What is the difference between an overload, a short circuit and earth leakage?

The three faults that trip a UK consumer unit, which protective device catches each one, and what the timing of the trip tells you about which you have.

Difficulty: beginner
Time: 10 min
Read: 4 min

Short answer

An overload is too much current drawn for too long — the MCB trips after a delay. A short circuit is live touching neutral or earth directly — the MCB trips instantly, usually with a bang. Earth leakage is a small current escaping to earth through damp or damaged insulation — only an RCD or RCBO catches that, and it can happen at a current far too small to bother an MCB.

Every trip in a house is one of these three. Telling them apart matters because they point at completely different causes: an overload points at what you plugged in, a short circuit points at damage, and earth leakage points at water or perished insulation. The clue is nearly always in the timing.

Overload, short circuit and earth leakage — and which device catches each

Step by step

Ask how long it took to trip.Immediate, with a bang or a flash, is a short circuit. A few seconds to several minutes after switching something on is an overload. Apparently random, sometimes with nothing on, is earth leakage.
Recognise an overload.The total current on one circuit exceeded what the cable can carry — three heaters and a kettle on one ring, or a workshop circuit doing too much. The MCB has a deliberate time delay so a motor starting does not trip it, so an overload takes a while.
Recognise a short circuit.Live and neutral, or live and earth, meeting directly. A nail through a cable, a crushed flex under a chair leg, a failed heating element, water inside a fitting. Current goes to hundreds of amps for a moment and the MCB opens instantly.
Recognise earth leakage.A small current, tens of milliamps, escaping through damp insulation, a wet outdoor fitting, a perished flex or a failing heating element. Too small for any MCB to notice, which is exactly why RCDs exist.
Match the fault to the device that caught it.MCB down only: overload or short circuit on that circuit. RCD down with a group of circuits dead: earth leakage. RCBO down: it could be either, so use the timing to decide.
Understand cumulative leakage.Every appliance with a filter — computers, induction hobs, EV chargers, LED drivers, washing machines — leaks a tiny amount to earth by design. Enough of them behind one 30mA RCD and the total sits close to the trip threshold, so any small extra tips it over. This is why RCBOs per circuit fix so many "random" trips.
Never respond by fitting a larger device.The MCB size is chosen for the cable in the wall. A larger one lets that cable overheat with nothing to stop it. If a circuit is genuinely too small for how the room is used, the answer is another circuit, installed by an electrician.

Tips

Water and heat are the two great causes of earth leakage: showers, immersion heaters, kettles, washing machines, outdoor lights and anything in a garage.
A short circuit that leaves a scorch mark or a smell is not a resettable event. Isolate that circuit and get it looked at.
If the same circuit overloads regularly during normal use, the honest fix is a dedicated circuit, not smaller appliances.

Common mistakes

Treating every trip as "the electrics being temperamental" — All three faults are the protection working correctly on a real condition. Repeated resetting without finding the cause leaves a genuine fault energised between resets.
Assuming an RCD trip means the appliance is broken beyond use — It often does, but not always — a damp appliance in an unheated garage may simply need drying out and then testing before it is condemned.

If it doesn't work

Trip happens instantly with a bang

Cause: Short circuit — damaged cable, crushed flex or a failed element — Fix: Leave that circuit off. Do not reset repeatedly. Identify which appliance or which part of the house and call an electrician.

Trip happens a minute or two after switching a heater on

Cause: Overload on that circuit — Fix: Reduce what is on that circuit at once. If normal use overloads it, ask an electrician about a dedicated circuit.

Trip happens with nothing obviously switched on

Cause: Earth leakage, often from a timed load or an accumulation across several appliances — Fix: Note the time of day it happens and what runs then. Cumulative leakage is diagnosed with a clamp meter by an electrician.

Questions people ask

Can one fault cause more than one type of trip?

Yes. A failing heating element commonly starts as intermittent earth leakage tripping the RCD, and later fails as a dead short that trips the MCB. Rising leakage over weeks is a good reason to replace an element before it fails properly.

Why does my MCB have a letter as well as a number?

The letter is the magnetic trip curve. Type B trips at three to five times its rating, and suits general domestic circuits. Type C trips at five to ten times, and is used where motors or large transformers cause big inrush currents. Changing one for the other is a design decision for an electrician.

Does a trip damage the device?

Occasional tripping is what the device is for. Repeated heavy short-circuit interruptions do wear an MCB, which is another reason not to keep resetting into a fault.