A nail that goes into a wall and nicks a wire.
Any kind of damaged cabling, really. Lamp cord that’s been abraded and has the conductors just barely touching each other, say.
I think jnglmassiv is questioning the efficacy of AFCIs.
I once read an article about house fires that were caused by electrical problems. The article said studies showed most were not caused by current overloads or short circuits, but by arcing, and primarily “series arcing” due to loose terminals and contacts. A traditional circuit breaker won’t detect a series arc, and nor will a GFCI, hence the reason for AFCIs.
Household arc fires are what I’m wondering about. There just aren’t many places high voltage, close distance/low, non-zero resistance, and fuel meet in a hazardous way.
The grape in the microwave is one way to induce an arc.
Parallel arcing can definitely occur. However, unlike series arcing, parallel arcing will often trip a traditional circuit breaker.
Drive two nails into a piece of wood, maybe 2” apart, connect the nails to a power cord, dampen the wood, and plug it into the wall. Should start arcing.
Maybe need to play with the distance apart and amount of dampness.
That sounds awfully close to breaking Commandment IV.
An easy way to generate arcs is to use a file. Connect one wire to the handle end using a big alligator clip, and drag the other wire across the teeth. (I wouldn’t do this with mains voltage or a car battery, though. Whatever you use, just make sure it doesn’t mind being short-circuited.)
Every single electrical connection is a potential source of arcing. Wire-to-wire or wire-to-terminal, if they are not made correctly (wire nuts too lose, screws not tightened to spec with a torque screwdriver, etc.) they can eventually work loose over time due to thermal cycling and come apart just enough to start arcing. Plug-to-socket, too, if the plug or socket has worn out over time and is no longer making secure contact.
We’re not talking about dramatic Jacob’s Ladder-style sparks here, just teeny-tiny (fractions of a millimeter long) arcs. That’s still able to cause enough heat to start a fire.
I had occasion to return a circuit breaker to Home Depot which I had determined was faulty - I had penciled an X on the side to make sure I took back the right one, and they cheerfully refunded my money when I explained the problem. A week later I noticed that X-marked breaker back on the shelf…
And they’ll let you return it as many times as you like!
Well, I am source of arcing, at least when I’m not wearing a grounding strap.
Yes, and if you were in a coal mine, you would expect to take normal care with what you wear and touch to avoid that risk.
Your electricity switches and nuts are exposed to much lower voltages, and much higher currents, and the nature of the danger is rather different.
I once had a new oven installed by a professional idiot. The line to the oven was stranded aluminum, and he connected it to the copper wires of the oven with an ordinary wirenut. After a while that breaker started tripping occasionally. When I pulled out the oven and saw the wires I understood why. The plastic part of the wirenuts had MELTED. There was definitely arcing going on in there.
The guy hardwired the branch circuit to the power cord of the oven? Besides being a significant Code violation, it’s something that I have never seen done (even by the most incompetent and clueless amateurs). My mind wobbles.
No. Built-in ovens (as opposed to a combination cooktop/oven “range” that slides in between cabinets) are almost always intended to be hard-wired. The come with a cable with bare wire ends that is meant to be permanently attached to the building wiring inside a junction box.
Example install guide: https://www.kitchenaid.com/content/dam/global/documents/202304/owners-manual-w11656312-revA.pdf (see page 18)
My bad, andrewm. You are correct. It didn’t even occur to me that this might be a built-in oven, as opposed to a common range.
And I HAVE seen those hardwired. I just had mental blinders on.
Yes, it was a built-in oven that is intended to be hardwired. The mistake was using wirenuts to connect aluminum to copper. Even I knew that that is considered extremely unsafe. I’m lucky I discovered it before it started a fire inside the wall. On the advice of an electrician, I replaced the wirenuts with Polaris connectors.
You obviously have never seen how much a frayed extension cord can arc.
You don’t need a grape in a microwave. All you need is an old and somewhat abused extension cord. Or maybe a screw terminal in an outlet that wasn’t properly tightened that has high current going through it. It won’t arc immediately, but once it heats up and the bad connection starts to char and separate, it will make all kinds of arcing (happened to me in the middle of the night once).
AFCIs were invented because things like a frayed extension cord often won’t draw enough fault current to blow a regular breaker. Frayed extension cords cause literally thousands of house fires every year. Arcing outside of a microwave with a grape in it is definitely not a rare thing.
And for what it’s worth, in case anyone wants to try this at home, you need to cut the grape almost in half, leaving just a bit of skin between the two halves. An intact whole grape won’t arc. 
Also for plugs partially pulling out of outlets behind curtains or next to bedding. Hence the original requirement for AFCIs was limited to living rooms and bedrooms.