- 2074 Bhadra · 4 marks
Purpose of earthing, earthed vs unearthed system
Short answer
The major purpose is to protect people by keeping enclosures near earth potential and to ensure fast fault clearance. In an earthed system, a fault causes a large current that trips protection, while an unearthed system allows supply to continue but causes healthy phases to rise to line voltage and may produce transient overvoltages.
What is the major purpose of earthing? Describe the consequences of earth and unearthed system.
Major purpose of earthing
Earthing connects the system neutral and the exposed metal parts of equipment to earth through a low-resistance path. Its main purposes are:
- Safety of persons: keeps metal bodies near earth potential so a fault does not cause a dangerous shock.
- Quick fault clearance: a large earth-fault current makes fuses, MCBs and relays trip quickly.
- Stable system voltage: the earthed neutral fixes phase-to-earth voltage and limits overvoltages.
- Safe discharge of lightning and static charges.
Consequences of earthed and unearthed systems
| Point | Earthed system | Unearthed system |
|---|---|---|
| Fault current on first earth fault | Large; protection trips | Very small (capacitive); nothing trips |
| Shock risk | Low; faulty part isolated fast | Healthy phases rise to line voltage; shock risk from second fault |
| Voltage of healthy phases | Stays near phase voltage | Rises to √3 × phase voltage |
| Overvoltages | Limited | Arcing ground gives transient overvoltages up to 5–6 times normal |
| Fault location | Easy (relay shows faulty feeder) | Difficult |
| Continuity of supply | Feeder trips on first fault | Supply can continue after one fault |
| Insulation needed | Phase voltage level | Line voltage level (costlier) |
Summary: an earthed system is safer and is used for almost all LV industrial and building installations. An unearthed system gives continuity during a single fault, but causes overvoltage, insulation stress, arcing grounds and hidden faults, so it is used only in special cases (some mines and process plants with insulation monitoring).
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- Earthing purpose and points to be earthed · Industrial Power Distribution and Illumination, chapter 2 · asked 2×
- Earth resistance factors and rod earthing diameter · Industrial Power Distribution and Illumination, chapter 2
- Equipment earthing objective and earth resistance factors · Industrial Power Distribution and Illumination, chapter 2 · asked 2×
- Reducing earth resistance and pipe earthing · Industrial Power Distribution and Illumination, chapter 2
- Earth resistance factors and lightning arrester earthing · Industrial Power Distribution and Illumination, chapter 2
- Low earth resistance and equipment earthing methods · Industrial Power Distribution and Illumination, chapter 2
- Chemical earthing and earth resistance · Switchgear and Protection, chapter 5
- Improper earthing and system grounding practice · High Voltage Engineering, chapter 2
- Short note: Earthing system · Basic Electrical and Electronics Engineering, chapter 4
- Major advantages of per unit system · Power System Analysis I, chapter 3
All Industrial Power Distribution and Illumination chapter 2 questions →