Short circuit

Causes for Short Circuit

Electric power systems are designed to be as fault-free as possible through careful system and equipment design, proper equipment installation and periodic equipment maintenance. However, even when these practices are used, faults do occur. Some of the causes of faults are as follows:

• Presence of animals in equipment

• Loose connections causing equipment overheating

• Voltage surges

• Deterioration of insulation due to age

• Voltage or mechanical stresses applied to the equipment

• Accumulation of moisture and contaminants
• The intrusion of metalic or conducting objects into the equipment such as grounding clamp,fish tape,tools,jackhammers or payloaders
• A large assortment of "Undetermined Causes"
  

Consequences of  Short Circuit

     When a short-circuit occurs in an electric power distribution system, several things can happen, such as the following:

• The short-circuit currents may be very high, introducing a significant amount of energy into the fault.
• At the fault location, arcing and burning can occur damaging adjacent equipment and also possibly resulting in an arc-flash burn hazard to personnel working on the equipment.
• Short-circuit current may flow from the various rotating machines in the electrical distribution system to the fault location.

Short circuit study calculates the maximum available short circuit current at various points throughout the system. Calculated values are then used to evaluate the application of protective devices, and to develop circuit breaker trip settings.

Purpose of Short-Circuit Calculations

• • Dimensioning of switching devices
  • Dynamic dimensioning of switchgear
  • Thermal rating of electrical devices (e.g. cables)
  • Protection coordination
  • Fault diagnostic
  • Input data for
    •  
      • Earthing studies
      • Interference calculations
      • EMC planning

Short-Circuit Calculation Standards

•  
  • IEC 60909:
    Short-Circuit Current Calculation in Three-Phase A.C. Systems
    • European Standard EN 60909
      •  
        • German National Standard DIN VDE 0102
        • further National Standards
    • Engineering Recommendation G74 (UK)              
      Procedure to Meet the Requirements of IEC 60909 for the Calculation of Short-Circuit Currents in Three-Phase AC Power Systems
  • ANSI IIEEE Std. C37.5 (US)
    IEEE Guide for Calculation of Fault Currents for Application of a.c. High Voltage Circuit Breakers Rated on a Total Current Basis.

Short-Circuit Calculations Standard IEC 60909

IEC 60909: Short-circuit currents in three-phase a.c. systems

Part 0:   Calculation of currents

Part 1:   Factors for the calculation of short-circuit currents

Part 2:   Electrical equipment; data for short-circuit current calculations 

Part 3:   Currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents flowing through earth

Part 4:   Examples for the calculation of short-circuit currents

Short-Circuit Calculations Scope of IEC 60909

•  
  • three-phase a.c. systems
  • low voltage and high voltage systems up to 500 kV
  • nominal frequency of 50 Hz and 60 Hz
  • balanced and unbalanced short circuits
    •  
      • three phase short circuits
      • two phase short circuits (with and without earth connection)
      • single phase line-to-earth short circuits in systems with solidly earthed or impedance earthed neutral
      • two separate simultaneous single-phase line-to-earth short circuits in a systems with isolated neutral or a resonance earthed neutral
        (IEC 60909-3)
  • maximum short circuit currents
  • minimum short circuit currents

Short-Circuit Calculations
Types of Short Circuits