Passive element failures-Potentiometers
Main article: Capacitor
Capacitors are characterized by their capacitance, parasitic resistance in series and parallel, breakdown voltage and dissipation factor; both parasitic parameters are often frequency- and voltage-dependent. Structurally, capacitors consist of electrodes separated by a dielectric, connecting leads, and housing; deterioration of any of these may cause parameter shifts or failure. Shorted failures and leakage due to increase of parallel parasitic resistance are the most common failure modes of capacitors, followed by open failures. Some examples of capacitor failures include:
Dielectric breakdown due to overvoltage or aging of the dielectric, occurring when breakdown voltage falls below operating voltage. Some types of capacitors "self-heal", as internal arcing vaporizes parts of the electrodes around the failed spot. Others form a conductive pathway through the dielectric, leading to shorting or partial loss of dielectric resistance.
Electrode materials migrating across the dielectric, forming conductive paths.
Leads separated from the capacitor by rough handling during storage, assembly or operation, leading to an open failure. The failure can occur invisibly inside the packaging and is measurable.
Increase of dissipation factor due to contamination of capacitor materials, particularly from flux and solvent residues.
Metal oxide varistors
Main article: Varistor
Metal oxide varistors typically have lower resistance as they heat up; if connected directly across a power bus, for protection against electrical transients, a varistor with a lowered trigger voltage can slide into catastrophic thermal runaway and sometimes a small explosion or fire. To prevent this, the fault current is typically limited by a thermal fuse, circuit breaker, or other current limiting device.