High currents also cause problems. Everyone is familiar with the common fuse - a piece of wire which heats and melts if too large a current flows in it, thus protecting power supplies from short-circuits and similar problems. But where there is a very high current density in very small conductors they may not get very hot - but may still, eventually, fail. The cause is electromigration 3 (sometimes called ion migration). This is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. This causes thin conductors carrying high DC currents to become thinner with time and eventually to fail.
But some components fail like fuses - wires, or the conducting tracks on semiconductor chips, melt. A common cause of the high currents which make this happen is high capacitor charge current. Consider a 1 μ F capacitor with an ESR of 1 Ω: if it is connected across 110 V, 60 Hz, mains, an ac current of about 41 mA rms will flow in it. But if it is connected to the mains at the exact time the voltage is at a maximum (110 √ 2 = 155.6 V) the only current limit is the ESR and a peak current of 155.6 A will flow, albeit for less than a microsecond. But this is long enough to destroy many small-signal semiconductor devices, and repeated current surges may damage the capacitor itself, especially if it is an electrolytic one.
This is a particularly common failure mechanism in the cheap low-voltage switching supplies ("wall-warts") used for charging small electronic devices - if plugged in at the wrong part of an ac cycle the rectifiers and capacitors carry a very large surge, which may eventually, when it has happened many times, destroy them. A small resistor in series with the rectifier limits this surge current and minimizes the problem 4.