What is the earthing principle?
Give fault and lightning current an easy way to ground, so exposed metal never stays at a dangerous voltage.
The earthing principle is to connect the exposed metal of equipment to the general mass of earth through a low-resistance path. Fault current divides between the paths available and flows mostly through the lowest-resistance one, so when a fault energises a case, the fault current flows down the earth conductor into the ground instead of through anyone touching it.
The same path gives lightning and surge currents somewhere safe to go. It only works if every joint is sound and the electrode stays in good contact with the soil, so corrosion, loose connections and dry ground that raise the resistance are hidden hazards found by regular earth-resistance testing.
- Bond the metalwork. Connect exposed metal parts to an earth conductor.
- Reach the ground. Connect the conductor to an electrode, such as a driven rod, in the soil.
- Keep it low. Maintain sound joints and test the resistance periodically.
Frequently Asked Questions
Q1What is the earthing principle?
Connect exposed metal parts to the general mass of earth through a low-resistance path. If a fault energises a metal case, the fault current flows through that path to earth rather than through a person.
Q2Why must the earth path be low resistance?
Current divides between the available paths and mostly takes the lowest-resistance one. A low-resistance conductor and electrode carry the fault current away and keep the voltage on the case low, while a person touching the case offers a far higher-resistance path.
Q3What are the parts of an earthing system?
The exposed metalwork being protected, the earth conductor connecting it, the electrode (such as a driven rod) in contact with the soil, and the soil itself. All connections must stay sound and low in resistance.
Q4How does earthing help with lightning?
Lightning and surge currents also need a path to ground. A well-connected earthing network gives them somewhere to go instead of travelling through cables and equipment.
Q5Why test the earth resistance?
Corrosion, loose joints and dry soil can raise the resistance over time. Periodic earth-resistance testing shows whether the earth path is still good enough, using the limits set by the project or applicable standard.
Q6Is earthing the same as bonding?
They work together but are not identical. Earthing connects the system to the ground, while bonding joins metal parts to each other so they stay at the same potential.
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