Short Circuit Currents And Circuit Breaker Selection

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Short circuit currents and Circuit breakers selection

TEPCO

Short Circuit Currents and Circuit Breaker Selection Part One: Short circuit and power system design Part Two: Short circuit study (calculation)

Part Three: Selection of circuit breakers

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Part One: Short circuit and power system design Goals of power system design  Ensure safety by Managing electrical hazards (electrocution, fire ,…)  Network protection against up normal condition (over load, short circuit, arcing faults,…) 

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Reduce cost by Reducing system down time  Optimum selection of system components 

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Part One: Short circuit and power system design 

Short Circuit Definition

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Cause of Short Circuit Insulation failure (mechanical stress, heat, fire ,…)  Wrong operation or connection 

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Part One: Short circuit and power system design 

Short circuit level affects Prevention of electric shock hazards  Amount of damage to cable insulation 



Circuit interruption success

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Part One: Short circuit and power system design 

110V

Prevention of electric shock hazards Line CB

Load

~

220V

PE

Neutral

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Part One: Short circuit and power system design 

Circuit interruption success

Never exceed circuit breaker interrupting capacity

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Part Two: Short circuit study 

Main Studies in Power System Load flow (Connected, demand loads, sizing cables and CB)  Short circuit 

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Short circuit limitation, development and types

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IEC Standards for short circuit calculation

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Performing short circuit study

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Part Two: Short circuit study 

Short circuit limitation Short circuit current is limited by supply and circuit impedance only 

I sc  

Vsupply Zsupply  Z circuit

Short circuit development 93kA 50kA

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Part Two: Short circuit study 

Short circuit types 

3-phase to ground fault



a b c n

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Single line to ground fault a b c n

Phase to phase fault a b c n

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Double line to ground fault a b c n

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Part Two: Short circuit study IEC Standards for short circuit calculation 

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IEC Organization 

Regional Committees (SASO in Saudi Arabia)



Technical Committees (Transformers, Cables, …)

IEC technical committee 73 publications 

IEC60909 Calculation of short circuit currents.

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IEC60865 Calculation of its effects

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Part Two: Short circuit study 

Performing short circuit study to IEC60909 

Network elements



Impact of induction motors on short circuit

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Input/Output of the study

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Software calculation (NetPlan)

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Performing short circuit study Network elements 

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MV network grid

500 MVA Isc   21kA 3 13 .8kV

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Impedance given in MVA

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For 13.8kV network impedance (available short circuit) 500MVA

Transformers 

 1600KVA  Isc    / 0.06  39kA  3  400 

Impedance given in per unit system (p.u.) 1p.u.100%

For 500KVA trafo , 0.04p.u.  4%  For 1600KVA & 1000KVA trafo , 0.06p.u.  6%  For 2000KVA trafo , 0.08p.u.  8% 

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Performing short circuit study Network elements  Cables & Busways

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Impedance given in milliohm

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Max short circuit at min temperature

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Min short circuit at max temperature

V A Isc   (    / A)   

Effect of supply voltage 

Voltage regulation affect short circuit level

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Max short circuit occur at max supply voltage (1.05Ur)

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Min short circuit occur at min supply voltage (0.95Ur)

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Performing short circuit study Network elements  Impact of induction motors on short circuit Motor impact depends on its size only

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Performing short circuit study 

Input/Output of Study 

Input: Network elements & connection between them

Output: Max/Min short circuit at each busbar & through each cable 

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Software calculation (Moeller NetPlan) Easy to use and learn  Short circuit calculation according to IEC60909  Editable network elements master database 

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Network element dimensioning according to IEC60364

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Performing short circuit study

Short Circuit on main busbar

Max Short Circuit on each bus

Performing short circuit study

Short Circuit on main busbar

Max Short Circuit on each bus

Part Three: Determination of breakers interrupting capacity 

Circuit interruption 

Arcs

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Circuit breaker operation

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Effect of short circuit current

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Circuit breaker types

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IEC Standards for circuit breakers

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Circuit interruption 

Arcs 

How much voltage appear on an Arc ?

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Arc voltage depend on two main factors

1) Arc length (20V/cm) 2) No. of segments in the arc (40V each) V(arc)=40+20x10=240V

V(arc)=2x40+20x10=280V

10cm

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Circuit interruption 

Circuit breaker operation CB

+ Varc -

L

+ -

Varc

+ VL -

Vs

dI Vs  Varc  L dt

Load

~

Vs

dI L  Vs  Varc dt

Iprocpective Ilimited

Current limiting is now common up to 630A

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Circuit interruption 

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Effect of short circuit current 

Repulsion force between contacts

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Very rapid temperature rise

Circuit breaker types NZM & PMC 1, 2, 3, 4 Ir = 20 ... 1600 A

MCB

MCCB

ACB (ICCB)

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IEC Standards for circuit breakers 

Circuit breaker for industrial areas IEC60947-3



Definition of breaking capacity

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Rated voltage 

Dramatic effect for current limiting circuit breaker

Non current limiting breakers also suffer from increased arc dissipation  Rated voltage is the phase to phase voltage 

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Conclusions 

Never exceed breaker interrupting capacity

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As a quick rule for interrupting capacity

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For 1000KVA select at least 36kA CB

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For 1600KVA select at least 50kA CB

Due to Motors

Interrupting capacity depend on system voltage and earthing

The End Thanks for listening TEPCO