One electrical circuit in my home (which powers the electrical sockets) is tripping my main RCD. The electrical circuit has an older style breaker, so it is the house's main RCD which trips first. If switch this circuit off using the breaker and then turn the main RCD on, the RCD will not trip. If I turn the breaker for the problem circuit back on, it sometimes trips the main RCD immediately and sometimes it will not trip the main RCD for several hours.

I have unplugged all the devices which were plugged into this circuit, so it doesn't seem to be being caused by a faulty appliance. (Shame that would be an easy fix. Bin a faulty appliance.)

Whilst it remained on, I went round each socket with a socket tester plug. This showed every socket was correct.

What are the chances that the breaker is faulty?

Rich, January 2023

Are you sure ALL loads have been disconnected?
Not just turned off but unplugged.
Yes, it's possible the breaker is faulty but it's also possible there's a wiring fault.
Many years ago I had a washing machine plugged into an unswitched outlet that was switched by a wall mounted switch above a worktop. One morning I discovered the walk switch was warm to the touch. Investigation showed the Neutral wire inside the switch box had been pressing against the head of the screw holding the box in the wall. Over time the insulation had gradually crept under the pressure and the neutral conductor had contacted the screw and therefore the (earthed) wall box. An earth leakage breaker would have tripped on that.

Eheh, January 2023