By An earthing system is only as good as its resistance to earth. A low resistance means fault current and lightning energy can flow safely into the ground; a high one means your protection may not work when it matters. Knowing how to check earthing resistance — correctly and safely — is one of the most useful skills for any electrician, facility manager, or plant maintenance team.
This guide walks through the standard testing method, the values you should aim for, and the mistakes that produce false readings.
Why earthing resistance matters
When a fault occurs, the earthing system must carry current away fast enough to trip the protective device and keep touch voltages safe. If the earth resistance is too high, the fault current is limited, the breaker may not trip quickly, and metal parts can stay dangerously live. For lightning and surge protection, high resistance means the energy has nowhere fast to go — raising the risk of side-flashes and equipment damage.
Resistance also drifts over time. Soil dries out, joints corrode, and electrodes age. That is why earthing should be tested at commissioning and then at least once a year.
What you need to test earthing resistance
The standard instrument is an earth resistance tester (also called an earth megger or earth tester). A digital clamp-on earth tester is convenient for quick checks on multi-electrode systems, but the most reliable and widely accepted method for a single electrode is the 3-point fall-of-potential test, which needs the tester plus two auxiliary spikes and connecting leads.
You will also need insulated gloves, and you must isolate the electrode under test from the installation before disconnecting anything.
The 3-point fall-of-potential method — step by step
- Make the area safe. Switch off and isolate the circuit. Disconnect the earth electrode under test from the system so you measure the electrode alone, not the whole network.
- Position the spikes. Drive the two auxiliary spikes into the ground in a straight line away from the electrode. Place the current spike (C) farthest out, and the potential spike (P) roughly 62% of the distance between the electrode and the current spike. A common practical spacing is the electrode at 0 m, P at about 15–20 m, and C at about 25–30 m.
- Connect the leads. Connect the tester’s terminals to the electrode under test, the potential spike, and the current spike as marked on the instrument.
- Take the reading. Run the test and note the resistance in ohms.
- Verify the reading. Move the potential spike about 3 m closer, then 3 m farther, and re-measure. If the three readings are close, the value is valid. If they differ a lot, the spikes are inside each other’s resistance zones — move the current spike farther out and repeat.
- Record and reconnect. Log the value with the date, then reconnect the electrode to the system.
What earthing resistance value is acceptable?
General guidance under Indian practice (IS 3043) is to aim as low as reasonably practicable. As a rule of thumb: below 1 ohm for large substations and power installations, below 5 ohms for most industrial and commercial buildings, and typically 5–10 ohms or lower for ordinary buildings and lightning protection. Sensitive electronics and data centres often demand 1 ohm or less. For a full explanation of target values, see our guide on earthing resistance value.
What to do if the resistance is too high
If your reading is above the target, you have several options. You can add more electrodes in parallel, install a deeper electrode to reach moist soil, or improve the soil around the electrode with a quality backfill compound that retains moisture and lowers resistivity. The most durable long-term fix is to switch to a maintenance-free chemical earthing electrode, which holds a low, stable resistance for years even in poor soil.
Common mistakes that cause false readings
The biggest errors are not isolating the electrode from the system (so you measure the whole network), placing the spikes too close together, testing right after rain (which gives an unrealistically low value), and corroded or loose test connections. Always repeat the potential-spike check to confirm the reading is genuine.
Frequently asked questions
How often should earthing resistance be checked?
At commissioning, then at least once a year — and after any major electrical work or lightning event.
Can I check earthing resistance with a multimeter?
No. A multimeter cannot inject the test current needed. Use a dedicated earth resistance tester.
What is a good earthing resistance value?
As low as practicable — commonly under 5 ohms for buildings, under 1 ohm for substations and sensitive equipment.
Why does my reading change between seasons?
Soil moisture changes resistivity. Dry summer soil raises resistance; that is why maintenance-free electrodes and backfill compounds are used to stabilise it.
Do I need to disconnect the electrode before testing?
Yes, for the fall-of-potential method — otherwise you measure the entire connected network, not the electrode.
Struggling to hit a low, stable earth resistance? Ashlok’s maintenance-free earthing electrodes and backfill compound are engineered to deliver reliable low-resistance earthing for years. Contact us for the right solution for your soil.