Unbonded Earth Electrodes at a 250 kVA Package Substation
A 250 kVA, 11 kV/415 V package substation serves a small industrial estate. The transformer neutral, transformer tank, LT panel body and lightning arrester are each connected to a separate 3 m galvanized iron pipe electrode. Measured resistances by the fall-of-potential method are 4.2 Ω, 3.8 Ω, 5.6 Ω and 4.9 Ω respectively; the electrodes are 8–12 m apart and are not bonded to each other. Soil resistivity is about 180 Ω·m, and the yard has 6 m × 4 m of available space. During a nearby lightning strike, the electronic energy meter and a PLC input card were damaged, and a technician received a shock from the panel body. The supply authority requires the transformer neutral earth to be ≤1 Ω. The contractor proposes adding salt and charcoal to each pit and driving the pipes deeper.
1. Assess the installation against the requirement and explain the likely cause of the damage and shock using the measured values.
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Compliance assessment
The transformer neutral earth is 4.2 Ω, which is more than four times the required ≤1 Ω. Each electrode is a separate earth, and they are not bonded. This is not one earthing system; it is four independent earths with different potentials.
Why the damage and shock occurred
During a lightning strike, the lightning arrester carries a large impulse current to its own 4.9 Ω electrode. The potential of that electrode rises to a high value, while the panel body and the transformer tank remain at the potential of their separate electrodes. The result is a large potential difference between the arrester earth, the panel body and the PLC reference. The surge can flash across to the electronic equipment and to a person touching the panel, causing the damage and the shock.
A second problem is the 4.2 Ω neutral earth. If an earth fault drives, say, 30 A into that resistance, the neutral and associated metalwork can rise by about 4.2 × 30 = 126 V relative to remote earth. That is far above a safe touch voltage and is enough to give a severe shock. The measured resistances are not in parallel because the electrodes are not bonded, so the low individual values do not add up to a low system resistance.
2. Estimate whether adding salt and charcoal can meet 1 Ω, and recommend an earthing arrangement within the 6 m × 4 m space.
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Salt and charcoal are not a dependable solution. They initially lower local soil resistivity by holding moisture and conducting ions, but they leach away, corrode the electrode and give unpredictable seasonal results. Driving the existing pipes deeper may help if a lower-resistivity layer exists, but the yard is small and the proposal does not address the separated earths.
Bonding first
Bond all four electrodes with a 25 × 3 mm GI strip or a 50 mm² copper conductor. The approximate parallel resistance is:
[R=\frac{1}{\frac{1}{4.2}+\frac{1}{3.8}+\frac{1}{5.6}+\frac{1}{4.9}}\approx1.13\ \Omega]
That is close to 1 Ω but still above the requirement, and the parallel formula is optimistic because the electrodes are close enough to interfere with each other.
Recommended arrangement
- Form a ring earth around the available 6 m × 4 m yard using 25 × 3 mm GI strip buried at about 0.6–0.8 m, bonded to all four existing electrodes.
- Add two or three additional 3 m rods at the corners, spaced as far apart as the yard allows, with bentonite or a proprietary backfill. With six electrodes in parallel and realistic mutual coupling, the resistance can be brought below 1 Ω, but it must be measured, not assumed.
- Bond the lightning arrester earth to the common earth grid. A separate unbonded lightning earth is a sparkover hazard to the panel and PLC.
- Bond the transformer tank, LT panel body, cable armour, structural steel and the neutral earth at the transformer to the common grid. Keep the neutral-earthing link at the transformer neutral only, in accordance with the earthing code, so that neutral current does not normally flow through the general mass of earth.
- Use equipotential bonding and, where needed, a 30 mA RCD for socket circuits to reduce touch-voltage risk.
With 1 Ω and a 1 kA impulse or fault current, the earth potential rise is still 1 kV, so bonding and equipotential grading matter as much as the resistance value.
3. Specify the tests and records needed to prove compliance, and state the relevant Indian standards or rules you would cite.
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Tests
- Fall-of-potential (three-pole) measurement of the common earth grid, with the current electrode far enough away and the potential electrode moved to check the plateau. Record the test layout and soil condition.
- Continuity and bonding resistance between the transformer tank, neutral, arrester, panel body, cable armour and the ring earth. A low-resistance ohmmeter should show a sound bond.
- Individual electrode resistance before and after bonding, so that the effect of the new electrodes can be assessed.
- Neutral-earth voltage and earth-leakage current at normal load, and a check that the neutral is earthed only at the designated point.
- RCD trip test on the socket circuits, if fitted, and a touch-voltage assessment at the panel.
Records
- As-built earthing layout with electrode positions, depths, conductor sizes and bonding points.
- Test results with instrument serial numbers, calibration status and weather/soil conditions.
- A compliance statement against the specified ≤1 Ω and the bonding requirements.
References: IS 3043 (Code of Practice for Earthing) and the earthing/connection-with-earth provisions of the Indian Electricity Rules, 1956, together with the CEA safety regulations and the supply authority's requirement. The design should also follow the manufacturer's instructions for the package substation and the lightning protection system.