A break in the low-resistance grounding path from a tool or electrical system will result in a ground-fault, and electrical current can instead take a path through the user to the ground. This unfortunate occurrence will result in either severe injuries or death.
A GFCI is a rapidly activated circuit breaker that can shut off electric power when there’s ground-fault in 1/40th of a second. It functions by measuring the amount of current traveling to and from equipment along the circuit conductors. If their amounts differ by about five milliamperes, the GFCI will interrupt the current.