Plant earthing: electrodes, strip sizes and what the inspector checks
A plant needs at least two separate body earths and two separate neutral earths under IS 3043, a measured resistance below about 5 ohms for a general LT installation, and a test reading taken in the dry season rather than after rain. In the red and lateritic soil around Bargarh the dry-season reading is the one that matters, and it is usually the reason a conventional pipe earth stops working in May.
- StandardIS 3043, Code of practice for earthing
- Minimum2 electrodes for body earth, 2 separate for neutral
- Target<5 Ω general LT · ≤1 Ω substation and generating
- Test whenEnd of dry season — the worst-case reading is the real one
What is the earthing actually for?
Two separate jobs, and confusing them is where most bad installations start.
Body earthing connects the metal enclosure of every piece of equipment back to earth, so that if a live conductor touches the frame, enough fault current flows to operate the protective device quickly. It is about disconnection time, not about "draining electricity away". If the earth path has too much resistance, the breaker does not see enough fault current, does not trip, and the frame stays live.
Neutral earthing connects the star point of the transformer or generator to earth, which fixes the system voltage relative to earth and gives earth faults a defined return path.
These get separate electrodes, and they are kept physically apart. A single earth pit doing both jobs is a common shortcut in small installations and it is wrong.
How many electrodes, and how big?
Two body plus two neutral is the floor, not the answer. The number rises with the fault current the supply can deliver and with the physical spread of the plant. A rice mill with the transformer at the gate and a godown 150 metres away needs earthing at both, bonded together into one grid.
Spacing matters as much as count. Electrodes driven close together share the same volume of soil and their resistances do not add up the way you would like — two pits a metre apart are barely better than one. Space them at least twice the electrode length apart, and further if you have the room.
| Item | Typical specification | Notes |
|---|---|---|
| GI pipe electrode | 40 mm dia, 2.5–3 m, medium class | Cheapest. Needs a watering funnel and actual watering |
| GI plate electrode | 600 × 600 × 6 mm | Larger contact area, harder to install deep |
| Copper plate | 600 × 600 × 3 mm | Better in aggressive soil, considerably dearer |
| Maintenance-free electrode | Typically 48–80 mm dia with backfill compound | Holds resistance through dry months. Best value in this district |
| Earth strip, main grid | GI 50 × 6 mm or 25 × 3 mm depending on fault level | Size for fault current and duration, not by habit |
| Equipment bonding | GI or copper strip / conductor to equipment size | Two separate connections to every major frame |
Why does the soil here matter so much?
Earth resistance is dominated by soil resistivity, and soil resistivity is dominated by moisture and dissolved salts. The red and lateritic soils common across western Odisha have a high resistivity to begin with, and between March and June they dry out and shrink.
Two things then happen to a conventional pipe earth. The soil resistivity itself rises sharply, and the soil physically pulls away from the electrode, so the contact area collapses. A pit that measured 3 ohms in September can measure 15 or 20 in May.
That is why the reading you take after the first rain is worthless for compliance. Measure at the end of the dry season. If it passes then, it passes all year.
Watering funnels that have never had water put down them. Almost every conventional pipe earth we are shown has a funnel and a chamber, and in almost every case nobody has watered it since the day it was commissioned. If you are not going to maintain a pipe earth, do not buy one — buy a maintenance-free electrode with a proper backfill compound and stop pretending.
Earth pits paved over during a floor extension. The chamber goes under new concrete, the test link becomes inaccessible, and from that day the earth cannot be tested or maintained. Keep every pit accessible and keep a drawing showing where they are.
And the one that actually hurts people: earth strip bolted to a painted frame. Paint is an insulator. The bolt looks like a connection and measures like an open circuit. Scrape to bright metal, use a serrated washer, and check continuity with a meter rather than with your eyes.
Getting the strip size right
Earth conductors are sized for the fault current they must carry for the time the protection takes to clear, not for the running load. A 25 × 3 mm GI strip is adequate on a small installation and badly undersized on a plant fed from a 1000 kVA transformer where the prospective fault current is far higher.
The practical version: tell us the transformer rating and its impedance and the clearing time of the upstream protection, and we will tell you the strip section. Guessing here produces an earth conductor that vaporises during the one fault it existed to handle.
What to keep for the inspector
- Earth resistance test report — value, instrument, method, date, and who took it.
- Earth pit layout drawing showing every pit and the grid interconnection.
- Accessible, visible test links at each pit so individual electrodes can be isolated and measured.
- Electrode type, size and depth for each pit.
- Continuity records from equipment bodies back to the earth bar.
- The previous years' readings. A rising trend is how you find a failing pit before it fails.
The short version
- Two body earths, two neutral earths, kept separate. That is the minimum, not the target.
- Space electrodes at least twice their length apart or they share the same soil.
- Below 5 ohms for general LT, tighter for substations.
- Test at the end of the dry season. That reading is the honest one.
- In Bargarh soil, maintenance-free electrodes usually beat pipe earths that nobody waters.
- Size the strip for fault current, and check bonds with a meter, not by eye.
Send us your transformer rating and a rough site plan and we will work out the electrode count, the strip section and the earthing material list, and quote it as one lot.