Dave, the way you worded your question is not how a circuit works.
Current flows out from the active wire and BACK through the neutral wire. With AC, this current reverses its direction 50 times a second (50Hz).
So, there HAS to be a current flowing in the neutral wire for anything connected to the circuit to work.
What the safety devices detect is a current IMBALANCE. In other words, exactly the same current that flows out has to flow back, if not there is an earth leakage somewhere.
During a short circuit (active to neutral) it is still the SAME current that flows in both wires. However, it is excessive and could melt them or the equipment connected to them.
What you need to do then is measure if the current is excessive and trip a circuit breaker. As Nigel said, the simplest way is a fuse.
Klaus