Earth Resistance Value: What Is Acceptable and How to Check It

For most low-voltage installations in India an earth electrode resistance of 5 ohms or less is the usual acceptance figure, with tighter limits for substations and sensitive equipment and a commonly cited 10 ohm ceiling for a lightning protection earth. The number that actually governs your site is whatever your project specification, the relevant standard edition and your electrical inspector require — so treat the table below as the map, not the contract.

What earth resistance value is acceptable?

Earthing resistance is the resistance between the earth electrode and the general mass of earth around it. The lower it is, the more readily a fault current or a lightning surge is carried away, and the lower the voltage that appears on exposed metalwork while it happens. Different applications tolerate different figures:

Two practical points that the bare numbers hide:

• Measure in the dry season. Soil resistivity swings with moisture. A pit that reads 3 Ω in September can read 8 Ω in May. The acceptance test should reflect the worst case, not the most convenient one.
• A low reading is not the same as a safe installation. Continuity of the bonding, the condition of the joints and the size of the conductor all matter. An excellent earth pit connected through a corroded lug protects nobody.

What does a “good” earthing value look like in practice?

As a working rule of thumb for an ordinary commercial or industrial building in India:

• Under 1 Ω — excellent. Expect this only with a well-designed multi-electrode or ring arrangement in reasonable soil.
• 1–5 Ω — good, and acceptable for most LV installations.
• 5–10 Ω — borderline. Acceptable for a lightning protection earth in many specifications; usually needs improvement for a power earth.
• Above 10 Ω — investigate. Either the soil is genuinely difficult or something in the installation is wrong.

How to read your megger / earth tester value

The figure on the instrument is only meaningful if the method matches what you are trying to measure. In short: the three-point fall-of-potential test measures a single, disconnected electrode; the four-point (Wenner) test measures soil resistivity for design, not the installed electrode; and a clamp-on tester measures a single electrode while it is still part of a multi-electrode system, without disconnecting it.

The most common reason two people get two different numbers from the same pit is that one of them left the electrode bonded to the rest of the system. That is what the earthing test link exists for — it lets you isolate the electrode for the test and restore the bond afterwards. Our step-by-step guide to how to check earthing resistance covers the procedure and the lead spacing in full.

Why is my earth resistance value high?

The resistance of an earth electrode is dominated by the soil immediately around it, so most high readings come down to soil or contact, not to the electrode metal:

• High soil resistivity — dry sand, gravel, laterite, rock or made-up ground. This is the single biggest factor.
• Seasonal drying — the moisture that carried the current in the monsoon is gone by summer.
• Too little electrode surface in contact with soil — a short rod, or a rod driven into a backfilled trench rather than undisturbed soil.
• Salt-and-charcoal backfill that has leached away — the classic failure of a conventional pit, which is why its reading climbs year on year.
• Corroded or loose connections — at the lug, the test link, or the electrode-to-conductor joint. This shows up as a reading that is erratic rather than just high.
• Electrodes spaced too close together — two rods a metre apart largely share the same volume of soil and give far less than half the resistance of one.

How to bring a high reading down

1. Go deeper before you go wider. Doubling the driven depth reduces resistance far more than adding a second shallow rod, and deeper soil holds moisture through the dry season.
2. Add electrodes at proper spacing. Space parallel electrodes at least their own driven length apart, otherwise their zones of influence overlap and you pay for metal you do not benefit from.
3. Use a low-resistance backfill compound around the electrode to improve contact with difficult soil and hold moisture. A stable, non-leaching compound is the point — see our comparison of backfill compounds for different soil types.
4. Replace a leaching conventional pit with a maintenance-free electrode. The difference between the two approaches, and when the swap is worth it, is set out in chemical earthing vs conventional earthing.
5. Fix the joints. Exothermic welded connections do not loosen or corrode the way bolted lugs do — see the exothermic welding kit guide.
6. Re-measure in the dry season and record it. A single reading is a snapshot; a log is evidence.

Frequently asked questions

What is the earth pit resistance value as per Indian standard?

IS 3043 is the Indian code of practice for earthing. It sets tighter limits for generating stations and sub-stations (0.5–2 Ω) and a general figure of 5 Ω for other installations. For a lightning protection earth termination, IS/IEC 62305-3 is the relevant document and a single earthing arrangement below 10 Ω is commonly cited. Confirm the current edition and your own project specification before accepting a value.

Is 5 ohms a good earthing value?

For a general low-voltage installation, yes — 5 Ω is the usual acceptance figure. For a sub-station, a data centre or sensitive equipment it is not low enough.

What should the earthing megger value be?

An earth tester (often called a megger after the brand) should read below the limit for your application — typically 5 Ω or less for an LV installation. Make sure the electrode is isolated at the test link before you read it, or you are measuring the whole bonded system rather than the pit.

How often should earth resistance be tested?

Annually is the common practice, and ideally at the driest point of the year so the record reflects the worst case. Conventional salt-and-charcoal pits need checking more often because their resistance drifts upward as the backfill leaches away.

Does a lower earth resistance always mean better protection?

Lower is better, but only up to the point where the rest of the installation is sound. Bonding continuity, conductor size and joint integrity decide whether that low-resistance path is actually available when a fault occurs.

Getting a stable earth value on a difficult site

Ashlok has manufactured earthing electrodes and backfill compounds in India since 1999, including for rocky and sandy sites where conventional pits will not hold a value. If your readings are drifting upward year on year, that is usually a sign the pit design — not the tester — is the problem.

See Ashlok safe earthing electrodes and the Baron backfill compound, or send us your site details and current readings.