When the oven takes the power out for the whole house

A woman in Barrack Heights had been getting the Sunday roast on by walking out to the switchboard, flicking the safety switch back up, and turning the oven on again quickly. Four weekends running. Sometimes it held long enough to cook. Sometimes she did it three times in an afternoon.

She apologised for wasting my time on something she thought was probably the switchboard being old.

It was the lower element in the oven, split at one end, and it had been putting current into the oven's metal shell every time it got hot. The safety switch had been doing its job perfectly for a month while everyone treated it as a nuisance.

A safety switch isn't a fuse

This is the bit worth understanding, because it changes how seriously you take a trip.

A circuit breaker watches how much current is flowing and cuts out if it's too much. That's an overload. A safety switch — an RCD, if you like the proper name — does something different. It compares the current going out on the active with the current coming back on the neutral, and those two numbers should be identical, because electricity that goes out has to come back.

If they don't match, some of it is going somewhere else. Through a metal chassis, through a puddle, through the earth wire, or through whoever is holding the handle. When the difference gets to about 30 milliamps the switch opens, and it does it in well under a tenth of a second, which is fast enough to matter.

So a trip is not the switchboard being fussy. It's a measurement. Something on that circuit is leaking, and the switch found it.

The pattern tells you a lot

Three ways this shows up, and they point in different directions.

It trips the instant you switch on. Something is hard down to earth. On an oven that's the element far more often than it's anything else. Switch it off at the wall, leave it there, and stop testing it.

It runs for a while, then trips once it's hot. This is the most common one and the most misread, because the appliance seems fine at first. Heat is opening a crack that's closed when cold.

It trips now and then, with no pattern anyone can see. The worst kind to chase. Usually moisture getting in somewhere, which means it follows the weather and the humidity rather than the appliance.

Why elements do this

An oven or a kettle element isn't a bare wire. It's a resistance wire running down the middle of a metal tube, packed all around with magnesium oxide powder that holds it in place and keeps it insulated from the sheath.

Magnesium oxide has one bad habit, which is that it loves water. Get a pinhole in the sheath, or a hairline crack where it bends, and the powder pulls moisture out of the air. Damp powder conducts. Now the element has a path to the metal it's supposed to be insulated from, and that path opens up as the whole thing heats and expands.

That's why these faults so often start after a house has been shut up for a fortnight, or after somebody has given the oven a proper wet clean, or on the first genuinely humid week of summer. Appliances that sit unused have their own version of this, which I've written about in what an empty house does to appliances.

Washing machines and dishwashers get there by a different route: the element sits in water by design, and it's the seal around it, or a chafed wire in the loom, or water tracking somewhere it shouldn't.

The local part of it

Salt air and earth leakage make a bad pair, and my honest read is that we get more of these calls than someone running the same van thirty kilometres inland would.

Terminal blocks corrode. Earth screws corrode. Cable glands and grommets perish and let damp air into places that were meant to stay dry. A machine living in a carport or an open laundry — normal enough around Warilla and Barrack Heights — is sitting in that air permanently rather than for the ten minutes a load takes.

Older freestanding cookers are the other repeat offender, because there's a lot of exposed metalwork behind the panel and forty years of steam has been at it. The old Westinghouse cookers still in a lot of houses around here are the ones I meet most, and it isn't a brand failing so much as an age one.

Working out which appliance it is

You can do this part yourself and it saves everyone time.

Unplug everything on the circuit. Everything, not just the suspects, and remember the microwave, the kettle and whatever's in the garage. Reset the switch. If it holds, plug things back in one at a time, using each one properly for a few minutes before moving to the next. The one that drops it is the one.

If it won't reset with everything unplugged, the fault is in the wiring rather than in an appliance, and that's an electrician's job, not mine.

Two things not to do. Don't keep resetting and running it, because every reset is another go at putting current somewhere it doesn't belong. And don't get talked into taping, wedging or bypassing anything at the board. I've seen that once and I'd rather not again.

Since the machine is off anyway, it's worth two minutes to check the model against the recall list. The ACCC keeps Australian product recalls in one searchable database, and household appliances land on it regularly.

Where the line sits

Inside the appliance is ours. Elements, thermostats, wiring looms, terminal blocks, motors, pumps, seals.

The powerpoint, the cable in the wall, the circuit and the switchboard belong to a licensed electrician. There's no grey area there and I won't pretend otherwise.

Wall ovens are where that line catches people out. A built-in unit usually has no plug on the end of it, and in this state taking fixed equipment off the supply needs a licence. So if the oven has to leave the cabinetry, the job has a sparky in it as well as me, and it's better to know that on the phone than on the day.

What it usually costs you

An element in an oven that's otherwise sound is a routine job and a part I'd normally have. Same with a heating element in a dishwasher or a washer, though those mean the machine comes apart further.

The awkward ones are a wiring loom that's perished in three places, or a control board that's spent a decade in damp air, on a machine that's already fifteen years old. You end up chasing leakage through a whole appliance instead of replacing one part, and I'd sooner say that before I take it apart than after. There's a longer piece on when a repair stops adding up.

Until someone gets there

Kill the power to it properly. Not the button on the front of the machine, but the powerpoint, or the isolating switch on the wall if it's built in, or that circuit at the board.

An appliance that trips a safety switch has already proved it can put current where a person could find it, and the switch is the only thing standing between those two facts. It's worth a weekend of cold sandwiches.

There's a $125 assessment fee for the visit itself, and no work happens until you've said yes to a firm repair price. Domestic appliances only. Get in touch or call 02 4229 3777, and tell me whether it trips straight away or once it's warmed up, because that alone narrows it down before I'm out the front.