hi I'm Veli from greenwood solutions today's presentation is on prospective
short circuit currents and at the end of this video you'll understand what they are
and how to calculate them from a transformer all the way down to a distribution board
but before we get stuck into it hit that subscription button let's get going
now in the case of a short circuit, the only limit to the current is the impedance
of the circuit and the available short circuit energy and this usually includes the impedance
of the supply source which in most cases is a substation transformer or a group of transformers
now the energy distributor the dnsp will usually advise the fault levels for the particular
transformer in question if the short circuit occurs close to the source's supply the only
limit to the current is source impedance a fault of zero impedance is often called a bolted fault
the impedance of the transformer is usually stated as a percentage of the primary rated voltage
that is necessary to cause full load current in the secondary if load terminals are
short-circuited so we're talking fault current situations and this is determined basically from
the transformer or group of transformers all the way down to the final circuit
we'll be talking from the transformer to the main switchboard that is feeding a distribution board
via submarines and effectively the impedance of the cable and the connections as you go down
into the system becomes higher and higher and therefore the fault current starts to decrease
the fault current calculations right at the transformer are based on certain assumptions
a common transformer impedance is five percent if five percent of supply voltage produces full
load current then with a secondary short circuit and normal supply voltage of a hundred percent
twenty times the full load current will be present so full load equals 100 over 5. the increasing
current demands of modern installations results in an increase in the capability of the supply source
to provide high values of short circuit current so let's assume a three-phase 400 volt 500 kva
transformer has an impedance of 5 supplying the 400 volt bus bars on a factory switchboard
the rated full load current is the kva of the transformer times a thousand
divided by in brackets square root of three times the rated voltage so we've got 500 which is the
kva of the transformer times a thousand divided by 1.73 times 400 and this gives you a figure
of 722.5 amps as the rated full load current
the available shortcut is 722.5 times 100 divided by the impedance percentage so 722.5 times 100
divided by 5 equals 14 450 amps and this is the available short circuit current look it really is
quite detailed stuff you have to get your head around impedance you have to get your
head around perspective short circuit currents and you have to get your head around some maths
in regards to the calculation as you come from the start being the transformer all the
way down to that last circuit under analysis so what does this all mean
so at any point in the circuit apart from the actual point of supply current would be less
than the value of 14 450 amps due to the impedance of the circuit between the source and the fault
now the most severe condition would be a three-phase fault at the supply terminals
and the calculations are usually based on this a fault between two phases would reduce current
to about 87 so you're looking at 12 570 amps a phase to neutral fault should not go above 50
of the most severe fault so in this case you're looking at 7225 amps
the short circuit to earth on an men system is the equivalent of a phase to neutral fault now
the current is further reduced by impedance of the earthing system between the fault
and the neutral link the 14 450 amps doesn't take into account impedance in the hv supply line and
the transformer primary their contribution is so small so the calculated values are
always on the safe side look at any point in the circuit apart from the point of supply
that perspective short second cut is going to be less obviously because the impedance of
the components involved now the calculations are always done with a safe a large safety
margin in mind as they should be so in a lot of cases the impedance from very small aspects of
the installation are not included in the in the calculations so again the transformers impedance
then you've got the consumer mains you look at the impedance of the cable there getting the
msb and then you work your way down all in this case all the way down to the distribution board
we have a look at is nz 3000 2018 version which states in clause 2.5.2 devices for protection
against both overload and short circuit currents that protective devices providing protection
against both overload and short circuit shall be capable of breaking any over current up to and
including the prospective short circuit current at the point where the device is installed and this
is really important in addition the device shall comply with the requirements of clauses 2.5.3
and 2.5.4 circuit breakers are not all the same the ability to break
a fault current or a prospective short circuit current varies depending on the
circuit breaker whether it's an mcb or a molded case circuit breaker so again what does this mean
the interrupting capacity must be adequate to enable the interruption of the highest value of
the current available at the point of installation of the protection this is a really important point
and the protection is at the commencement of the circuit
this is usually the main switchboard or at a distribution board the value of the current
is called the pscc and it must be interrupted before the conducted temperature reaches its limit
the pscc must be calculated at every relevant point of the electrical installation basically
where the protective devices are installed fault level estimation must commence at the
source of supply really important so you have to find out the transformer rating in question
the energy distributor will provide information stated as amperes per phase
or an mva figure million volt amps if supplied with a million volt amp figure
you have to do a calculation so let's say you get the figure of 10 mva from the distributor
the perspective current for phase is 10 times 10 to the power 6 which is a million
divided by in brackets the square root of 3 times 400 equals 14 450 amps
any equipment mounted on the msb must have full short circuit capacity at that point but the pscc
and therefore the fault current will be less if the switchboard is fed by the consumer main
protection for sub-circuits or sub-mains that originate at the msb
must comply with these requirements but the fault level at for example a distribution
board fed by the submarines would be reduced due to the impedance of the sub mains itself
let's look at the 500 kva transformer example connected through a large consumer mains to an msb
which feeds a db through a sub mains originating at that msb we know that the pscc for the 500k
transformer is 14 450 amp and we know that we've done the counts so the impedance per phase of the
supply z1 is 230 volt divided by 14450 and that gives us an impedance of 0.0159 ohms
assume that the impedance per phase of the consumer mains is 0.0037 ohms
and the impedance per phase of the sub main z3 is 0.026 ohms
so the fault level at the msb is 230 volts divided by in brackets the impedance of z1 0.0159 plus
the impedance at z2 0.0037 and that equals 11 735 amps per phase
now the fault level at the distribution board is again the same kind of calculation but we add
another impedance so it's 230 volts divided by 0.0159 ohms plus 0.0037 ohms plus 0.026
ohms and that gives you a figure of 5044 amps per phase as a prospective short circuit current with
prospective short circuit currents it's really important to get the data in regards to the
impedance of the transformer under consideration and then you're looking at the complete
impedance of the consumer mains and the impedance of the sub main so effectively when calculating
what fault currents are at different points of the installation it's a process of starting from
the top being the transformer or transformers and working your way all the way down in the in
this case we're talking to the all the way to the distribution board so at every step you're doing
the calculations where the protective device is actually installed now here's a diagram so
effectively step one establish that current perspective short set current at the source
step two if we establish the perspective short circuit current based on the impedance
of the consumer mines step three
determine the prospective short circuit current at the distribution board
take into consideration the impedance of the supply the consumer mains and the submarines
available pscc supply source is determined from information from distributor or calculation
two the fault level at the msb is determined by using the cable or bus bar impedance and neglect
other sources for a safety margin three fault level at any other position is reduced again
due to the impedance of the sub mains or the sub-circuit fault level
conclusion in the case of a short circuit the only limit to the current is the impedance
of the circuit the pscc is determined at the start of the circuit at the protection
device all calculations allow a safety margin thanks so much for watching our presentation
on prospective short circuit currents if you have any questions any enquiries any answers
please feel free to contact us and before you go hit that subscription button see you later you