Overcurrent Protection and Circuit Devices

Overcurrent Definition

  • Overcurrent: Condition where electric current exceeds intended levels through a conductor, causing excessive heat.

Causes of Overcurrent

  • Short circuits

  • Excessive load

  • Incorrect design

  • Arc faults

  • Ground faults

Overcurrent Protection Mechanisms

  • Common devices:

    • Fuses

    • Circuit Breakers (CBs)

    • Residual Current Devices (RCDs)

Fuses

  • Basic protection device that melts to break the circuit when current exceeds its rating.

  • Ratings include:

    • In: Nominal current rating (max current without disconnection).

    • Ia: Disconnection current rating (current causing disconnection in a set time).

    • Breaking Capacity (kA): Maximum current fuse can disconnect safely.

Issues with Older Fuses

  • BS 3036 rewirable fuses lack reliability and protection.

  • Incorrect wire ratings may lead to inadequate protection.

  • Must consider age and installation conditions affecting operation.

Modern Fuses (BS 88)

  • Fixed time current curves, high reliability.

  • High-breaking capacity for industrial applications (up to 80 kA).

  • Types include:

    • gG: General circuit applications

    • gM: Motor-rated circuits

Circuit Breakers (CBs)

  • Thermomagnetic devices for carrying and interrupting current.

  • Ratings include:

    • In: Nominal current rating.

    • Ia: Disconnection current.

    • Icn: Fault current causing potential damage.

    • Ics: Fault current threshold for maintain serviceability.

Types of Circuit Breakers

  • MCBs: Miniature Circuit Breakers; protect final circuits.

  • MCCBs: Moulded-case Circuit Breakers; for larger distribution circuits.

Operation Mechanism of CBs

  • Combination of thermal (overload) and magnetic (short circuit) tripping mechanisms.

  • Advantages include easy reset after faults, no need for replacement.

MCB Characteristics

  • Typically rated for short-circuit current: 6 kA to 10 kA.

  • Time-current curves illustrate activation response:

    • Faster acting than BS 88 fuses.

Types of MCBs

  • Type B: Trips at 3-5 times rated current (domestic use, minimal inrush).

  • Type C: Trips at 5-10 times rated current (commercial use, small motors, inrush).

  • Type D: Trips at 10-20 times rated current (industrial use, high inrush).