Residual Current Protection, Plainly
It does not protect the circuit. It protects the person, and only from one specific way of dying.
What it detects
An imbalance between the live and neutral conductors, which means current is returning by some other route. Often that route is a person. The device trips in tens of milliseconds, which is fast enough to prevent ventricular fibrillation in most circumstances.
What it does not do
It will not protect you from a shock between live and neutral, because that current is balanced and looks entirely normal to the device. Grabbing both conductors is exactly the scenario it cannot see.
It is also not overcurrent protection. The breaker and the residual current device do different jobs and neither substitutes for the other.
Nuisance tripping is usually real
Persistent tripping is generally a genuine leakage fault rather than a faulty device, and cumulative leakage across several appliances is the usual cause. Splitting the load across more devices fixes the symptom; finding the appliance fixes the problem. The HSE electricity pages are a good plain summary of the duties involved.
