The short answer
On Forno arcing or a trip is most often metal in the cavity. Ranked that way because a built-in microwave is held square by its trim kit, so the install is part of the appliance here rather than something that happened once.
What you observe
- The breaker trips when the machine starts — Either a shared circuit under load or an internal fault, and the first is far more common.
- A loud hum with no heating — Points at the high-voltage side, which is a technician job and not an owner one.
- NEAR MISS: the food is simply not getting hot, with no smell and no sparks — That is a heating fault rather than a safety one, and it has its own page.
Causes, most likely first on Forno
- Metal in the cavity — First on a fitted machine, which is only ever as good as its installation. Foil, a fork, a metal-rimmed plate or a twist tie will arc visibly and loudly. It is the most common cause of sparking by a wide margin and it usually leaves the machine undamaged if it is stopped promptly.
- The machine was run empty or with very little load — Second: on a fitted machine this is the next thing genuinely yours to check. A magnetron needs something to absorb its energy. Running with an empty cavity, or heating a single small item, reflects that energy back and damages the machine, sometimes with visible arcing.
- The waveguide cover is soiled or damaged — Third. The small panel on the cavity wall absorbs splashes, and food burnt onto it arcs, chars and eventually burns through.
- An internal fault: magnetron, diode, capacitor or transformer — Last, and it is the reason the order above matters. It trips the breaker on its own circuit, hums loudly without heating, or smells hot with a clean cavity.
Check in this order, and why
- Stop the machine and look at what is actually in the cavity — It is the most common cause, it is instantly visible, and it is entirely avoidable.
- Ask whether it was run empty, or with a very small item, in the last few uses — It explains damage with nothing metallic in the cavity at all.
- Look at the small panel on the cavity wall for charring or a hole — Repeated sparking from one spot names this cause outright.
What an owner can safely check
- Stop the machine, remove anything metallic, and inspect the cavity for scorch marks — Scorching on the cavity wall means it happened more than once and needs looking at.
About 10 min
- Never run the machine empty; keep a cup of water in it if children use it — If it has been run empty repeatedly, have it checked before relying on it.
About 5 min
- Wipe the waveguide cover gently with a damp cloth once the machine is cold — A charred or holed cover must be replaced before the machine is used again.
Tools: damp clothAbout 10 min
- STOP HERE if the next step needs a meter on live terminals — That is where an owner check ends and a service call begins.
Before you open anything
Safety
Never open the outer casing of a microwave. The high-voltage capacitor inside can hold a lethal charge long after the machine is unplugged, and that risk does not fall with care or experience. Stop using the machine and switch it off at the breaker if you see arcing, smell burning or find the plug or outlet warm, and never reset a breaker that has tripped twice on the same appliance.
When to call a technician
Call, or replace the machine, when it trips a breaker with nothing else on the circuit, when it hums without heating, when it smells hot with a clean cavity, or when the waveguide cover is charred or holed. Call an electrician rather than an appliance technician if the plug or the outlet is warm or discoloured, because the fault is then in the wiring.
Nearest pages of this class
What this page does not cover
This page covers the brand and the microwave class, not one family or model line. Where a family-specific or model-specific page exists for your unit, that page is the more specific answer and it takes precedence over this one. Not here: part numbers, wiring colours and component locations, which differ by family.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
