Solar Plant Earthing Design: Complete Calculation Guide (IEEE 80)
Earthing is the one part of a solar plant that nobody sees and everybody depends on. It is the buried network of conductor, electrodes and bonds that gives fault current a controlled path back to the source, holds every piece of exposed metal at the same potential, and keeps the voltage a person can be exposed to during a fault inside survivable limits.
This guide explains how a solar plant earthing system is actually calculated — not estimated. Every number below comes from an equation with a named input, and every design criterion is stated instead of assumed.
Why Earth Resistance Alone Is Not a Design
The single most common mistake in solar earthing is treating "less than 1 ohm" as the design target.
Grid resistance is a useful project criterion. It is not a safety criterion. A plant can have a grid resistance of 0.08 Ω and still be dangerous, because the quantity that injures people is not resistance — it is the voltage difference across the body during the fault.
Two people standing in the same switchyard experience:
- Touch voltage — hand on an earthed structure, feet on the ground about one metre away
- Step voltage — one foot to the other, one metre apart, standing on the ground
Both depend on soil resistivity, grid current, grid geometry, burial depth, the surface layer and the fault clearing time. None of them is determined by resistance on its own.
A correct earthing design therefore has to answer four separate questions:
1. Will the conductor survive the fault thermally?
2. What is the grid resistance and the resulting ground potential rise?
3. Is the attainable mesh (touch) voltage below the tolerable touch voltage?
4. Is the attainable step voltage below the tolerable step voltage?
Only when all four are satisfied is the design safe.
Step 1 — Soil Resistivity
Soil resistivity ρ (ohm-metre) scales almost everything downstream: grid resistance, GPR, mesh voltage and step voltage move roughly in proportion to it. A geometry that is comfortably safe at 10 Ω·m can fail outright at 200 Ω·m.
It must be measured, normally by the Wenner four-pin method, at several spacings and several locations across the site. Where results vary strongly with probe spacing, the site is layered and a two-layer model should govern the issued design.
Two further values matter:
- Surface-layer resistivity ρs — crushed rock or gravel spread over walking areas, typically 2000–5000 Ω·m dry
- Surface-layer thickness hs — commonly 100–150 mm
The surface layer is one of the cheapest and most effective safety measures available, because it raises the tolerable touch and step voltage substantially without changing the buried grid at all.
Step 2 — Fault Data and Grid Current
The full system fault current is not the current that flows into the earth grid.
Part of the earth-fault current returns through overhead earth wires, cable sheaths, neighbouring earthed structures and the transformer neutral path. Only the remainder passes through the grid into the soil. That share is the fault-current division factor Sf:
```
```
Where:
| Symbol | Meaning | Typical |
|---|---|---|
| If | System earth-fault current | From the system study |
| Sf | Fault-current division factor | 0.4 – 1.0 |
| Df | Decrement factor (DC asymmetry) | 1.0 – 1.3 |
| IG | Grid current | Result |
Example: 25 kA × 0.7 × 1.0 = 17.5 kA flowing into the grid.
Two different times are used and they are frequently confused:
- tc — fault duration, used for conductor thermal sizing. Take the backup clearing time.
- ts — shock duration, used for tolerable touch and step voltage. Take the primary clearing time.
Using tc for both is conservative on conductor size and unconservative on nothing; using ts for both undersizes the conductor.
Step 3 — Conductor Thermal Sizing
The conductor must carry the fault current for the full duration without reaching a temperature that damages it or its joints. IEEE Std 80 gives:
```
```
Where A is in mm², I in kA, tc in seconds, Ta the initial conductor temperature and Tm the maximum allowable temperature. TCAP, αr, ρr and K₀ are material constants.
Indian practice frequently uses the CBIP / IS 3043 shortcut instead:
```
```
with k depending on material and permitted temperature rise (roughly 80 for GI, 205 for copper).
Both are valid. What matters is that the result is then increased by a stated design margin — commonly 20–30% for corrosion allowance over plant life — and rounded up to a standard flat size such as 50 × 6 mm, 65 × 8 mm or 50 × 10 mm.
For 17.5 kA at 1 s in GI, the requirement lands close to 480 mm², which drives a 50 × 10 mm strip. The same plant with a 0.5 s clearing time needs roughly 30% less steel. This is why quoting a "standard" earthing conductor size without stating fault current and duration is meaningless.
Step 4 — Grid Geometry
The buried grid is a rectangular mesh of dimension Lx × Ly with conductors spaced D metres apart, buried at depth h (typically 0.6 m).
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```
Spacing is the main lever a designer has. Halving D roughly doubles the buried conductor and sharply reduces mesh voltage, because the potential inside each mesh has less room to rise between conductors.
Step 5 — Grid Resistance (Schwarz Equations)
For a grid combined with vertical electrodes, IEEE 80 uses the Schwarz set:
```
```
The mutual term Rm is what stops designers from simply adding rods in parallel and claiming an unrealistically low resistance. Rods placed closer together than their own length screen each other, and the combined resistance improves far less than the count suggests.
Step 6 — Ground Potential Rise
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```
At 17.5 kA and 0.08 Ω, GPR is about 1400 V. The whole earthing system, and everything bonded to it, rises to that potential relative to remote earth for the duration of the fault.
GPR is not a pass/fail number by itself, but it drives two decisions:
- Whether telecom, SCADA and LV circuits leaving the plant need isolation transformers or fibre
- Whether the fence, if bonded, needs its own gradient control at gates and corners
Step 7 — Tolerable Touch and Step Voltage
The surface layer derating factor:
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Then, for a 50 kg body:
```
```
For 70 kg the constant becomes 0.157. The body-weight assumption should be stated in the design basis, not buried in a spreadsheet.
Notice how strongly the surface layer helps: 100 mm of 3000 Ω·m crushed rock can more than double the tolerable touch voltage compared with bare soil.
Step 8 — Attainable Mesh and Step Voltage
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```
Where Km is the mesh geometry factor, Ks the step geometry factor, Ki the irregularity factor (0.644 + 0.148n), and:
```
```
The design is safe when:
```
Em ≤ E_touch and Es ≤ E_step
```
If it is not, the remedies in order of practical effectiveness are: reduce conductor spacing D, increase or thicken the surface layer, add electrodes, extend the grid beyond the fence line, and reduce the fault clearing time.
Step 9 — Solar-Specific Earthing
A solar plant differs from a conventional substation in that most of its metal is spread across hundreds of acres.
Module mounting structures (MMS) are bonded row by row with a continuous conductor, each block tied into an array perimeter grid at a stated maximum spacing — 20 to 30 m is common as a project criterion, not a standard requirement. The array grid is then interconnected to the main plant grid at multiple points so no structure depends on a single path.
Earth pit quantity is calculated, not assumed. A defensible count is built from:
- Array perimeter ÷ maximum earth-connection spacing
- Grid perimeter ÷ maximum earth-connection spacing
- Corner points
- Dedicated pits for each transformer body and neutral connection
- Two connections for each inverter station and each MV panel
- One dedicated pit per lightning arrester set
- Fence sections and gates
An answer like "10 earth pits for a 6 MW plant" with no derivation is not engineering.
Inverters and transformers each require at least two separate and distinct connections, taken from different directions so a single damaged conductor cannot isolate the equipment.
Step 10 — Transformer Body vs Neutral Earthing
These are two different functions and must never be merged in the calculation:
| Body (equipment) earthing | Neutral (system) earthing | |
|---|---|---|
| Purpose | Keeps the tank and metalwork from becoming live | Provides a return path so earth faults are detected and cleared |
| Current | Fault current only during a fault | Defines the earth-fault current magnitude |
| Typical material | GI flat | Copper flat |
| Connections | ≥ 2, separate and distinct | ≥ 2, separate and distinct |
Both are sized by the same thermal equation, using the transformer earth-fault current and its clearing time, plus the design margin.
Step 11 — Verification and Testing
A design is not complete until it is testable. Commissioning should record:
- Continuity of every bond, structure row and equipment connection
- Individual earth-electrode resistance for each pit
- Overall grid resistance by the fall-of-potential method
- Where required, measured touch and step voltage
- Photographic records of joints before backfilling
Documented as part of the PV system commissioning record.
Standards Commonly Applied
- IEEE Std 80-2013 (and Cor 1-2015) — the calculation methodology used throughout this guide
- CBIP Manual on Substation, Publication No. 223 — Indian substation earthing practice
- IS 3043 — Code of practice for earthing
- CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 — statutory requirements including separate and distinct neutral connections
- IEC 62548-1 — PV array design requirements
- IEC 60364-5-54 — Earthing arrangements and protective conductors
- IEC 60364-7-712 — PV power supply installations
- IEC 62305 series — Lightning protection and equipotential bonding
- IEC 62446-1 — PV system documentation, commissioning and inspection
Where project specifications, utility requirements or statutory regulations differ, the governing project or AHJ requirement takes precedence.
Worked Summary — 6 MW Ground-Mounted Plant
| Parameter | Value |
|---|---|
| Soil resistivity ρ | 10 Ω·m |
| Surface layer | 3000 Ω·m, 100 mm |
| Fault current If | 25 kA, 1 s (thermal), 0.5 s (shock) |
| Division factor Sf | 0.7 |
| Grid current IG | 17.5 kA |
| Grid | 70 × 50 m, 5 m spacing, 0.6 m depth |
| Conductor required | ≈ 477 mm² → 50 × 10 mm GI |
| Buried conductor Lc | ≈ 1520 m |
| Grid resistance Rg | ≈ 0.08 Ω |
| GPR | ≈ 1400 V |
| Tolerable touch / attainable mesh | ≈ 674 V / ≈ 121 V — SAFE |
| Tolerable step / attainable step | ≈ 2204 V / ≈ 146 V — SAFE |
The margins here are wide because the soil is exceptionally good. On a 200 Ω·m site with the same geometry, the mesh voltage would rise by a factor of twenty and the design would need much tighter spacing and a thicker surface layer.
Frequently Asked Questions
What is solar plant earthing?
The buried conductor grid, electrodes and bonding network that provides a controlled fault return path, equalises potential across all exposed metal, and limits touch and step voltage during a fault.
How is the earthing grid size calculated?
Conductor length from grid dimensions and spacing; cross-section from fault current and duration through the IEEE 80 thermal equation; then verification against resistance, GPR, mesh voltage and step voltage.
How many earth pits does a solar plant need?
It is derived from the array and grid perimeter divided by the maximum earth-connection spacing, plus corners and every equipment connection point. For a typical 6 MW plant this lands in the dozens, not in single figures.
Is one earth pit enough?
No. A single pit gives no redundancy and cannot control potential gradients across a plant. Transformers alone require multiple independent connections.
What earthing conductor size should I use?
There is no universal answer. It follows from fault current, fault duration, material and design margin. State those four and the size is determined.
What is grid current?
The part of the earth-fault current that actually returns through the grid into the soil: IG = If × Sf × Df.
What is the fault-current division factor?
The fraction of total earth-fault current entering the grid rather than returning through earth wires, sheaths or adjacent earthed structures. It comes from a current-division study.
What is GPR?
Ground potential rise, GPR = IG × Rg — the voltage the entire earthing system reaches relative to remote earth during a fault.
What is touch voltage?
The potential difference between a hand on earthed metal and the feet on the ground about a metre away.
What is step voltage?
The potential difference between two feet a metre apart on the ground surface.
What is mesh voltage?
The worst-case attainable touch voltage inside a grid mesh, Em = ρ·IG·Km·Ki / LM.
What is the difference between transformer body and neutral earthing?
Body earthing protects against the enclosure becoming live; neutral earthing establishes the system earth reference so faults can be detected. Different purpose, separately sized, separately connected.
Why is soil resistivity so important?
It scales resistance, GPR, mesh voltage and step voltage almost proportionally. It is the single most influential input in the entire calculation.
Is below 1 ohm always safe?
No. Low resistance with high fault current can still produce dangerous mesh and step voltages. Safety is decided by the voltage checks.
How are rods included in IEEE 80?
Through the Schwarz equations for R₁, R₂ and the mutual resistance Rm, combined as Rg = (R₁R₂ − Rm²)/(R₁ + R₂ − 2Rm).
Which standards apply?
IEEE 80 for calculation, CBIP 223 and IS 3043 for Indian practice, CEA regulations for statutory compliance, IEC 62548-1 / 60364-5-54 / 60364-7-712 for PV installations, IEC 62305 for lightning, IEC 62446-1 for testing.
How is MMS earthing connected to the main grid?
Rows are bonded continuously, blocks tie into an array perimeter grid at the specified spacing, and the array grid interconnects with the main plant grid at several points.
What tests are required after installation?
Bond continuity, individual electrode resistance, overall grid resistance by fall-of-potential, and where required measured touch and step voltage.
Calculations in this guide are design aids. They do not replace a soil investigation, a system fault study, approved drawings, utility requirements or verification by a qualified electrical engineer.