The earth return path is a vital part of how the circuit breakers (MCB‘s) or fuses in the consumer unit (CU) work to disconnect the supply in the event of a fault. This is known as protection by automatic disconnection of supply, or ADS. The CPC (earth) provides a low resistance path back to the CU for any potential fault current, hopefully preventing that harmful current from flowing through a person in the worst case scenario. That’s why a CPC should be present at all points of the fixed wiring and should ideally run alongside the live conductors of each circuit.
ADS is like the bare minimum requirement for protection from electric shock and RCD’s provide an additional layer of protection on top of that.
Some faults don’t create enough current to allow the MCB’s or fuses to operate, or they allow more current than is typically survivable to flow through a person before they do disconnect the supply. Residual Current Devices (RCD’s) provide additional protection against these by being more sensitive to smaller current imbalances in the circuits they protect. They monitor the outgoing and return current on the line and neutral conductors of a circuit or group of circuits and trip if the imbalance is more than 30mA (other ratings are available, but 30mA is required for additional protection). They don’t need a CPC to operate but having one provides that low resistance path back to the CU for the fault current, causing the RCD to see the imbalance in the live conductors (L + N) quickly and trip accordingly.
Electricity is sort of like water in that it usually follows the path of least resistance. In the event of a fault with a CPC present, the electricity will flow down that and should trip either the RCD or MCB/fuse before anyone gets hurt. Without a CPC it’ll flow through the next path of least resistance which might well be the unfortunate human being who’s just stuck a nail through a cable above a light switch.
Hypothetically (really don’t try this at home), grabbing the end of a single line conductor (usually brown or red in the UK) won’t cause an MCB or fuse to trip or rupture before you’ve had a very nasty, possibly fatal, shock. They might not even operate at all in some circumstances. If we fit a 30mA RCD as well it will trip quickly enough to prevent that same shock being fatal in most cases.