rooms that were divided at different times. Lighting runs sometimes borrow a neutral from a neighbouring circuit — a practice that was once common and that causes baffling, dangerous behaviour when an RCD is later fitted. Rubber-insulated and lead-sheathed legacy cable still appears, and its insulation fails progressively with age and heat, which produces exactly the maddening intermittent that shows up only when a circuit is loaded.
Then there is the access problem that defines flats specifically: no loft, no crawl space, and floors that are structural and often shared with the neighbour below. Where a house lets you drop into a void from above, a flat frequently forces you to go through a finish. That is why an experienced electrician spends the first part of a fault-finding visit looking for the routes previous trades used, rather than the routes a drawing would suggest — because there are no drawings, and the previous trades left the clues.
Intermittent faults and why they cost more
An intermittent is not a harder fault. It is a fault that will not stand still. Thermal faults — a loose terminal that only misbehaves once it is warm — appear after an hour of load and vanish overnight. Moisture faults track the weather. Faults in a communal supply may follow the pattern of the whole building's usage rather than yours. The honest position is that these sometimes need more than one visit, and anyone who promises a first-visit fix on an intermittent they have not seen is telling you what you want to hear.
What shortens it is data. A log of when it happens, what was on, whether it was raining, and which device tripped can turn a two-visit job into one. Keep the log.
Blocks, communal supplies and access
In a mansion block, some of what looks like your fault is not in your flat. Meters and intake equipment often sit in a communal cupboard or a basement room controlled by the porter or the managing agent, and works there need booking. If the problem is upstream of your meter, it is generally a matter for the freeholder or the distributor rather than for you, and the first useful thing an electrician can do is establish which side of that line the fault sits on. That determination alone is often the whole value of a first visit — it decides who pays.
Practicalities follow the usual Central London pattern: restricted working hours in many leases, booking a lift and protecting it, no parking outside, and materials carried in by hand. It is worth telling whoever attends about porter access and booking rules before the visit rather than at the door.
What we do, and what it costs
Our electrical work starts from £80; the published set prices are on the electrical page. For fault finding we are straight about the structure: the first block of time is diagnostic, and it either finds the fault or tells you precisely what is needed to find it. Once we know what the fault is, you get a fixed price to repair it before we repair it. We would rather tell you at the end of a diagnostic hour that the fault sits in the floor of the flat above and needs the neighbour's cooperation than quietly start cutting into your ceiling.
If the diagnosis points at an installation that is past repair rather than a single failed component, we will say so, and our guide to rewiring a period property explains how that decision is reached. If you are a landlord and the fault surfaced through an inspection, coded observations translate into work in their own way. For portfolios and managed blocks, recurring faults are usually a symptom of deferred maintenance, and they are cheaper handled through planned maintenance than as a run of call-outs. In W2 the same team covers our Bayswater electrician work.
To get it looked at, send us the details — the symptom, which device trips, when it started, and anything you have already ruled out. That last part is the most useful thing you can give us.