Earthing at home: the difference between the earth wire, the breaker and the RCD, and why you need all three

"The washing machine gives a shock", "the iron tingles", "the breaker didn't trip": all things you hear when the protection system is incomplete. Protection at home has three parts that work together: the earth wire, the breaker, and the RCD.

The binding reference in Saudi Arabia is the Saudi Electrical Code SBC 4011. The examples in this guide come from international guides and only explain the principle; the values required for your building are set by the code and the designer.

1) Who does what?

For breakers and 30 mA sensitivity: breakers and RCDs.

Why neither replaces the other

• Fault protection needs both: the Schneider guide states that protection against an insulation fault is achieved by automatic disconnection of the supply if the exposed conductive parts of equipment are properly earthed, at two levels: earthing all exposed conductive parts with an equipotential bonding network, then automatic disconnection of the faulty section2.
• An RCD detects leakage whatever its path: it works by detecting residual currents to earth, which may or may not pass through a person, and disconnects fast enough to prevent injury3. In a TT system, automatic disconnection is achieved by an RCD, with all exposed conductive parts connected to a common earth electrode4.
• Why "additional protection": the guide calls a high-sensitivity RCD (30 mA or less) additional protection because the other preventive measures are not infallible: lack of maintenance, carelessness, worn insulation, accidental contact and immersion in water3.

2) TT and TN in brief

• TT: all metalwork in the building connects to the building's own earth electrode. The fault path runs through two earth electrodes, so the fault current is generally too small to operate breakers or fuses, and an RCD is essential in this system4.
• TN: metalwork connects directly to the earthed point of the supply through protective conductors, so any insulation fault becomes a phase-to-neutral short circuit with a large current the breaker clears; the guide notes RCDs may also be necessary5.

Which system your home has depends on the supplier and the electrical design; ask your electrician.

3) A worked example (from the Electrical Installation Guide)

In a TT example: supply earth electrode 10 Ω, building electrode 20 Ω, giving a fault current of only 7.7 A and a fault voltage on the casing of 154 V, which is dangerous. An ordinary 16 A breaker will not trip at that current, while a 300 mA RCD trips in about 30 ms4.

Source for the table and example: the Electrical Installation Guide4. The figures illustrate the principle only; the values required for your building are set by the code and the designer.

4) How to request a documented test

1. Hire a licensed electrician or firm, and ask for a written report, not a verbal opinion.
2. Ask that it includes: the earthing system type, the earth measurement result, continuity of protective conductors to sockets, and RCD trip-time tests.
3. Ask for the panel circuits to be labelled and the RCD-protected circuits identified.
4. Keep the report with the house documents. See new building handover.

5) When to call an electrician immediately

• A tingle when touching an appliance, tap or metal door.
• The RCD trips and will not reset, or does not trip when the test button is pressed.
• Sockets without an earth terminal in a kitchen, bathroom or laundry.
• A burning smell or heat at a socket or panel. See electrical problems.

The binding reference in Saudi Arabia is the Saudi Electrical Code SBC 401, as noted at the top of this guide.