Earthing and grounding

Earthing and grounding design per IEEE 80-2013

Earth-mat and ground grid design for AIS and GIS substations in uniform soil: tolerable voltages, conductor size, grid resistance, mesh and step voltages. The documents are a layout, typical details and a report.

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The earth mat with the mesh voltage of 578 V and the tolerable touch voltage of 685 V, per IEEE 80-2013

Capabilities

Calculations and documents

The grid is checked against the tolerable touch and step voltages of IEEE 80-2013. The results go to the layout, the report and the bill of materials.

Tolerable voltages and sizing

Surface derating factor per IEEE 80 §7.4 Eq. 27
Tolerable touch and step voltages per IEEE 80 §8.3: Eq. 29 and 32 (50 kg) or Eq. 30 and 33 (70 kg)
Conductor size per IEEE 80 §11.3.1 Eq. 37
Decrement factor and grid current per IEEE 80 §15.10 Eq. 79 and §15.1 Eq. 63–64

Grid results

Grid resistance per IEEE 80 §14.2 Eq. 52
Ground potential rise per IEEE 80 §16
Mesh voltage per IEEE 80 §16.5.1 Eq. 80–91, and step voltage per §16.5.2 Eq. 92–94
Mesh spacing selected by the design
Electrodes at the perimeter, surge arresters, instrument transformers, neutral points, lightning masts and shield-wire towers

Inputs

Soil resistivity entered or from Wenner measurements, with a season factor
Surface layer resistivity and thickness
Fault current and durations, current split factor and X/R
Burial depth, ambient temperature and electrode size

Documents

Layout in DXF and PDF
Typical details in DXF and PDF
Calculation report in PDF: inputs, calculation trail, results and quantities
Quantities in the bill of materials

Standards sets

Materials and criteria per standards set

The IEEE 80-2013 method applies on both sets. The materials, sizes, body weight and words follow the standards set of the project.

Item	IEC, CEA and CBIP	ANSI, IEEE and NESC
Words on the documents	IEC, CEA and CBIPEarthing, earth mat	ANSI, IEEE and NESCGrounding, ground grid
Verdict	IEC, CEA and CBIP50 kg body, 70 kg for information	ANSI, IEEE and NESC70 kg body, 50 kg for information
Conductor	IEC, CEA and CBIPMild steel rod, with a corrosion allowance	ANSI, IEEE and NESCAnnealed soft-drawn copper, 4/0 AWG to 1000 kcmil
Electrodes	IEC, CEA and CBIP40 mm × 3 m mild steel rods	ANSI, IEEE and NESC3/4 in × 10 ft copper-clad steel rods
Burial depth, new project	IEC, CEA and CBIP0.6 m	ANSI, IEEE and NESC18 in
Connections and typical details	IEC, CEA and CBIPWelded joints, riser, rod electrode and electrode pit	ANSI, IEEE and NESCExothermic connections, riser, rod electrode and test well
Sheets	IEC, CEA and CBIPA1, metric	ANSI, IEEE and NESCANSI D, feet and inches, from 1″ = 20′

Sample project

Results, 220/33 kV pooling substation

The mesh and step voltages are below their tolerable limits for a 50 kg body. Sample design per IEC, CEA and CBIP.

Mesh voltage (Em)IEEE 80 §16.5.1 Eq. 80

578 V

Tolerable touch voltage (50 kg)685 V

Step voltage (Es)IEEE 80 §16.5.2 Eq. 92

1451 V

Tolerable step voltage (50 kg)2246 V

Design soil resistivity (ρ): 150 Ω·m
Grid size: 155.0 m × 133.4 m
Conductors: 40 × 35, mesh spacing 3.97 m
Electrodes: 144, 40 mm × 3 m
Grid current (IG): 31.85 kA
Grid resistance (Rg): 0.476 Ω
Ground potential rise (GPR): 15.15 kV

Calculation trail

Equation, input values, result and clause per step

The calculation trail shows how the design reaches its results, in the order of the IEEE 80 design procedure.

1Soil resistivityIEEE 80 §13.4.2 (uniform soil)
$ρ = ρ_{0} \times k_{season}$
ρ0 100 Ω·m (entered), season factor 1.5ρ = 150 Ω·m
2Surface deratingIEEE 80 §7.4 Eq. 27
$C_{s} = 1 - \frac{0.09 ( 1 - ρ / ρ _{s} )}{2 h _{s} + 0.09}$
ρs 3000 Ω·m, hs 0.10 mCs = 0.705
3Tolerable touch and step voltages (50 kg)IEEE 80 §8.3 Eq. 32, Eq. 29
$E_{touch, 50} = (1000 + 1.5 C_{s} ρ_{s}) \frac{0.116}{t _{s}}, E_{step, 50} = (1000 + 6 C_{s} ρ_{s}) \frac{0.116}{t _{s}}$
ts 0.50 sEtouch = 684.6 V, Estep = 2246.3 V
4Conductor sizeIEEE 80 §11.3.1 Eq. 37
$A = \frac{I}{\frac{TCAP \cdot 1 0 ^{- 4}}{t _{c} α _{r} ρ _{r}} ln \frac{K _{0} + T _{m}}{K _{0} + T _{a}}}$
Steel 1020, I 50 kA (rating), 20.24 kA calculated, tc 1.00 s, Tm 500 °C, Ta 50 °C (ambient)A = 608.9 mm² → Ø 40 mm MS rod (+6 mm corrosion)
5Grid currentIEEE 80 §15.10 Eq. 79, §15.1 Eq. 63–64
$D_{f} = 1 + \frac{τ}{t _{f}} (1 - e^{- 2 t_{f} / τ}), τ = \frac{X}{ω R}, I_{G} = D_{f} S_{f} 3 I_{0}$
X/R for Df 20, tf 0.50 s, Sf 0.6, 3I0 50 kADf = 1.062, IG = 31.85 kA
6Grid resistance (Sverak)IEEE 80 §14.2 Eq. 52
$R_{g} = ρ [\frac{1}{L _{T}} + \frac{1}{20 A} (1 + \frac{1}{1 + h 20/ A})]$
A 20 677 m², LT 11 193 m, h 0.60 mRg = 0.476 Ω
7Ground potential riseIEEE 80 §16, design procedure, step 6
$GPR = I_{G} R_{g}$
IG 31.85 kA, Rg 0.476 ΩGPR = 15.15 kV The rise of earth potential is above the tolerable touch voltage, so the mesh and step check follows.
8Mesh voltageIEEE 80 §16.5.1 Eq. 80–91
$E_{m} = \frac{ρ K _{m} K _{i} I _{G}}{L _{M}}$
n 37.37, Km 0.224, Ki 6.174, LM 11 438 mEm = 577.6 V ≤ 684.6 V
9Step voltageIEEE 80 §16.5.2 Eq. 92–94
$E_{s} = \frac{ρ K _{s} K _{i} I _{G}}{L _{S}}$
Ks 0.415, Ki 6.174, LS 8438 mEs = 1450.6 V ≤ 2246.3 V
10Spacing searchDesign rule
$D = \frac{L _{x}}{n _{x} - 1}, n_{x} = ⌈ L_{x} / S ⌉ + 1$
start 10 m, step 0.5 m, minimum 3 mtrial S = 4.00 m → 40 × 35 conductors, D = 3.97 m, 144 electrodes

Drawings

Earth-mat layout and typical details

Documents of a sample project, generated by SubstationLayout and marked as samples. Not for construction.

The sample earth-mat layout — Earth-mat layoutSSL-E-EAR-001 · Rev 0 · 1 page · PDF · 53 KB
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The sample earthing typical details — Earthing typical detailsSSL-E-EAR-002 · Rev 0 · 1 page · PDF · 30 KB
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Document

Earthing calculation report

The report states the inputs and their sources, the calculation trail, the results against their limits and the quantities.

Earthing calculation reportSSL-C-EAR-001 · Rev 0 · 4 pages · PDF · 84 KB

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