Understanding Earthing and Bonding Safety

Aug 19, 2024

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!