Why your RCD trips, and how to test it and understand the earthing in your board
An RCD that trips is almost never faulty: it is telling you current is leaking. Learn what the RCD and earthing do, why a circuit breaker does not protect you, and how to test your RCD in a minute.
In a family home, the RCD in the board tripped every time someone turned on the electric shower (an instant water heater that is very common in Brazil). The resident had heard that all he had to do was take the RCD out, because it was too sensitive. When we opened the shower, the heating element was broken and touching the casing, with the water making its way to the metal valve.
The RCD was not faulty. It was stopping the next person in the shower from getting a shock.
A breaker and an RCD do different jobs
The circuit breaker protects the wire, not the appliance, and even less the person. It deals with overload and short circuits and works in amperes. A dangerous shock starts at a few tens of milliamperes, hundreds of times less. To the breaker, the current running through your body is invisible.
The device that sees that current is the RCD, or residual current device. It works like a doorman counting who comes in and who goes out. All the current that goes out along one wire of the circuit must come back along the other: along the neutral, in a 127 V circuit, or along the other phase, in a 220 V circuit between phases.
When part of it does not come back, it is escaping along another path: through the earth wire or through someone's body. If the difference reaches 30 mA, the RCD cuts the circuit in a fraction of a second, fast enough to greatly reduce the risk to the heart.
In the case of that 5,500 W shower on 220 V, 25 A went out along one phase and only 24.96 A came back along the other. The missing 40 mA went through the earth. Since 40 mA is more than 30 mA, the RCD tripped.
Earthing: an easier path than your body
Any appliance with a metal casing, such as an electric shower, washing machine, refrigerator or pump, can develop a fault that puts voltage on the casing. Earthing (grounding) is a green and yellow wire, the protective conductor (PE), that connects that casing to a metal rod buried in the ground, normally next to the service entrance.
The two work together. With earthing and no RCD, the leakage goes on, heating things and leaving the casing live. With an RCD and no earthing, the RCD only notices the leakage when it passes through someone. Together, the fault is detected before it becomes an accident.
That is why NBR 5410 (the Brazilian standard for low-voltage installations) requires a 30 mA RCD on circuits serving bathrooms, kitchens, laundries, service areas, garages and outdoor sockets, and a protective conductor on every circuit. The technical basis here is Brazilian: outside Brazil, follow your local electrical code.
How to test the RCD in your board
- Open only the board door, never the inner cover that hides the wires.
- Look for a device wider than the breakers, with a button marked T or "test".
- Read the label: it should show IΔn 30 mA. If it shows 300 mA or 500 mA, it protects against fire, not against shock to people.
- Warn everyone at home: the protected circuits will switch off.
- Press the T button once. The RCD should trip immediately.
- Reset the lever. If it goes up and stays, the RCD is working.
- Write down the date and repeat once a month.
While the RCD is off, walk around the house and note what lost power. Those are the circuits it protects.
Where your part ends
Anyone can test with the T button. If the RCD trips on its own, switch the circuits off one by one to find out which one causes the trip, and note when it happens and what was on. Finding the leakage in that circuit is a job for a licensed professional.
These also belong to the professional: an RCD that does not trip on the test or will not reset (switch off the main breaker if there is any risk), anything inside the board, installing an RCD and working on the earthing. Two things never: removing or bridging the RCD, and connecting the earth wire to the neutral at the socket. And the RCD does not protect someone who touches phase and neutral at the same time, so it never replaces switching the circuit off before working on it.
To keep going
The checklist Earthing and RCD: testing your distribution board brings together what you need to check in front of your own board. It arrives as a PDF in your inbox.
Test yours this month.
Marcelo Nogueira Souza — E-Nogueira, since 2014