Electrical Safety and Protection Mechanisms

Posted on Dec 6, 2024 in Technology

Direct and Indirect Contact

In public distribution networks employing a TT system, the neutral ground of the transformation center and all bodies are crucial. Direct contact occurs when people or animals come into contact with an active directory installation. Indirect contact occurs when people or animals touch the body of a receiver with an insulation fault.

Overcurrent

Overcurrent refers to a current exceeding the carrying capacity of a line. This can be caused by three factors:

  • Overloading: Due to high consumption or a defect in the insulation.
  • Shorts: Two active parties entering into connection through negligible resistance.
  • Lightning: High tension caused by lightning or a phase line defect.

Overcurrent Protection

Overcurrent protection utilizes cutting devices to detect and automatically interrupt excessive currents. Rapid reaction time is essential, especially for short circuits with high currents. Overloads involve lower currents. Commonly used cutting devices include circuit breakers and fuses.

Fuses

Fuses protect against overloads and short circuits.

Fuse Characteristics

Fuses are defined by three characteristics:

  • Rated or Gauge Current: The maximum current that can flow through the fuse.
  • Breaking Capacity: The maximum intensity the fuse can safely interrupt.
  • Time-Current Curves: Graphically represent the relationship between current and time.

Fuse Types

  • gG/gL Type: Distribution fuses for overload and short-circuit protection.
  • aM Type: Motor fuses exclusively for motor protection.
  • Cylindrical Fuses: Used in protective boxes, general protective boxes, and distribution boards for motors.
  • Diazed Fuses: Commonly known as “bottle” fuses, often used as safety fuses in buildings.
  • NH Fuses: Also known as “blade” fuses. Low-voltage fuses with high breaking capacity.

Circuit Breakers

Parts of a Circuit Breaker

  • Magnetic Trip: When the current exceeds the electromagnet’s threshold, it rapidly opens the contacts.
  • Thermal Trip: A heated foil distorts, causing the circuit to open.

Tripping Characteristics

  • Rated or Gauge Current: The maximum current the circuit breaker can withstand.
  • Breaking Capacity: The maximum current the circuit breaker can interrupt. Some breakers have lower breaking capacities than others.
  • Time-Current Curves: Define cut-off times based on current. Manufacturers provide these curves for devices with the same features, regardless of caliber. Circuit breaker selection depends on both size and the type of receiver or facility being protected.

Differential Switch

A differential switch protects against direct/indirect contact, installation faults, and fire hazards. In an earth fault, input and return currents differ, creating a force that opens the differential contacts. Grounding significantly enhances differential switch performance.

Tripping Class

A differential switch must cut power quickly to ensure safety. It should be associated with a circuit breaker with a nominal value equal to or greater than the protective device’s rating.

Selectivity

Chronometric or Time-Based Selectivity

Achieved by using an upstream protective device with a time delay. This allows downstream devices sufficient time to disconnect. Not suitable for small power plants.

Current-Based Selectivity

Achieved by using upstream protective devices with higher current ratings than downstream devices.

Surge Protection

  • Transient Surges: Short-duration surges caused by switching, network maneuvers, or lightning strikes. Can destroy equipment.
  • Permanent Surges: Caused by neutral rupture, leading to reduced lifespan, immediate destruction, or fire hazards.

Technical Characteristics of Surge Limiters

  • Rated Discharge Current: The peak value of discharge current.
  • Maximum Discharge Current: The peak current the limiter can handle.
  • Protection Voltage: The limiter’s terminal voltage value.
  • Rated Voltage: The reference voltage for limiter operation.
  • Residual Voltage: The voltage across the limiter during operation.
  • Maximum Permissible Voltage: The maximum continuous voltage applicable to the limiter’s terminals.

Grounding Installations

Grounding limits voltage to ground on metal frames, ensuring protection performance and eliminating risks from electrical equipment malfunctions.

Grounding Components

  • Electrodes: Buried items like rods, bare wires, plates, rings, or metallic armor.
  • Grounding Conductors and Terminals: Every installation requires a grounding terminal with sections of 16mm², 25mm², 35mm², or 50mm².