The short answer
Here is what the symptom means on a Black+Decker microwave: a trip is the protection doing its job, and the circuit is a more likely culprit than the oven. Testing the oven on its own circuit is free and settles most of it; a trip on a dedicated circuit ends owner diagnosis.
What you actually see
- Sparking or arcing inside the cavity — Visible sparks are a stop-now condition whatever caused them.
- The plug or the cord is warm to the touch — A warm plug is a poor connection making heat, and it gets worse.
- Scorch marks on the cavity wall or the waveguide cover — Scorching marks where arcing has already happened.
Causes, ranked for this microwave
- Shared or overloaded circuit — Ranks first because a microwave is a heavy load, and a socket shared with a kettle or a toaster will trip on the combination rather than on a fault. This is the entry most often blamed correctly and diagnosed carelessly, so take the extra minute.
- Damaged cord, plug or outlet — Second because it also produces nuisance tripping that is blamed on the oven rather than on the connection feeding it. Confirm it before moving on, because the entries below read differently once it is ruled out.
- Metal in the cavity — Third because foil, a metal-trimmed dish, a twist tie or a fork is the most common cause of arcing by a wide margin, and it is entirely preventable. Rule it in or out properly here; a maybe at this position costs an hour further down.
What to check, in order
- First — the circuit the oven is on — Note what else was running each time it tripped. That pattern is the diagnosis.
- Second — the cord, plug and socket — Unplug the oven and run the cord through your hand end to end, feeling for flats and nicks and checking the pins for discolouration.
Checks an owner can do safely
- Step 1. Give the oven its own circuit — Move it to a socket with nothing else on the run, then note whether the tripping stops.
Tools: noneAbout 15 min
- Step 2. Stop the oven and unplug it — At the first spark, smell or trip, stop the cycle and unplug the oven. Nothing else on this page happens with the oven live.
Tools: noneAbout 1 min
- Step 3. Inspect the waveguide cover — Look at the small panel on the cavity side wall for grease, scorching or a hole. Wipe it gently with a damp cloth.
Tools: damp cloth, flashlightAbout 10 min
Before you open anything
Safety
Unplug the appliance or switch off its breaker before removing any panel. The high-voltage capacitor in a microwave can hold a lethal charge long after the cord is out, and it is not discharged by waiting. Nothing inside the outer cabinet is an owner-serviceable part, and no fault justifies running the oven with the case open or with a door switch held closed by hand.
When this becomes a service call
Stop and call when the next thing to do meets any of these. The case is never opened. A microwave holds a dangerous charge after it is unplugged, and every internal fault on this page is a service or replacement decision.
Sources
General guidance- The Control of Hazardous Energy (Lockout/Tagout) — OSHA 1910.147 and manufacturer service manuals, fetched
- Article 210 — Branch Circuits — NFPA 70 (National Electrical Code), fetched
Nearest pages
What this page does not cover
One limitation worth stating up front: this page is written for the Black+Decker microwave range rather than for your particular unit. It does not name families, model numbers or control revisions. Where a page exists that does, it is more precise than this one and it is linked under Related pages.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
