When a Residual Current Device (RCD) trips, it’s doing exactly what it was put there to do, cut the power to stop people getting hurt.
But if it keeps tripping and there’s no obvious reason why, it’s not just frustrating, it’s a signal that there’s something amiss with your wiring, the appliances you’ve got, or the way the power is balanced in your system.
Before you start pulling out the fuses or blaming your kettle, it’s worth taking a step back and trying to figure out what’s really going on behind that flicking switch.
What An RCD Actually Does (And What It Doesn’t Do?)
An RCD is always on the lookout for the difference between the current coming into a circuit and the current flowing out of it
If a small bit (we’re talking thirty milliamps) of that current leaks out to earth, that could be down to a dodgy appliance, damp insulation, or a damaged cable, for instance, the RCD will cut power in a split second.
That rapid shut-off is what saves lives, it stops people getting a nasty shock or a fire starting due to an electrical fault
But what RCDs don’t do is stop short circuits or overloads, that’s down to your MCB.
So, if your RCD is tripping, it’s picking up on an earth leakage fault, which could be related to wiring, dampness, or faulty equipment. The tricky bit is working out what the actual culprit is.
9 UK-Specific Reasons Your RCD Keeps Tripping
1 – Moisture: The Silent Cable Killer
Outdoor sockets, pond pumps, EV chargers, and patio lights are the ones that always seem to cause problems.
Moisture creeps into the cable joints or fittings, especially if the cable was installed a while back and the weatherproofing rating wasn’t high enough.
Here’s how you can tell if it’s the culprit: the RCD suddenly starts tripping whenever it’s damp outside, or even after you’ve given the patio a good clean with the power hose.
The first step is to isolate the circuit at the consumer unit and get it checked out. A proper insulation resistance test should be enough to track down the dodgy cable.
2 – Neutral To Earth Fault
These are classic faults, and one that will keep you up all night trying to figure out what’s gone wrong.
What’s happening is that there’s some kind of unintended path that current is flowing through, and it’s not doing its job properly.
Here are a few ways this can happen:
- Your neutral wire has made contact with the earth terminal in a socket back box somewhere.
- There’s a damaged flex inside some appliance that’s shorting out the neutral and earth conductors.
The RCD picks up on this imbalance and trips, even if nothing seems to be going on.
3 – Faulty Appliances
Sometimes the problem isn’t the wiring, it’s the appliance itself.
Common culprits include:
- Those old washing machines or dishwashers that have been running for years, and the heating element is starting to get a bit dodgy from years of damp.
- That fridge compressor seems to leak a bit of current when it’s on.
- Any power tools or lawn gear that’s been stored in the shed for months & is starting to get mouldy.
To figure out which one is the culprit, try unplugging everything one by one, resetting the RCD, and plugging them back in one at a time. If it trips when you plug the next one in, you know the one that’s causing it.
4. Shared Neutrals
Old consumer units can be a real minefield, especially if they were tampered with without a full rewire.
The shared neutral is the problem, it causes the RCD to get a bit confused between the circuits.
You’ll usually notice it when two of the circuits share the same RCD, and turning one off still trips the other one. Safety first, so get a pro in to run some tests with a proper insulation resistance meter.
5. Nuisance Tripping: When Leakage Adds Up
These days, it’s not uncommon for a house to have dozens of devices that are constantly leaking a tiny bit of current to earth, chargers, LED lights, computers, the lot.
Each one individually is harmless, but when you add them all up, those little leaks can be enough to tip the RCD over the edge.
If you have a house full of sensitive electronics, a modern Type A RCD or RCBO might be the answer, they can handle DC leakage better than the older units.
6. A Faulty RCD Or Consumer Unit
It’s not impossible, the RCD itself can just fail. Age, wear and tear, and even a bit of mechanical failure can cause the RCD to go rogue and start tripping under normal conditions.
Get a qualified electrician in to check it out. They should be able to run some tests to see if the RCD is up to scratch, if not, they can sort it out for you.
7 – Water Ingress Inside Walls Or Junction Boxes
You’d be surprised how often minor leaks in roof spaces or walls end up causing electrical chaos.
Condensation around bathroom extractor fans or leaking roof tiles can lead to dampness inside junction boxes.
Over time, that moisture lowers insulation resistance, and the RCD interprets it as an earth fault.
Infrared thermography or moisture meters can help trace such hidden culprits without tearing into walls.
8 – Incorrectly Wired Extension Or DIY Alterations
If someone’s extended a ring or added a spur incorrectly, they may have crossed neutral or earth connections without realising. That’s why an RCD may start tripping right after “a bit of DIY.”
Even small mistakes like connecting two lighting circuits on a shared neutral can create persistent nuisance trips.
9 – Faulty Heating Elements Or Immersion Heaters
Immersion heaters, underfloor heating mats, and towel rails are known sources of intermittent earth leakage, especially in older homes with hard water or corrosion. Sometimes the fault only appears once the system heats up.
Testing resistance when cold won’t always catch it, a thermal test under load usually reveals the leak.
How To Safely Diagnose An RCD That Keeps Tripping
Let’s be blunt: DIY testing electrical systems has limits. But there are a few safe steps to narrow things down before calling a professional.
Step 1: Identify What’s On The Tripped RCD
Check the consumer unit and note which circuits it protects, often marked “sockets,” “lighting,” “upstairs,” etc.
If you’re unsure, turn them off one by one and reset the RCD. See which one causes the trip.
Step 2: Isolate The Suspect Circuit
Once you’ve found the faulty circuit, unplug every appliance on it. Reset the RCD. If it holds, plug devices back in gradually. The fault will often show itself at this stage.
Step 3: Check For Moisture Or Obvious Damage
Look for outdoor connections, garden sockets, or damp walls. If the RCD trips after rain, that’s a clue moisture’s involved.
Step 4: Avoid The Guessing Game
Don’t start rewiring or swapping breakers. The fault could lie deep inside the insulation or at a junction box.
Call a qualified electrician with the proper testing kit, it saves hours of trial and error and avoids risk.
How Electricians Professionally Diagnose RCD Faults
A good spark will methodically go through the following tests:
- Continuity and Polarity Checks : Confirms correct connections throughout the circuit.
- Insulation Resistance Testing : Checks for leakage through cable insulation.
- Earth Leakage Clamp Testing : Measures how much current is escaping to earth under live conditions.
- RCD Trip Time Testing : Confirms the RCD trips within BS 7671’s safe limits.
These tests narrow the fault to a specific cable, circuit, or appliance, usually within an hour or two.
When To Call A Pro?
It’s always best to call a pro, If your RCD:
- Trips instantly after reset.
- Trips randomly, even when nothing’s running.
- Trips after rain or humidity changes.
- Or protects key circuits like showers, cookers, or EV chargers.
DIY fixes can miss underlying insulation failures or shared neutral faults. A Part P registered electrician has the tools to measure exact leakage levels and verify compliance with BS 7671.
Simulate Faults Safely Before Facing Them Live
If you’re in the trade or training to enter it, TradeFox offers realistic fault simulation environments, perfect for electricians and apprentices who want to understand scenarios like RCD tripping, insulation failure, or circuit overloading without ever touching live gear.
It’s hands-on learning without the hazard.
Conclusion
An RCD that keeps tripping isn’t just a nuisance, it’s your electrical system raising a flag. Sometimes it’s moisture, sometimes a tired appliance, sometimes a wiring quirk from 20 years ago. But it’s never something to ignore.
Diagnose what you can safely. Then let a certified electrician handle the rest. Because when an RCD cuts out, it’s not failing, it’s protecting.
