How electrical fault finding works

The first tool is a set of questions. What stopped working, which rooms, what was running, what the weather was doing and whether it has happened before. A safety switch that drops out when it rains points somewhere very different from one that trips when the kettle and oven run together.

Then comes the switchboard. Which device has operated says a lot. A circuit breaker trips on overload or a short circuit. A safety switch trips when current leaks to earth, the kind of fault that can hurt someone, and Energy Safe Victoria says it turns the power off within 0.03 of a second. A board with several circuits behind one safety switch takes more narrowing down than one where each circuit has its own.

Labels help, but in older homes they're often wrong, especially after a few renovations. Part of a fault-finding visit is working out what each breaker actually feeds.

The core of fault finding is elimination. With everything unplugged on the affected circuit, does the fault still happen? If not, the cause is probably an appliance, and plugging them back in one at a time usually finds it. If it does, the problem is in the fixed wiring: a power point, a light fitting, a junction or the cable itself.

Where one safety switch covers several circuits, the electrician isolates them one by one, switching breakers off until the safety switch holds. That points to the circuit. From there, the circuit is split at points and junctions along its run until the faulty section is found.

This is why the power goes off in parts during a visit. You should be told before anything is switched off, so fridges, freezers, home offices and medical equipment can be planned around.

Most of the evidence comes from standard tests done with the power off. An insulation resistance test applies a test voltage between the conductors and earth and measures how well the insulation is holding. Cable that's damp, chewed, crushed or perished shows up as a low reading, often before anything has started tripping.

Continuity tests check that conductors are unbroken end to end and that the earth path is sound. A high reading on a run that should be low points to a loose or corroded joint, the classic cause of a warm power point or lights that flicker.

With power restored, the electrician checks polarity, confirms the safety switch trips as it should and checks the fault has gone. These are measurements, not hunches, and it's reasonable to ask what the readings showed.

Intermittent faults come and go with heat, moisture and load. A circuit can test clean on a dry morning and trip on a wet night, or a joint can hold until it warms up under a heater. In Melbourne's west, outdoor circuits that trip after rain or when the sprinklers run are a common example.

Dead tests often catch the weakness anyway, because degraded insulation reads low even when it isn't actively faulting. Opening junctions and fittings along a suspect run finds connections that look fine until they're touched. Sometimes the fault only appears with one appliance, or one combination of appliances, running.

When it still hides, your notes become the best tool there is. Two or three dated entries can turn a guess into a pattern.

Energy Safe Victoria advises householders who lose power to check whether the safety switch has tripped, reset it, and call a licensed electrician if it won't reset. Reset it once. If it drops straight back out, leave it off. Each extra reset sends power back into whatever caused the trip.

Unplug what's easy to reach on the dead circuit, note what was running and photograph the switchboard without removing any covers. Clear a path to the board and the manhole. Mention anything recent: a new appliance, someone drilling into a wall, a storm.

What not to do matters as much. Don't hold or tape a breaker on, don't open the switchboard or remove fittings, and don't run extension leads from another room around a dead circuit. Energy Safe Victoria says electrical work isn't a DIY job, even changing power points or light switches.

Once the cause is found, the repair might be a burnt terminal remade, a failed breaker or safety switch replaced, an appliance retired, or a length of damaged cable replaced. The circuit is then retested, because a second weak joint further along the same run is common.

In Victoria, repair counts as electrical installation work, so a licensed electrician must provide a Certificate of Electrical Safety when it's done. For work on things like power points and lights, Energy Safe Victoria says your copy is due within 30 days.

Testing sometimes finds more than the fault you called about, such as ageing insulation across several circuits or a board with no safety switch. That should be explained, made safe if it's dangerous, and priced separately. We give the price before work starts, so anything beyond the original fault is your decision.

Fault finding chases a problem you've noticed until the cause is found. A safety check inspects and tests the whole installation for problems that haven't shown up yet, and ends with a written report. They use many of the same tests but answer different questions.

If the fault turns out to be ageing wiring in an older house, a wider check may be the sensible next step. Energy Safe Victoria advises owners of houses over 30 years old to have their wiring checked by a licensed electrician, and lists old switchboards without a safety switch, flickering lights and sparks from switches or power points as tell-tale signs of faulty wiring.