Why is leakage current dangerous?

Leakage current is current that escapes the normal phase-to-neutral path and returns to earth, through the earthing conductor or, in the worst case, through a human body. Just a few tens of milliamperes through the body can be dangerous to the heart. A persistent leak through damaged insulation can also heat materials and start a fire without tripping the circuit breaker, because the current is too small to trigger it.

A residual current device (RCD) compares the current going out with the current coming back and cuts the supply as soon as the difference exceeds its threshold. For the protection of people, 30 mA devices are typically used. An RCD does not replace earthing; the two work together.

A small amount of leakage is normal in electronic devices because of their filters. But when many computers or a UPS are connected to the same circuit, the leakage currents add up and the RCD may trip for no obvious reason. The solution is for an electrician to spread the loads over more circuits, never to remove the RCD. Also press its test button periodically to make sure it works.