Three different faults all get called a short circuit. They behave nothing alike, and each one is caught by a different device in your switchboard. One of them is caught by nothing at all unless you have the right device fitted.
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In a healthy circuit the current leaves the switchboard on the active, does its work inside the appliance and returns on the neutral. The appliance is the thing that decides how much flows, because it is the resistance in the path. A kettle has very little and draws a lot. A phone charger has a great deal and draws almost nothing.
A short circuit is what happens when that resistance disappears. An active conductor touches a neutral directly, with nothing useful in between, and the only thing left limiting the current is the resistance of the cable itself, which is almost nothing. The voltage has not changed. The resistance has collapsed. So the current rises to an enormous figure in a fraction of a second, and keeps rising until something opens the circuit.
That is the bang, the flash and the instantaneous trip, and they are all one event. It is also why a short circuit is loud and an earth fault often is not, which brings us to the useful part of this article.
Three separate things get called a short circuit in ordinary conversation, and they are not the same fault. A genuine short circuit, an overload, and an earth fault behave completely differently, are caught by different devices, and mean very different things about the state of your house. Telling them apart takes about thirty seconds and it changes what you do next.
Every step along this scale is ten times the one before it, which is the only honest way to show currents this far apart on one line.
Scroll to see the full scale →
What it means: the numbers that matter to a human body sit at the far left of this scale, in thousandths of an amp. Around 40 milliamps sustained for more than two seconds can cause ventricular fibrillation, and a safety switch is required to open at 30, which is the entire margin. A circuit breaker is rated in whole amps, so it sits three decades to the right and is completely blind to everything in between. That gap is not a design flaw. It is why the two devices are different devices, and why a house with breakers but no safety switches is protected against fire and not against electrocution.
Two questions answer it almost every time: how quickly did it go, and which device moved.
| Fault | What has happened | What it looks like | What catches it |
|---|---|---|---|
| Short circuit | An active conductor touches a neutral, or another active, directly | Bang, flash, instant trip, sometimes a burnt smell | The breaker's magnetic trip, in milliseconds |
| Overload | Nothing is broken. A healthy circuit is being asked to carry too much | Trips after minutes, often at the same time of day | The breaker's thermal trip, slowly |
| Earth fault | Active meets earth: a metal case, damp, damaged insulation, or a person | Often nothing visible at all. The safety switch goes and the breaker does not | The RCD, at 30 mA |
The speed is the giveaway. A circuit breaker has two entirely separate mechanisms inside the same body. A bimetallic strip heats up and bends when the current is above its rating, which takes seconds or minutes depending on how far above, and that is the overload half. A magnetic solenoid snaps the contacts open when a very large current appears, which happens in milliseconds, and that is the short circuit half. A breaker that goes the instant you switch something on is telling you a different story to one that goes twenty minutes into dinner.
Which device moved is the other half of the diagnosis, and it is the more important one. If the safety switch is the thing that opened, current was going somewhere it should not have gone, and the somewhere it goes is often a metal case, a wet floor or a person. Treat that as more urgent than a breaker trip, not less, even though nothing visible happened.
Flexible cords, at the two ends. By a wide margin the most common cause, and the one people least expect, because the cable itself looks fine along its length. A cord fails where it flexes: right at the plug and right where it enters the appliance. Vacuum cleaners, hair dryers, extension leads and anything on wheels are the usual suspects.
Water where there should not be any. Not always dramatic. A kettle base with a slow leak, an outdoor lead lying in a puddle after rain, a bathroom heater that has been steamed for ten years, a fridge defrost drain that has quietly blocked. Water is the classic earth fault rather than a short circuit, which is why the safety switch goes and the breaker sits still.
Nails and screws. A picture hook, a shelf bracket, a new architrave, a bathroom renovation. Cable in a wall does not announce itself, and a fastener that clips the insulation without going all the way through can sit there for months before it becomes a fault. This one is worth mentioning to anybody about to drill.
Rodents. Common in Sydney roof spaces and under floors, and they chew insulation for reasons that have nothing to do with electricity. It is one of the reasons a periodic look in the roof space is worth doing, particularly in a house where the wiring is original.
Loose terminations. A connection that is not tight heats up every time the circuit is used, cools when it stops, and works itself progressively looser over years of that cycle. It shows up first as intermittent behaviour and discolouration rather than as a trip, and it is the reason a switchboard that smells faintly of hot plastic is worth a look now rather than later.
Age. Rubber and fabric insulation from before about 1960 goes hard and crumbles, particularly where it has been warm, which is exactly where it is most likely to be disturbed. If a house is on its original wiring, faults stop being events and start being a pattern. That is a different conversation, covered on our rewiring page.
Search this subject and the majority of what comes back describes a different country's electrical system in a different country's vocabulary. It is not wrong, it is just not about your house, and the mismatch causes real confusion when somebody is trying to follow advice or buy a part.
The American active conductor is called a hot wire. Earth is called ground, so an earth fault becomes a ground fault, and the device that catches it is a GFCI rather than an RCD or safety switch. Their supply is 120 volts at 60 hertz on ordinary circuits, so the currents for a given appliance are roughly double ours and their cable sizes are quoted in gauge rather than square millimetres. Their protection philosophy differs too: GFCI protection is commonly built into individual outlets, where here the residual current device usually sits in the switchboard covering whole circuits.
The physics is identical everywhere. The vocabulary, the voltages and the rules are not. If an article uses the words hot, ground or GFCI, it is worth reading for the principle and not for the specifics.
Four steps, and the last one is the one people skip.
Unplug everything on that circuit first
Before touching the board, switch off and unplug everything the circuit feeds. Resetting into whatever caused the trip is how a fault that had been safely interrupted gets a second go.
If you are not sure what is on the circuit, that in itself is worth fixing, because an unlabelled board turns every future fault into a treasure hunt.
Reset once, and only once
With everything disconnected, reset the device. If it holds, the fixed wiring is probably fine and something you unplugged is the cause. If it trips again immediately with nothing connected, stop there.
Repeated resetting into a live fault is genuinely how nuisance trips turn into fires. The device operated for a reason and it will keep operating.
Add things back one at a time
Plug items back in one by one, switching each on and giving it a moment before moving to the next. The one that trips it is your answer, and you have just done for free what an electrician would charge to do.
Watch for the item that only trips it under load: a washing machine on the spin, a heater on high, a fridge when the compressor starts.
Know when to stop
Stop and call somebody if the safety switch is what went, if there is a burnt smell or discolouration at the board, if it trips with everything disconnected, or if it is happening more often than it used to.
An intermittent fault that is becoming more frequent is not settling down. It is a connection or a piece of insulation working its way toward failure.
Everything above assumes the earth fault has something watching for it, and in plenty of Sydney houses it does not. The tell is the test button. A residual current device has one, a circuit breaker does not, and if there are no test buttons anywhere on the board then nothing in the house is protecting a person from the currents at the left of that scale.
Where a switchboard is altered or its circuit protection replaced, residual current protection becomes mandatory for the circuits it supplies under AS/NZS 3000, at a maximum of 30 milliamps on final subcircuits in a domestic installation. What goes into a modern board, and what the devices actually do, is covered on our switchboard upgrade page.
Finding an intermittent fault is testing rather than guessing, and it is the part worth paying for. Any electrical work is certified: we lodge a Certificate of Compliance for Electrical Work within 7 days of the compliance test, which since 1 July 2026 goes through the BCNSW eCert portal.
It is current finding a path it was never meant to take, one with almost no resistance in it. Normally the current leaves on the active, does its work in the appliance and returns on the neutral, and the appliance is what limits how much flows. When an active conductor touches a neutral directly, that limit disappears. The voltage has not changed, the resistance has collapsed, so the current rises enormously in a fraction of a second until something opens. The bang, the flash and the instant trip are all the same event.
No, and most of the time it is not. A circuit breaker has two separate mechanisms inside it. A bimetallic strip that heats and bends handles overload, which is too much load on a perfectly healthy circuit and takes anywhere from seconds to minutes. A magnetic solenoid handles short circuit current and acts in milliseconds. If the breaker goes the instant something is switched on, that points at a fault. If it goes a few minutes into cooking dinner, that is almost always the circuit being asked to carry more than it was built for.
They protect different things. A circuit breaker watches how much current is flowing and opens when it is more than the cable in your wall can carry safely, so it protects the building. A safety switch, properly called a residual current device, compares what goes out on the active with what comes back on the neutral, and opens when the difference reaches 30 milliamps, so it protects the person. A breaker cannot detect current leaking to earth through somebody, because in breaker terms that current is far too small to notice. There is more on both devices on our switchboard upgrade page.
Because most of it is written for the United States. The American active is called a hot wire, earth is called ground, an earth fault is a ground fault, and the device that catches it is a GFCI. In Australia the same things are the active, the earth, an earth fault and an RCD or safety switch, and the supply is 230 volts at 50 hertz rather than 120 at 60. The physics is identical and the vocabulary is not, which matters when you are trying to work out what a tradesperson said or which product to buy.
Once, yes. Switch off and unplug everything on that circuit first, then reset it. If it holds, plug things back one at a time and the one that trips it is your culprit. If it trips again immediately with nothing connected, stop: the fault is in the fixed wiring rather than in an appliance, and repeatedly resetting into a fault is how a nuisance becomes a fire. If it is the safety switch rather than the breaker that went, treat that as more urgent, not less, because it operated to protect a person.
Usually age, heat or water. Flexible cords fail where they flex, right at the plug and at the point they enter the appliance, and eventually the conductors inside touch. Heating elements fail internally. Motor windings break down after enough thermal cycles. And anything that gets wet where it should not, a kettle base, an outdoor lead lying in a puddle, a leaking appliance, can produce a fault path in seconds. It is why one appliance can trip a circuit that has behaved perfectly for twenty years.
Attending your job is a chargeable callout. We confirm what it costs when you book, and the price for the work itself is agreed before we start, so there are no surprises on the invoice.
Why watts and amps are the same fact in two units in an Australian house, and the one sum that turns a label into an answer.
What to do when it is an older board with fuses rather than breakers, and what not to do.
When an appliance arrives needing more than a standard point can give, and why that is a circuit question.
If the answer is that the board needs work: safety switches and modern protection are a switchboard upgrade, original wiring behind repeated faults is rewiring, and the cable feeding the board is consumer mains. See every post on the blog.
Upfront pricing, with the price agreed before we start. We test to find the fault rather than replacing parts until it stops.
