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Prospective Short Circuit Currents
Tuesday, February 9, 2021
Today we’re exploring ways of determining prospective short circuit currents from a transformer to a distribution board, as we recently introduced over on our YouTube channel.
*Note: For detailed calculations; please consult the (cable) manufacturer’s data tables and/or use dedicated power engineering software.
So without further ado, let’s get started...
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. This usually includes the impedance of the supply source, usually a substation transformer or group of transformers.
Now to impede is to resist and effectively this is part of what we are looking at when examining prospective short circuit currents.
Some key points to begin with are that the energy distributors will usually advise the fault levels of the transformer in question. Now if the short circuit occurs close to the source of supply, the only limit to the current is the source impedance. A fault of zero impedance is often called a bolted fault and this is what can happen at the transformer.
Now the impedance of a transformer is usually stated as the % of the primary rated voltage
that is necessary to cause a full load current in the secondary if the load terminals are short circuited.
A common transformer impedance is 5% so if a 5% of supply voltage produces full load current, then, with a secondary short circuit and normal supply voltage of 100%, 20 x times the full load current will be present.
Factory site example
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. For example we have a factory with the following:
- It is being fed by a three phase, 400 volt, 500 kVA transformer
- This transformer has an impedance of 5%
- Transformer supplying the 400V busbars on the factory switchboard
Factory site example; full load current
So the transformer is 500 kVA, we convert to VA by multiplying by 1000 and then divide by the square root of three x the nominal voltage, 3 phase of 400 V.
Now, to calculate the Short Circuit Current, multiply the rated full load current by 100 and then divide by the actual percentage impedance of the transformer. Ta da!
So what does this all mean?
At any point in the circuit (apart from the actual point of supply), the current would be less than the value of 14,450 A, due to the impedance of the circuit between the source and fault.
The most severe condition would be a three phase fault at the supply terminals and calculations are usually based on this. The next fault is less severe, basically a fault between two phases that reduces the current to about 87%, so 12,570 A.
Finally there is the phase to neutral fault which should not go above 50% of the most severe fault, so 7,225 A.
Short circuit to earth on MEN
A short circuit to earth on a Multiple Earthed Neutral (MEN) system is the equivalent of a phase to neutral fault, and the current is further reduced by the impedance of the earthing system between the fault and the neutral link.
The available short circuit current above (14,450 A) doesn’t take into account impedance in the HV supply line and the transformer primary. As their contribution is so small, calculated values are always on the safe side.
Prospective short circuit current, the standards
When we look at AS/NZ 3000:2018 it 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 and
- Shall be capable of breaking any overcurrent up to and including the prospective short-circuit current
- At the point where the device is installed.
*In addition the device shall comply with the requirements of Clauses 2.5.3 and 2.5.4.
... And 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.
Protection is at the commencement of the circuit and this is usually the main switch board or at a distribution board.
The value of the current is called the Prospective Short Circuit Current (PSCC), and it must be interrupted before conductor temperature reaches its limit (refer to the AS/NZS 3008 Standard for your circumstances). This is where you must look at the current carrying capacity of the cables under consideration in addition to location, temperature ratings etc.
How to calculate PSCC
So PSCC must be calculated at every relevant point of the electrical installation—basically everywhere the protective devices are installed. Fault level estimation must commence at the source of supply and then you work your way down.
Firstly, we will need to gather some information such as the transformer rating in question and to do this you will need to seek out the relevant DNSP to obtain information stated as amperes per phase, or Million Volt-Amperes (MVA),
Let’s say we get a 10 MVA figure from the DNSP, the prospective current/phase is = 14,450 A.
Transformer, large consumer mains, MSB
Any equipment mounted on 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 mains so any protection for sub circuits or sub mains that originate at MSB must comply with these requirements and the fault level at, e.g. a DB fed by the sub mains, would be reduced due to impedance of the sub mains.
Let’s look at the 500 kVA transformer example, connected through a large consumer mains to a MSB, which feeds a DB through sub mains originating at the MSB:
Available PSCC at the supply source is determined from the distributor or via calculation. The fault level at the MSB is determined by using cable or busbar impedance and neglecting other sources to maintain a safety margin.
Fault level at any other position is reduced again due to impedance of sub mains or sub circuit fault level.
- In the case of a short circuit the only limit to the current is the impedance of the circuit.
- PSCC is determined at the start of the circuit, at the protection device.
- Calculations should always allow a safety margin.