Earthing & Bonding

Quick pick (TL;DR)

Choose the system earthing type (TN-S is the usual industrial default).
Size the earth conductor for the fault current & clearing time with S = √(I²t)/k.
For MV substations use a mesh/ring grid and verify touch & step voltages to IEEE 80.
Bond all exposed and extraneous metal; keep instrument/clean earth separate from power earth at a single point.

Step 1 — System earthing type

Type	Character	Use for
TN-S	Separate N & PE	Industrial LV
TN-C-S	PEN then split	Distribution
TT	Local electrode	Remote sites
Resistance-earthed	Limited EF current	MV process

Step 2 — Conductor sizing

Earth conductor, 4 kA / 0.5 s

Adiabatic formula S = √(I² · t) / k Given I = 4000 A , t = 0.5 s k = 143 (Cu, XLPE-covered) Step 1 — Compute S = √(4000² × 0.5) / 143 S = √(8.0 × 10⁶) / 143 S = 2828 / 143 = 19.8 mm² Step 2 — Round up → next standard size = 25 mm² main grid often 95 mm² Cu for robustness

Adiabatic gives the thermal minimum; grids use bigger sizes for strength & low resistance.

Step 3 — Grid & safety voltages

MV grid ≤ 1 Ω; LV ≤ 5–10 Ω.
Verify touch/step to IEEE 80.
Bond all metal; single-point clean earth.

Earth conductor calculator (adiabatic)

Minimum conductor cross-section

S = √(I²·t)/k. Thermal minimum — grids use larger for robustness.

Fault current (A)

Duration (s)

Conductor (k)

Thermal only — grids use 70–95 mm² Cu; add an IEEE 80 touch/step check.

Common mistakes

Sizing earth conductor by load.
No touch/step check on MV grids.
Multiple earths on instrument screens.

Requisition & datasheet

Purpose, type, fault, soil resistivity.
Target R, conductor, electrode, bonding.

Frequently asked questions

What should I know about Earthing & Bonding?

Choose the system earthing type (TN-S is the usual industrial default). Size the earth conductor for the fault current & clearing time with S = √(I²t)/k. For MV substations use a mesh/ring grid and verify touch & step voltages to IEEE 80. Bond all exposed and extraneous metal; keep instrument/clean earth separate from power earth at a single point.

Which standards or codes apply?

IEC 60364-5-54 (LV earthing & protective conductors), IEEE 80 (substation grounding, touch/step), BS 7430, IEC 62305 (lightning).

Standards — IEC 60364-5-54; IEEE 80; BS 7430; IEC 62305.