What are the common defects of capacitors?

Answer 1

Basic defect - it ceases to insulate between its two terminals and passes current that it should not.

Short circuit - the insulating material (dielectric) has failed and is allowing the two metal "plates" to come in contact. Depending on the circuit, effects range from simple loss of performance to failure to operate to destruction of other components. Easily detected by lifting/disconnecting one lead and measuring resistance.

Leakage - less severe than a short circuit, but the capacitor is allowing current to flow that should not. Usually an internal fault (partial failure of dielectric), but a faulty case may cause leakage, and other components connected to the capacitor may be the cause, making the capacitor appear faulty. Less likely to cause damage than short circuits, but severe leakage may cause other damage, moderate leakage may cause lack of performance or stop operation altogether. Some circuits are highly sensitive to leakage even if it is less than microamps (millionths of an amp). Resistance measurements will often help but (i) the leakage may only be apparent with considerable voltage applied - more than common meters do, and (ii) some types, especially electrolytics, exhibit leakage from new, so you need to know how much is acceptable.

Open - the capacitor does not allow any signal to pass. The most common symptom is loss of operation/ loss of performance, but some circuits can suffer damage if the capacitor is a critical part of its operation. Resistance measurements are of no use, so you need to use a capacitance meter or test by replacing with a good capacitor. If the new one fixes the problem, you've found the fault.

Intermittent - may be open or shorted. Intermittent leakage is possible but much less common. Tuning capacitors with moving plates often suffer intermittent shorts due to dust/metal fragments or bent plates (shorts), or faulty grounding "wipers" that have oxidised (rusted) or gotten greasy/dirty (opens). Tuning capacitor shorts can often be confirmed by inspection or resistance measurement, opens by inspection/cleaning.

Drift - the capacitor's actual capacitance increases or decreases, usually slowly, to a point where the equipment no longer works as designed. Observing equipment performance may confirm this fault, if not then measurement will be needed. Drift may happen with excessively high or low operating temperatures, and the equipment may work perfectly in the service shop at normal room temperature.