Improper earthing and system grounding practice

Why improper earthing leads to high electric potential

When an earth fault occurs, the fault current $I_f$ flows through the earth electrode of resistance $R_g$. The electrode and all connected metal parts rise to the ground potential rise (GPR):

$$V = I_f R_g$$

• If the earthing is poor (high $R_g$ due to dry soil, corroded or loose connections, too few electrodes), $V$ becomes large. For example, $I_f = 1000$ A with $R_g = 10\ \Omega$ gives $V = 10$ kV on the equipment frame, while with $R_g = 1\ \Omega$ it is only 1 kV.
• High $R_g$ also limits the fault current, so fuses or relays may not trip, and the frame stays live for a long time.
• If the frame is not earthed at all, a body touching it becomes the only path to earth and gets the full phase voltage.
• Poor earth-mat design gives steep surface voltage gradients, so step and touch potentials exceed safe values.
• In ungrounded systems an earth fault raises healthy-phase voltage to line value, and arcing grounds can produce 5–6 pu overvoltages.
• Lightning current through a high tower footing resistance causes high tower-top voltage and back-flashover.

So improper earthing creates dangerous potentials on equipment and ground, threatens people and damages insulation.

System grounding in common practice

1. Generators: neutral earthed through a high resistance or a distribution transformer with a secondary resistor, to limit stator earth-fault current to a few amperes.
2. Transmission (66 kV and above): solid (effective) earthing of transformer neutrals, so that healthy-phase voltage stays below 80% of line voltage ($X_0/X_1 \le 3$, $R_0/X_1 \le 1$). This allows reduced insulation and surge arrester ratings. NEA's 132/220/400 kV systems are effectively earthed.
3. Distribution (11/33 kV): solid earthing, or resistance/reactance earthing to limit fault current; resonant (Petersen coil) earthing is used in some countries for overhead networks.
4. LV (400/230 V): neutral solidly earthed at the transformer (TN or TT systems); consumer installations have equipment earthing and RCDs.
5. Delta systems: a zig-zag earthing transformer provides an artificial neutral.
6. Substations: an earth mat (grid) of copper or GI conductors buried 0.5–1 m deep, with earth rods, bonding all equipment frames, fences and structures, covered with crushed gravel; designed per IEEE 80 so that step and touch voltages are safe.
7. Transmission lines: overhead earth wires bonded to each tower; tower footing resistance kept low (below about 10–20 Ω) with rods or counterpoises.