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Overcurrent protection

Overcurrent protection

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When the first small power systems were set up, the need to add automatic protection was soon realised. Equipment responsive to excess current (in the first place by fuses) was the obvious solution to the difficulties which had arisen and still today, by far the majority of all circuits are protected by this means. Selective action was soon needed and the graded overcurrent system has evolved to give discriminative fault protection. Overcurrent protection should not be confused with overload protection which is related to the thermal capability of plant or circuits, whereas overcurrent protection is primarily provided for the correct clearance of faults. Very often, however, settings are adopted which make some compromise in order to cover both of these objectives. Overcurrent protection is achieved by the use of fuses, by direct-acting trip mechanisms on circuit breakers or by relays.

Chapter Contents:

  • 8.1 Introduction
  • 8.2 Types of overcurrent system
  • 8.2.1 Overcurrent and earth-fault protection systems
  • 8.2.2 Grading of current settings
  • 8.2.3 Grading of time settings: the definite-time system
  • 8.2.4 Grading by both time and current: inverse-time overcurrent systems
  • Fuses
  • Delayed action trip coils
  • Fuse-shunted trip coils
  • Inverse-time overcurrent relays
  • 8.3 Selection of settings
  • 8.3.1 System analysis
  • 8.3.2 Grading of relay settings
  • Grading for definite-time relays
  • Grading for inverse-time relays
  • Grading with 'very inverse' relays
  • Graphical method of grading
  • 8.3.3 Current transformer requirements
  • Burdens
  • Variation of burden impedance
  • Additional burden
  • Significance of leads
  • Burden of earth-fault schemes
  • Effective setting
  • Time-grading of earth-fault relays
  • Phase-fault stability
  • 8.3.4 Sensitive earth-fault protection
  • 8.3.5 High-set instantaneous overcurrent relays
  • 8.3.6 Relay co-ordination with fuses
  • 8.4. Directional control
  • 8.4.1 Directional relays
  • 8.4.2 Connections for directional phase-fault relays
  • 30° relay connection: m.t.a. = 0°
  • 60° relay connection: m.t.a. = 0°
  • 90° relay connection
  • 8.4.3 Directional earth-fault relays
  • Polarisation by residual voltage
  • Polarisation by neutral current
  • Dual polarisation
  • 8.4.4 Grading of ring mains
  • 8.4.5 Multiple-fed ring mains
  • 8.4.6 Parallel feeders
  • 8.5 Bibliography

Inspec keywords: overcurrent protection; relays; power system protection; electric fuses; circuit breakers

Other keywords: direct-acting trip; automatic protection; fuse; overcurrent protection; circuit breakers; overload protection; small power systems; discriminative fault protection; relays; thermal capability

Subjects: Power system protection

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