Earthing is the part of the electrical installation you see least and that protects most: when something fails, it is what stops a fault becoming a real shock. It isn't a minor item that can be handled with a single rod without checking anything.
What the earth rod does
The earth rod is the electrode driven into the ground to establish the installation's earth connection. It is sold in lengths of 1.5 m or 2 m.
The important point: a single rod is not always enough. Earth resistance depends on the moisture and composition of the ground, not on a fixed number of rods. It must always be measured with an earth tester — in Spain the maximum allowed is 37 Ω for installations with a 30 mA RCD. If the ground is very dry or rocky, more than one rod connected in parallel may be needed to bring the resistance down to that limit.
Which conductor to use in each section
- Bare cable: for the buried section up to the rod.
- Insulated green-yellow conductor: for the visible section and for the connections to boxes and accessories.
The usual sizes are 16, 25 or 35 mm², according to the sizing of the installation. When buying the conductor, a 10% margin for joints is added to the measured length.
Worked example: for 20 metres of earth conductor: 20 × 1.10 = 22 m to buy. This is the same calculation the PresupuestoYa calculator applies automatically for the Electrician trade.
Why it isn't an optional item
Earthing is mandatory under the Low Voltage Electrotechnical Regulation, and works together with the RCD: if there is a current leakage to a metal frame (a casing, a pipe), the earth gives a low-resistance path for that current to flow so the RCD detects it and cuts the supply before someone receives it by touching the appliance.
Frequently asked questions
How many earth rods does a home need? It depends on the resistance measured with the earth tester, not on a fixed number — if one rod doesn't get below 37 Ω, more must be added in parallel.
What's the difference between bare cable and green-yellow cable? Bare is used on the buried section up to the rod; insulated green-yellow, on the visible section and on the connections to boxes and accessories.
How do you measure whether the earthing is correct? With an earth tester, checking that the resistance doesn't exceed the 37 Ω required on installations with a 30 mA RCD.
Conclusion
Earthing isn't solved by simply driving in a rod: the real resistance of the ground must be measured and the number of rods adjusted until the regulatory limit is met. Quoting it with the right conductor margin and the earth-tester check included is what ensures the protection really works when needed, not just on paper.
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