How do I find which appliance is tripping my RCD?

This is a process of elimination. First, switch off or unplug every appliance on the affected circuit. Reset the RCD — if it now stays on, one of the appliances was causing the fault. Plug them back in one at a time, waiting a few minutes each time. When the RCD trips again, the appliance you just plugged in is the likely culprit. Common offenders are washing machines, dishwashers, electric showers, kettles, and tumble dryers. If the RCD trips with nothing plugged in, the fault is in the wiring and you need an electrician.