How do earthing and a residual current device (RCD) work? Why does it trip?
Most 'the breaker trips when I turn on the water heater' faults aren't the breaker at all, but the RCD. One idea is enough to understand when and why it trips: if the current going out doesn't come back, it's leaking somewhere.
What does it do? A residual current device (RCD, also called an RCCB or GFCI) passes the L and N conductors together through a toroid. In a healthy circuit the outgoing and returning currents are equal and their fields cancel. If some current leaks to earth through the frame and PE, or through a person's body, a difference (IΔ) appears between L and N; once it exceeds 30 mA the RCD opens L and N together within 300 ms at most, within 40 ms at 150 mA. An RCD doesn't limit the current, it limits the time; nor does it protect against overcurrent. Earthing (PE) holds the frame at earth potential in a fault and gives the leak a far easier path than a body. Together they protect you.
Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.
Phase 1/4: Normal
The current going out on L passes through the appliance and comes back on N unchanged. Both wires pass through the toroid, so their magnetic fields cancel: nothing is induced in the toroid's winding and the RCD stays closed.
1. L current
The appliance current (≈8.7 A for a 2 kW heater).
On the scope
Multimeter≈8.7 A AC
🔍 How to spot it on a board
•A 2- or 4-pole switch in the panel with a 'T' test button and IΔn 30 mA (or 300 mA) printed on it
•Its type mark: AC, A (also sensitive to pulsating DC), B (also sensitive to smooth DC, for VFDs and EV chargers)
•A green-yellow PE wire screwed to the appliance frame; earth contacts in the socket
🔧 How to measure it
•Press the T button monthly: the RCD must trip at once.
•Which appliance trips it: unplug everything, reset the RCD, plug appliances back in one at a time.
•Unplugged, megger L+N (together) to PE/frame at 500 V: > 1 MΩ is good, < 0.5 MΩ is leaky. Disconnect electronic boards before testing.
•Clamp L and N together with a leakage clamp: the circuit's total standing leakage should be below 9–10 mA. Frame-to-PE resistance should be < 0.3 Ω.
⚠ Common faults
•A damp heater element (water heater, washing machine, oven): trips at first switch-on, sometimes clears once warm; replace the element.
•Many computers, TVs and chargers on one circuit: Y capacitor leakage adds up and the RCD trips 'for no reason' now and then.
•A neutral mixed with another circuit or with earth, a damp outdoor junction box; a broken PE won't trip the RCD, but it's the most dangerous fault.
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