Lecture Notes: Earthing and Bonding

Introduction

• New playlist introduced on earthing and bonding.
• Topics to cover:
  • What is earthing?
  • Importance of earthing.
  • Designing earth mats for substations/structures.
  • Designing earthing bonding schemes for AC and DC traction.

Objectives of Earthing Bonding

1. Safety of a Person:

   • Example: Washing machine connected to a transformer.
   • MCB (Miniature Circuit Breaker) tripping current is set at 10A.
   • Importance of direct earthing connection to prevent electrical shock.

2. Protection of Installation:

   • Earth provides a return path during faults, leading to breaker tripping.
   • Prevents overheating of installations.

3. Electromagnetic Compatibility (EMC):

   • Earthing bonding is essential for ensuring EMC.
   • Serves as the foundational principle for EMC.

Current Flow and Fault Conditions

• Current always returns to its source.
• If the phase wire touches the equipment frame:
  • Human body can become part of the circuit if someone touches the frame.
  • Body impedance considered (approx. 1000 ohms), resulting in current flow (e.g., 230mA).
  • Breaker does not trip, leading to a dangerous situation.

Example of Earthing Connection

• Connecting equipment frame to earth reduces impedance:
  • With a 1-ohm earth electrode, most current flows through this low resistance path.
  • Resulting current could be around 230A, causing the MCB to trip.

Definitions and Concepts

• Earth Electrode: Conductive part embedded in the earth for electrical contact.
• Local Earth: Area near the electrode; potential may not equal zero.
• Zone of Influence: Region affected by the local earth.
• Remote Earth: Region outside the zone of influence considered at zero voltage.

Body Impedance and Current Impact

• Body Impedance: Affects the current flow; includes internal and skin impedance.
  • Internal impedance depends on current path.
  • Skin impedance relies on touch voltage, duration, moisture, area of contact, pressure, and temperature.

Impedance in Different Scenarios

• At higher voltages (above 200V), skin breakdown occurs, reducing skin impedance impact.
• Example: At 100V, AC impedance = 990 ohms, DC impedance = 1100 ohms (dry conditions).

Current Impact Thresholds

1. Threshold of Perception:
   • Current flow noticeable by tingling sensation (1mA).
2. Threshold of Let Go:
   • Current flow allows removal of hand (1-6mA).
3. Threshold for Immobilization:
   • Current flow causes muscle contraction, preventing hand removal (9-25mA).
4. Threshold for Ventricular Fibrillation:
   • Dangerous currents leading to heart failure (60-100mA).

Current Response Curves

• AC and DC current thresholds vary based on time and magnitude.
• AC thresholds for perception and let go are time-dependent; after certain points, they become magnitude-dependent.

Conclusion

• Understanding the relationship between earthing, current flow, and body impedance is crucial for safety.
• Next lecture will cover touch and step potentials using the discussed values.

Thank you for listening!