Calculation of short-circuit currentsLay Meng HortE-mail:
[email protected]: 012 405 877 ELECTRICITE DU CAMBODGE
INSTITUTE OF ELECTRICAL SCIENCE 2Intro!ction Lay Meng
HortContentsI. IntroductionII. Calculation of Isc by the impedance
methodIII. Calculation of Isc values in a radial network using
symmetrical compoment3Intro!ction Lay Meng HortI.
IntroductionSizing an electrical installation and the required
equipmentdetermining the characteristics of the equipment required
to withstand or break the fault current.
Intro!ction Lay Meng HortI. Introduction!he ma"imum
short#circuit current$ used to determine!he breaking capacity of
the circuit breakers!he making capacity of the circuit breakers!he
electrodynamic withstand capacity of the wiring system and
switchgear%Intro!ction Lay Meng HortI. Introduction!he minimum
short#circuit current$ essential when selecting the time#current
curve for circuit breakers and fuses$ in particular when&Cables
are long and'or the source impedance is relatively high.(rotection
of life depends on circuit breaker or fuse operation$ essentially
the case for !) and I! electrical systems.*Intro!ction Lay Meng
HortI. IntroductionCharacteristics of short-circuitsDuration
+self#e"tinguishing$ transient and steady#state,Origin-echanical
+break in a conductor$ accidental electrical contact between two
conductors via a foreign conducting body such as a tool or an
animal,Internal or atmospheric overvoltagesInsulation breakdown due
to heat$ humidity or a corrosive environment1. The main types of
short-circuits.Intro!ction Lay Meng HortI. Introduction/ocation
+inside or outside a machine or an electrical
switchboard,Short-circuits can be:(hase#to#earth +012 of
faults,(hase#to#phase +3%2 of faults,. !his type of fault often
degenerates into a three phase fault!hree#phase +only %2 of initial
faults,0Intro!ction Lay Meng HortI. Introduction4Intro!ction Lay
Meng HortI. IntroductionConsequences of short-circuitsAt the fault
location, the presence of electrical arcs, resulting inDamage to
insulationWelding of conductors5ire and danger to life6n the faulty
circuit7lectrodynamic forces$ resulting in# 8eformation of the
busbars# 8isconnection of cables31Intro!ction Lay Meng HortI.
IntroductionExcessive temperature rise due to an increase in Joule
losses, with the risk of damage to insulationOn other circuits in
the network or in near-! networks"oltage dips during the time
re#uired to clear the fault, ranging from a few milliseconds to a
few hundred millisecondsShutdown of a part of the network$ the
e"tent of that part depending on the design of the network and the
discrimination levels o9ered by the protection devices33Intro!ction
Lay Meng HortI. IntroductionD!namic instailit! and$or the loss of
machine s!nchronisationDisturances in control $ monitoring
circuits32Intro!ction Lay Meng HortI. IntroductionStandardised Isc
calculations!he Isc value is obtained by applying 6hm:s
law&33Intro!ction Lay Meng HortI. IntroductionEample: Calculate
Isc3 + (arameters of line should select from table
below,.3Intro!ction Lay Meng HortCodeN0.AreaNumberof wiresDiameer
R0!0"# $R%os +xSin) /kmCos mm2"0.mm #$%m #$%m 0.&5 0.&0
0.85 0.8010 10 7 4.05 '.24 0.4243.23 3.31 2.40 2.0%1( 1( 7 5.12
2.02 0.4102.1% 2.11 3.43 3.0*25 25 7 (.40 1.2& 0.'&(3.3%
3.33 3.33 3.2.40 40 7 8.0& 0.80& 0.'811.04 1.04 1.04 1.00('
(' 7 10.2 0.514 0.'((1.*1 1.*2 1.*3 1.*3100 100 1& 12.&
0.'25 0.'4&1.2 1. 1.* 1..125 125 1& 14.5 0.2(0 0.'421.3%
1.30 1.1 1.31(0 1(0 1& 1(.4 0.20' 0.''41.31 1.33 1.3% 1.3*200
200 1& 18.' 0.1(' 0.'271.2* 1.24 1.33 1.33250 250 1& 20.5
0.1'0 0.'201.22 1.2* 1.20 1.31'15 '15 1& 2'.0 0.104 0.'1'1.21
1.23 1.2% 1.2.400 400 1& 2(.0 0.082 0.'051.3. 1.23 1.23 1.2%450
450 1& 27.5 0.072 0.'011.3* 1.21 1.22 1.2500 500 1&
2&.0 0.0(5 0.2&81.3% 1.34 1.23 1.23E#uivalent impedance for
AA% &'E% ()*+,-3%Intro!ction Lay Meng HortII. !emple of
CambodiaT&a'( )ou
