Prospective Short Circuit Test and Prospective Fault Current Test (PSC & PFC Test)

short circuit test

Prospective Short Circuit (PSC) and Prospective Fault Current (PFC) are both intended to calculate the highest current that will stream within a fault loop path during the occurrence of an electrical flaw as needed by rules.   

The Prospective Short Circuit Current(PSC) is the utmost current that could flow between Line and Neutral conductors on a single-phase supply or between Line conductors on a three-phase supply. A PSC test calculates the current that will flow in the event of a short circuit fault between the live conductors. That is, Line and Neutral on a single-phase installation or Line to Line/ Line to Neutral on a three-phase installation.  

Prospective Fault Current (PFC) is the common term used for the highest amount of current that will stream under fault conditions. The PFC will continuously be the highest at the source of the installation as the impedance/resistance is always the lowest there. So as a regulation, if it’s not too extreme at the mains it will be fine everywhere else. This is because it will decrease due to the increase in resistance as we move further away from the origin. (ohms law I = V/R). 

Because of the type of dissimilar supplies, you would assume to find a PSC value greater than a PFC value on both TT and TN-S systems, yet on a TNC-S system both the PFC and PSC readings should be same. 

What is done During PFC and PSC Tests? 

PSC is decided by the voltage and impedance of the supply system. It’s far of the order of some thousand amperes for a well-known domestic mains electrical set up, but can be as little as a few milliamperes in a separated extra-low voltage (SELV) device or as excessive as hundreds of heaps of amps in huge industrial strength systems 

PFC is conducted at the source of the installation, like the main switch or at other switchgear connected straight to the tail from the electricity distributor’s metering device. Where a calculation is made at a point in the installation other than the source, such as a piece of switchgear served by a distribution circuit, it would not be the highest value for the installation.  

Particular guardianship should be exercised during the testing summons, as flaw conditions are most severe at the origin of an installation, where this test is performed. The earthing conductor, main protective bonding conductors and circuit protective conductors should all be connected as for normal operation during these trials, because the presence of these and any other latitude ways to earth may reduce the impedance of the earth flaw loop and so increment the level of prospective fault current. 

PSC will be greater than the PFC. Prospective fault current and short circuit current of a circuit is automatically calculated when making a loop impedance test. The calculation uses a nominal circuit voltage, not the actual circuit voltage. 

Why PSC and PFC are Done? 

Pfc and psc test is necessary for choosing the correct protective device for the circuit because it’s going to carry the maximum fault current flowing in a circuit. Regulation 612.11 of BS 7671 requires that the prospective fault current under both short circuit and earth fault conditions be determined for every relevant point of the installation. 

How is PSC and PFC Performed? 

The minor supply voltage used in the calculation is automatically chose depending on the real circuit voltage. The instrument uses the following voltage values: 

Prospective Short Circuit Test Procedure 

  • PFC tester or the Prospective short circuit function of a multifunctional tester such as the Megger 1553 is chosen, and we make sure that the supply is ON, but the Main Switch is in OFF position. 
  • The test leads are joined on the incoming side of the Main Switch, one test lead on Line and another on the Neutral terminals of the Main Switch. 
  • TEST switch is pushed and a note of the value (kA) is made. 
  • For three phase installations each phase is tested separately and the measured reading (test between Line 1 and Neutral, then Line 2 and Neutral and last Line 3 and Neutral) is doubled. 
  • Some test meters need that the third (usually green) lead to be connected on the Neutral during this test. Please refer to the test meter manufacturer’s instruction. 

Prospective Fault Current Test Procedure 

  • We use PFC tester or select the PFC function of a multifunctional tester such as the Megger 1553, and it has to be made sure that the supply is ON, but the Main Switch is in OFF position. 
  • Next, we connect the test leads on the Line and Neutral terminals of the Main Switch, as well as on the Earth terminal. 
  • The TEST switch is pushed and we make note of the reading (kA).
    For three phase installations each phase is checked separately and the measured reading is doubled (L1 – N – CPC, L2 – N – CPC, L3 – N – CPC). 
  • Having obtained these values by the measurements described above, we will select the highest value and write it down on the Electrical Installation Certificate as the value of PFC. 
  • The value of PFC obtained is compared with the breaking capacity of all the protective devices within the installation. The breaking capacity of the protective devices should be greater than the value of PFC. 

Benefits of PSC and PFC Tests

  • They give accurate results as its live testing.
  • The testing is simple and not much calculations are needed. 
  • Increased safety for employees and third parties.
  • Decreased Insurance Premiums.
  • Asset Data management and tracking systems. 
  • Small repairs of equipment made on-site to reduce down time.