Electrics keep tripping? How I hunt down the appliance that's doing it

I've written before about what an RCD is and why it trips... the short version being that when your power cuts out, a safety switch has usually spotted a small leak of current and shut off to keep you safe. That post answers "what's going on". This one answers the follow-up I get asked on the doorstep every week: right, but how do I find the thing that's causing it?

Nine times out of ten the culprit is a single tired appliance, and you can often track it down yourself, safely, with nothing more technical than your own two hands. Here's how I go about it... and where I reach for a tester when the simple method runs out of road.

Why one appliance can knock out a whole floor

Appliances leak a tiny bit of current to earth as they wear out. On its own that leak is harmless, but the RCD is watching the total, and once enough of it stacks up on one circuit, the switch decides something's wrong and drops the lot.

The classic one I see in the shared houses and Victorian terraces around Slough is a whole floor going dark from one knackered appliance. In a house of five with a fridge, a kettle, chargers and a mystery tumble dryer all on the same tired circuit, it only takes one to tip the balance... and everyone loses power, not just whoever owns the dryer. That's not a wiring fault, it's arithmetic. Find the one appliance and the maths comes good again.

The one-at-a-time test, done properly

You'll have seen the quick version of this. Here's the thorough version, which is the one that actually works:

Unplug everything on the dead part of the house. Not most things... everything. Every charger, lamp, telly, kettle, the washing machine, the fridge, the lot. Anything left plugged in is a suspect you've let walk free.
Reset the switch. Push the RCD back up. If it holds, good... the problem is one of the things you just unplugged, not the fixed wiring. If it trips straight back with nothing plugged in, stop here and skip to the wiring section below.
Plug things back in one at a time... and actually use them. This is the bit people rush. Plug the first appliance in, switch it on, and give it a few minutes. A fault often only shows up under load, so let the kettle boil, the washing machine get going, the fridge kick its compressor in. Rush it and you'll walk straight past the culprit.
When it trips, you've found your suspect. The last thing you plugged in and ran is very likely the one. Leave it unplugged and carry on through the rest to make sure it's the only one... occasionally there are two.

A quick tip: tick each appliance off on a notepad as it passes... four rooms deep with the power gone again, you'll be glad to see what's already cleared. And don't leave the guilty one unplugged and forget about it: a leaking appliance won't heal itself, so get it checked or replaced.

The appliances that are usually guilty

If you want to weight your suspicions before you start, the repeat offenders have two things in common: heat and movement.

Anything with a heating element... kettles, washing machines, dishwashers, tumble dryers, immersion heaters, ovens, even a toaster. The element works in a damp, hot environment and eventually starts leaking to earth.
Anything with a motor... washing machines and dryers again, older fridges and freezers, pumps, extractor fans, power tools.
Outdoor and garage kit... outside sockets, garden lights, pond pumps, hot tubs, that extension lead living in the shed. Moisture is the enemy here, which is why trips often turn up the morning after heavy rain.

The washing-machine-mid-cycle pattern

This one fools people. The machine fills, spins, behaves itself perfectly... then thirty or forty minutes in, everything trips. Cue panic that it's the wiring, when it's almost always the heating element.

A washing machine only draws serious current when it heats the water partway through the cycle. A cracked element sits dry and quiet at the start and only leaks to earth once it's energised and hot. So the trip that lands "randomly" mid-wash isn't random at all... it's the machine reaching the heat stage. Same logic for a dishwasher that trips near the end, or a shower that only trips running hot.

Instant trips versus intermittent ones

How it trips tells me almost as much as what trips it.

Instant trips (the switch won't even stay up, or drops the second an appliance is switched on) usually mean a solid, constant fault. Easier to pin down: it's consistent, so the one-at-a-time test flushes it out quickly.

Intermittent trips (fine for days, then twice in an afternoon) are the horrible ones. They're tied to a condition: the appliance getting hot, the weather turning damp, a particular programme running. If yours is intermittent, think about what was happening each time it went. Same appliance? Same time of day? Just after rain? That pattern is the thread I pull on, and it's worth jotting down before I arrive.

When the simple test doesn't crack it... the tester comes out

Sometimes the unplugging test comes back clean and the thing still trips, or the fault is so intermittent you can't reproduce it to order. That's where I stop guessing and start measuring.

I'll safely isolate the board and run an insulation resistance test on each circuit... it puts a test voltage down the cables and measures whether current is escaping where it shouldn't, finding a leak in the fixed wiring that no amount of unplugging ever will. I'll also use a clamp meter to read the actual earth-leakage current on a live circuit, then split that circuit to corner which leg it's on. It's methodical rather than magic: divide in half, test, keep the faulty half, halve it again.

I'll be honest about the limits: I can't diagnose an intermittent trip down the phone, and neither can anyone else. If it's only playing up once a week, the fair thing is to measure what's really happening rather than sell you a guess... which is exactly what my fault-finding service is for.

When it's the wiring, not an appliance

A few signs point away from appliances and towards the fixed installation... the ones I'd want to see:

It trips with everything unplugged, or won't reset at all.
It started after building work, damp, or a leak... a screw through a hidden cable while someone put up shelves is a classic.
The tripping comes with any burning smell, buzzing, warmth or scorch marks at a socket or the fuse box.

That last one changes things. A trip is normally your electrics protecting you, but combined with heat or a smell it can be the first sign of a damaged cable or failing connection... not a next-week job. Switch off at the main switch if you safely can and call my emergency line. If there's smoke, sparking or anyone's had a shock, ring 999 first.

Still can't find it?

If you've unplugged your way through the whole house and it's still tripping, you've done the hard part... you've proved it's not a simple appliance, which tells me exactly where to start.

I'm a NAPIT-registered electrician based in Slough, covering Windsor, Maidenhead and across the Thames Valley, and I trace faults like this most weeks. For a free, no-obligation quote, get in touch at bphelectrics.co.uk and I'll come and find it properly.