Earthing: IS 3043, pits, electrodes, conductor sizes, market practice vs standard

Earthing is invisible to the customer and rarely tested properly, so it is often done badly. A good earthing system does three things:

1. Protective earthing: if a live conductor touches a metal part (module frame, structure, inverter body), fault current flows to earth and the protective device trips, so nobody gets a shock.
2. Equipotential bonding: all metal parts that someone can touch at the same time are held at the same voltage, during a fault and during a lightning surge.
3. A path for surges: SPDs and the lightning protection system need a low-impedance route to earth.

Point 2 is the one most often forgotten: earthing is about bonding, not only a low number on the meter.

What the standards say

Three ideas run through all of these:

• One bonded earthing system: several electrodes are fine, but connected to each other and to the building's main earthing terminal.
• Conductors sized for the fault current they must carry, and made of materials that survive in soil.
• Measured and recorded, with a test link that lets you measure each electrode later.

Electrodes and soil

For a single vertical rod or pipe electrode, IS 3043 gives:

R = (100 ρ ÷ 2πL) × ln(4L ÷ d)

with ρ the soil resistivity in Ω·m, and L (length) and d (diameter) in centimetres.

Worked example

A 3 m (300 cm) electrode of 40 mm (4 cm) diameter:

• Moist garden soil, ρ = 20 Ω·m: R = (2,000 ÷ 1,885) × ln(300) = 1.061 × 5.70 = 6.0 Ω
• Typical dry urban fill, ρ = 50 Ω·m: R = 2.65 × 5.70 = 15.1 Ω
• Sandy soil in Rajasthan, ρ = 200 Ω·m: about 60 Ω

One 3 m electrode will not reach a low value in dry soil. The options are a longer electrode, several electrodes in parallel (spaced at least twice their length apart so they do not share the same soil), or an earth-enhancing backfill around the electrode to lower the local resistivity. Two electrodes spaced well apart give a bit more than half the resistance of one.

Resistivity rises as soil dries, so measure in the dry season, or allow a margin if you measure in the monsoon.

What value to aim for? There is no single number for every rooftop system in IS 3043. Many DISCOM and tender specifications ask for 5 Ω or less for each pit (the PM Surya Ghar technical specification says earth resistance shall never exceed 5 Ω, Annexure 3 cl. 8.1.4), and IS/IEC 62305-3 recommends below 10 Ω for the lightning earth where possible. A well-bonded system with a modest resistance is safer than an isolated pit with a very low one.

Sizing conductors

Protective conductors must carry fault current until the device trips. IS 3043 and IEC 60364-5-54 use the adiabatic equation:

S = I × √t ÷ k

with S in mm², I the fault current in amps, t the disconnection time in seconds, and k a material factor (143 for PVC-insulated copper as a separate conductor, 95 for PVC-insulated aluminium).

Example: fault current 2,000 A, cleared in 0.4 s, PVC copper: S = 2,000 × 0.632 ÷ 143 = 8.8 mm², so use 10 mm² copper. In aluminium it would be 13.3 mm², so 16 mm².

For PV systems the common practical sizes are:

Common market practice in India

On most Indian residential roofs you will see something like this:

• Three "separate" earth pits, labelled DC, AC and LA, each with a 1–3 m copper-bonded or GI "chemical earthing" rod in a bag of earth-enhancing compound, and often not connected to each other or to the building's existing earth.
• 16 mm² aluminium PVC-insulated "LA cable" (or 25 × 3 mm GI strip) from the roof to the pits, with the aluminium sometimes taken right down into the pit and clamped to a copper-bonded rod.
• Module frames daisy-chained with 4 mm² green wire, sometimes clamped onto anodised aluminium frames with an ordinary washer.
• An earth resistance figure written on the handover sheet that nobody measured, or measured once in the monsoon with a clamp tester.

Many DISCOM templates and tender specifications themselves ask for "three separate earth pits". PM Surya Ghar asks for three separate pits only for systems of 10 kW and above (Annexure 3, cl. 8.1.5), and its LA conductor minimum of 16 mm² aluminium (cl. 8.2.4) is where the "LA cable" habit comes from (next lesson).

What is wrong with it, and why it matters

• Unbonded pits work against each other. During a lightning strike or a fault, the soil around one pit can rise by thousands of volts relative to another pit a few metres away. If the inverter's DC side is tied to one pit and its AC side to another, that difference appears across the inverter and its SPDs. IS/IEC 62305-3 and IS 3043 both call for the electrodes to be bonded into one system. Read "three separate pits" as three electrodes, and bond them, either below ground or at the main earth bar. If a specification explicitly forbids bonding, raise it in writing with the DISCOM engineer and record the answer.
• Aluminium does not belong in soil. IS 3043 and IEC 60364-5-54 do not allow aluminium for buried earth conductors because it corrodes quickly in moist soil. An aluminium-to-copper joint in a damp pit corrodes even faster. After two or three monsoons the "earth" can be an open circuit.
• 16 mm² aluminium is not a lightning down conductor. IS/IEC 62305-3 Table 6 asks for at least 50 mm² for copper and 50–70 mm² for aluminium (70 mm² for tape). See the next lesson.
• Anodising is an insulator. A plain washer on an anodised frame may give a high-resistance bond. Use the module maker's earthing holes with serrated (star) washers or listed earthing clips.
• Daisy chains break. If the earth wire runs from frame to frame, removing one module for a warranty claim leaves the rest of the row unearthed. Run a continuous bonding conductor along the structure and tap each row onto it.

Not every market-style system will fail; on many sites nothing has happened yet. But the risks are real: shocks from frames during an insulation fault, inverter and SPD damage in storm season, and a weak position if an inspector or insurer ever looks closely.

Key points

• Earthing protects people (fault clearing), equalises voltages (bonding) and gives surges a path; bonding is as important as resistance.
• Single-rod resistance depends mainly on soil resistivity: one 3 m rod reaches 6 Ω in moist soil but 15–60 Ω in dry or sandy soil.
• Size protective conductors with S = I√t ÷ k; bond array frames with at least 6 mm² copper and LPS connections with 16 mm² copper.
• Common practice of unbonded separate pits, buried aluminium and 16 mm² aluminium down conductors does not meet IS 3043 or IS/IEC 62305.
• Bond all electrodes into one system, use test links and inspection chambers, and measure in the dry season.

Check yourself

1. Estimate the resistance of a 3 m, 40 mm electrode in soil of 30 Ω·m.
2. Why are unbonded "DC", "AC" and "LA" pits risky for the inverter?
3. A fault of 1,500 A is cleared in 0.2 s. What PVC copper protective conductor size is needed?