Earthing and Grounding
Calculation (Excel)

An earthing and grounding calculation sizes the earth grid for a substation, switchyard, or process plant electrical system so that step voltage, touch voltage, and ground potential rise (GPR) under maximum fault current remain within the tolerable limits per IEEE 80 (Guide for Safety in AC Substation Grounding). It records soil resistivity, fault current, clearance time, grid geometry (area, conductor spacing, parallel conductors), conductor cross section, ground rods, calculated grid resistance using the Schwarz or Sverak equations, GPR, step voltage, touch voltage, and the tolerable safety limits for a 50 kg or 70 kg body.

Required columns: calculation reference and revision, project and substation identifier, single line diagram reference, soil resistivity for upper and lower layers, upper layer thickness, surface layer material and resistivity (crushed rock typically 2500 to 5000 ohm-m), maximum fault current symmetrical, fault clearance time, X/R ratio, decrement factor (Df) and current division factor (Sf), grid area, conductor spacing (D), number of parallel conductors (N), conductor material and cross section per IEEE 80 sizing equation, number and length of ground rods, calculated grid resistance, GPR, calculated step and touch voltage, tolerable step and touch voltage limits, and compliance status.

Pathnovo, an engineering document intelligence platform, builds the earthing calculation by extracting fault current and clearance time data from the relay coordination study, reading soil resistivity from the geotechnical report, and consuming the substation general arrangement for grid area and conductor layout. The platform applies the IEEE 80 Schwarz or Sverak grid resistance equation, computes step and touch voltage, and produces a draft calculation in your EPC template. Drafts are reviewed by the electrical engineer before issue. See the P&ID extraction workflow for upstream electrical data capture.

Conductor cross section is sized per IEEE 80 Clause 11 using the fault current magnitude, fault duration, ambient temperature, maximum allowable conductor temperature (copper 1083 deg C for fusing, GI 700 deg C, aluminium 660 deg C), and the material thermal coefficient. The sizing equation balances the I-squared-t energy against the conductor heat capacity. Typical EPC scope uses 95 mm² copper or 100 x 6 mm GI flat for 40 kA, 1 second fault. The calculation template captures the inputs and the resulting minimum cross section, plus the procurement size selected after rounding up to the next standard cross section.

Step voltage is the voltage between a person's feet 1 m apart, touch voltage is the voltage between a person's hand and feet 1 m from the equipment. IEEE 80 derives tolerable limits from the body current limit equation Ib = k / sqrt(ts) where k is 0.116 for a 50 kg body and 0.157 for a 70 kg body, and ts is the fault clearance time. The surface layer resistivity (crushed rock) and the derating factor Cs are applied to compute the final tolerable step and touch voltage. The calculation Pass / Fail compares calculated values to these limits. Refer to the ASME Section VIII reference for related plant electrical safety code interfaces.

The earthing calculation sizes and proves the earth grid: conductor cross section, grid resistance, GPR, step and touch voltage. The bonding schedule (also called the earthing schedule) is the field document listing every piece of equipment that must be bonded to the grid: each motor, vessel, structural steel column, tank shell, lightning protection downcomer, and cable tray run, with the bond cable size, bond point, and inspection record. The calculation is performed once per substation. The bonding schedule is a per-tag list used by the construction crew during installation and signed off during pre-commissioning walkdowns.

On a brownfield Indian PSU revamp, the existing earthing system is the baseline. Soil resistivity is re-measured during the revamp survey because soil conditions change with construction and excavation. New equipment additions are bonded to the existing grid, and the calculation is re-run with the new fault current (revamps typically increase fault current due to added transformer capacity). For PSU refineries the calculation references CEA Regulations 2010 and IS 3043 in addition to IEEE 80. The OISD-118 compliance checklist covers the related plant safety audit interface for refinery earthing.