About the Earth Mat Conductor Sizing
Earth mat conductor sizing determines the minimum cross-section of the bare copper or galvanised steel conductor used in a solar substation's buried earthing grid — ensuring the conductor can carry the maximum earth fault current for the protection operating time without exceeding its adiabatic temperature limit.
Formula
A_min = I_fault × √t / K
- I_fault
- Maximum earth fault current flowing through the conductor (A rms)
- t
- Fault duration = protection operating time (seconds)
- K
- Material constant: 143 for bare copper, 94 for aluminium, 78 for galvanised steel
- A_min
- Minimum conductor cross-section (mm²)
How to calculate it
- 1Determine the maximum earth fault current from the short-circuit study (single-line-to-ground fault).
- 2Determine the fault duration from the main earth fault relay operating time (slowest backup protection).
- 3Apply the formula to get A_min.
- 4Round up to the next standard size: 25, 35, 50, 70, 95, 120 mm² for copper.
- 5Check the minimum size for mechanical strength: IS 3043 recommends a minimum of 25 mm² copper for buried earth mat conductors.
Worked example
Earth fault current = 10 kA, relay clears in 0.5 s, copper conductor: A_min = 10,000 × √0.5 / 143 = 10,000 × 0.707 / 143 = 49.4 mm². Select 50 mm² bare copper (HDC) as the standard next size. This is consistent with IS 3043 requirements for a 33 kV solar substation.
What you can calculate
- Conductor size
- IS 3043:2018
- Thermal limit
- SC current
Standards & references
Frequently asked questions
Should I use copper or galvanised steel for the earth mat conductor?
Copper is strongly preferred for solar substations: it has lower resistivity (better fault current conduction), better corrosion resistance in Indian soil conditions, and lower long-term maintenance. Galvanised steel is cheaper but corrodes more quickly (5–10 year coating life in many Indian soils) and has a significantly higher resistivity — requiring a much larger cross-section for the same performance. IS 3043 recommends copper for critical installations.
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