IEC 60909 Maximum and Minimum Prospective Short Circuit Current

A further feature of the calculator is that you can obtain a maximum and minimum prospective short circuit current in accordance with IEC 60909: Short circuit currents in three-phase AC systems. To use the DGUV Box Test Algorithm it will be necessary to have both maximum and minimum figures and the voltage correction factors given below are as stated in DGUV I 5188E. The IEEE 1584 guide calculates an arc variation factor which will reduce the arcing current in order to take into account fluctuations in fault currents.

This is calculated as follows.

• The maximum short circuit current is found by applying a voltage factor c to the estimated prospective short circuit current above which was based upon conductor temperature of 20℃. The voltage factor is based upon a 6% tolerance giving a factor of cmax =1.05.
• The minimum short circuit current is found by calculating the estimated prospective short circuit current based upon fully loaded conductors which are operating at design temperature maximum of 90℃ and the applying a voltage factor cmin. The voltage factor is based upon a 6% tolerance giving a factor of cmin = 0.95.

To summarise, the following is a list of data entry and output fields from the calculator.

TRANSFORMER DETAILS SECTION

Transformer Size (kVA)
Select the transformer size from a dropdown list of distribution transformers from 112.5 kVA up to 3000 kVA. If there is no transformer and the source is at low voltage from a utility company, select “NA” and go directly to the cable details section.

Primary Voltage (kV)
Enter the primary voltage to the transformer up to a maximum of 20 kA.

Prospective Short Circuit Current (kA)
Enter a figure for the prospective short circuit current at the primary of the transformer.

Secondary Voltage (volts)
Enter the secondary three-phase line to line voltage at the transformer low voltage terminals between 208 and 600 volts.

Transformer Impedance (%)
Enter the % transformer impedance figure which can be found on the transformer nameplate.

Supply Source X/R Value
The calculator assumes a typical high voltage source X/R value of 15 but a specific X/R ratio can be entered.

CABLE DETAILS SECTION

System Voltage (volts) (No transformer)
If there is no transformer used, enter the low voltage three-phase line to line system voltage in volts.

Enter Source Prospective Short Circuit Current (kA) (No transformer)
If there is no transformer used, enter a figure for the prospective short circuit current for the low voltage source.

Supply Source X/R Value (No transformer)
If there is no transformer used, enter a source X/R value, otherwise a default value of X/R = 1 will be used.

Number of Cables
Enter the number of cables that will be used between the supply source and the equipment up to a maximum of three. A data entry block will be displayed for each cable. Cable one will always be the nearest to the supply source as in the single line schematic shown earlier.

Number of Cores
Enter the number of cores for each cable used.

Conductor Material
Select conductor material as aluminium or copper from the drop-down field.

Cable Size (mm2)
Choose a cross sectional area of each conductor from the drop-down list from between 16 mm2 up to 1000 mm2.

Cables in Parallel
Choose the number of cables installed in parallel from the drop-down list.

Cable Installation Method
Choose the cable installation method from the drop-down list.

Cable Length (m)
Enter the length of the cable in metres.

OUTPUTS

Prospective Short Circuit Current at LV Terminals (kA)
This is the symmetrical prospective short circuit current at the low voltage source, transformer low voltage terminals or low voltage utility mains.

X/R ratio at Equipment
This is the X/R ratio at the Equipment.

Prospective Short Circuit Current (kA)
This is the symmetrical prospective short circuit current at the equipment after calculating impedances of the supply source and all cables. This figure can be used to input directly into the IEEE 1584 incident energy calculator by selecting the “Use PFC in Incident Energy” option button.

IEC 60909 MAX PSCC at Equipment (kA)
As described in this chapter an output is given for the symmetrical maximum prospective short circuit current as calculated using IEC 60909 correction factors.

IEC 60909 MIN PSCC at Equipment (kA)
As described in this chapter an output is given for the symmetrical minimum prospective short circuit current as calculated using IEC 60909 correction factors.