Arcing

Current normally flows through a solid path. When wire is damaged, a connection is loose or insulation breaks down, current can jump a small gap as an arc. The heat can ignite nearby material. An arc fault breaker, known as an AFCI, is built to detect this and shut off the circuit.

OSHA's rule for electric equipment says parts that produce arcs or sparks in ordinary operation must be enclosed or isolated from combustible material. If you see sparks from an outlet or hear sizzling, stop using it and call a licensed electrician. See electrical warning signs.