A tripping fuse board is one of those problems that ranges from mildly annoying to genuinely dangerous — and the difference isn’t always obvious from the symptom. Sometimes it really is a knackered kettle. Sometimes it’s water dripping into a bathroom light. This guide walks through what’s actually happening when an RCD or MCB trips, the three most common causes in homes across Havant, Waterlooville, Portsmouth, Petersfield and the surrounding villages, how to safely narrow down the culprit yourself, and — importantly — the warning signs that mean you should stop DIYing and call a qualified electrician the same day.
What’s actually happening when the board trips
There are two separate protective mechanisms inside a modern consumer unit, and knowing which one has tripped is the first step in diagnosis:
- Overcurrent protection — the MCB, or the overcurrent side of an RCBO. This trips when a circuit is drawing more current than the cable can safely handle. The cause is either an overloaded circuit (too many appliances running at once) or a short circuit (live and neutral touching each other somewhere in the wiring or an appliance).
- Earth-leakage protection — the RCD, or the RCD side of an RCBO. This trips when current is going somewhere it shouldn’t: leaking to earth via damaged insulation, a wet fitting, a faulty appliance or, in the worst case, a person.
Older split-load consumer units, still common in properties last rewired in the 1990s or early 2000s, group several circuits under a single RCD. When it trips you lose multiple circuits at once, which makes it much harder to pinpoint the fault. Modern full-RCBO boards — the standard for new installs since the 18th Edition amendments — protect each circuit independently, so you only lose the affected circuit and diagnosis is dramatically easier.
The three most common causes in local homes
1. A faulty appliance
Overwhelmingly the most common cause. Deteriorating heating elements in kettles, washing machines, dishwashers, tumble dryers and immersion heaters are the classic offenders — they develop small internal insulation faults that leak to earth intermittently. Cheap phone chargers, older fridges, garden power tools brought in from a damp shed and cheap Christmas lights are also frequent culprits. The tell-tale sign is that the trip happens when a specific appliance switches on or starts a heating cycle.
2. An overloaded circuit
More common in older properties — the Victorian terraces in Southsea and older parts of Havant town, and the 1930s semis around Cosham and Waterlooville — where “a socket outlet” was originally specified for a table lamp, not a 3kW kettle plus a microwave plus an air fryer plus a phone charger. The give-away here is consistent tripping when the same combination of high-draw appliances is running (kettle + toaster + microwave, or shower + immersion together). The fix is usually to spread the load, or in stubborn cases to add a new dedicated circuit.
3. Water or moisture
Very common in Hampshire coastal properties. Coastal humidity, wind-driven rain finding its way into loft spaces, and simple condensation all take their toll. Common triggers: bathroom light fittings after a hot shower, garage sockets after heavy rain, garden sockets after any rain, kitchen extractor fans with water ingress. The pattern is telling — trips that happen after weather events or after specific activities (showering, laundry) point to moisture.
Less common but more serious causes
4. Deteriorating fixed wiring
Cables installed before around 1970 — rubber or lead-sheathed — degrade internally as the insulation breaks down. Eventually they leak to earth and trip the RCD. If you’re in an older Southsea, Fratton, Old Portsmouth or original Havant town centre property that hasn’t been rewired in more than 40 years, this is a realistic possibility. It usually presents as trips that happen at random, unrelated to which appliances are running.
5. Loose connections
A loose neutral or earth terminal — often inside a socket, a ceiling rose or the consumer unit itself — can cause intermittent tripping that’s notoriously hard to diagnose without proper thermal-imaging or test equipment. Symptoms include trips that happen when the property warms up (or cools down), or trips that follow physical activity (a door slamming, someone walking heavily upstairs).
6. A failing RCD or RCBO
Protective devices themselves have a finite lifespan and can develop faults that make them trip erroneously — sometimes at extremely low currents. If everything else on the circuit checks out clean and the device is more than about 15 years old, sometimes the fix is simply to replace the RCBO.
How to safely diagnose which one it is
You can do a reasonable amount of narrowing-down yourself before calling anyone — as long as you stop the moment anything feels wrong. A safe, methodical order:
- Note exactly which switch tripped — is it a main RCD (usually a larger switch at the top), an MCB (a smaller single-circuit switch) or an RCBO (has a small test button and covers one circuit)? Write down what you see.
- Unplug everything from sockets on the affected circuit.
- Turn off every light switch on the affected lighting circuit.
- Reset the tripped switch. If it holds, the fault is in something you unplugged or switched off.
- Plug appliances back in one at a time. Wait 10–15 minutes between each — some faults only show up when the appliance actually cycles on. The appliance that trips the board is your culprit.
- If the switch trips again immediately with nothing plugged in and everything switched off, the fault is in the fixed wiring or the protective device itself — stop and call an electrician.
- If the switch holds for hours and then trips at random, you’re most likely dealing with moisture ingress or a heating element cycling. Worth calling an electrician for proper testing rather than continuing to guess.
A few absolutes: never open the consumer unit; never remove socket faceplates while trying to diagnose; never wet-finger anything electrical to see if it’s live. If you have any doubt at all about what you’re seeing, isolate the affected circuit and call a professional.
Warning signs that mean STOP and call the same day
Any of the following, and you should stop diagnosing, isolate the main switch and call an electrician the same day — not the next day, not next week:
- A burning smell coming from any socket, switch or the consumer unit itself
- A warm or hot socket, switch or consumer unit
- Brown, black or discoloured scorching around any electrical fitting
- Sparks or a crackling sound from a socket, switch or the board
- Any evidence of water inside an electrical fitting
- Buzzing or humming from the consumer unit
- A circuit that won’t reset at all, no matter what you unplug
- Multiple circuits tripping simultaneously and repeatedly
These are all indicators of either an active fire risk or a genuine shock hazard. The £120–£180 for a same-day call-out is a rounding error next to a house fire.
What a proper diagnostic visit looks like
If you’ve narrowed it down to “something in the fixed wiring”, or you’re just done trying to guess, here’s what a competent electrician actually does on a diagnostic visit — worth knowing so you can spot corners being cut:
- Uses a calibrated insulation resistance tester (a “megger”) to identify the exact circuit that has an insulation fault, and often the exact section of that circuit.
- Uses an RCD tester to verify the protective device is tripping at the correct current and within the correct time — this tells us whether the RCD itself is faulty.
- Runs earth loop impedance tests at every accessible point on the suspect circuit.
- Systematically isolates half-circuits at the consumer unit — physically splitting a ring, for example — to bracket in on the fault location.
- On more complex jobs, uses a thermal imaging camera to hunt for loose connections in consumer units and larger installations.
A straightforward diagnostic visit is usually 1–2 hours; an intermittent fault can take 3–4 hours or more of methodical testing. Typical diagnostic cost across Havant, Portsmouth and Petersfield in 2026 is £80–£180 for the visit itself, with any repair quoted separately once the fault has been located.
Frequently asked questions
Is it OK to keep resetting a tripping RCD?
No. Every trip is telling you something. Repeatedly resetting a faulty RCD can eventually damage the device itself, and if the underlying fault is a live-to-earth leakage, you’re resetting a genuine safety mechanism into a live fault.
Why does it only trip at night?
Two likely causes: overnight moisture accumulation (dew inside a fitting, condensation inside a poorly sealed light) or a heating cycle from an appliance that runs at night (fridge, freezer, immersion on a timer).
I’ve unplugged everything and it still trips. What now?
The fault is in the fixed wiring or the protective device itself. Time to call an electrician — that’s not something to keep poking at.
The whole house tripped, not just one circuit — what does that mean?
Usually a fault on the main RCD, or an incoming supply issue. If you can smell burning or feel heat anywhere on the board, treat it as urgent.
How much does an electrician charge for a call-out?
£80–£180 for a diagnostic visit across this area, day rate. Out-of-hours emergency call-outs are typically £120–£250 depending on time of night.
Book a diagnostic call-out
Our local electricians cover Portsmouth, Havant, Waterlooville, Cowplain, Horndean, Petersfield and Rowlands Castle, with same-day diagnostic appointments in most cases and 24/7 emergency response for genuine safety issues. If you’re seeing any of the warning signs above, don’t wait — call us the same day.