Question:
What is the difference between a PSC and a PFC test?
Answer:
First of all and to avoid confusion, both the PSC and PFC tests can be abbreviated by the following terms:
PSC
PSC – Prospective Short Circuit
PSCC – Prospective Short Circuit Current
PFC
PFC – Prospective Fault Current
PEFC – Prospective Earth Fault Current
Both PFC and PSC tests are designed to calculate the maximum current that will flow within a fault loop path during the event of an electrical fault as required by regulation 612.11 of BS7671:2008.
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.
A PFC test calculates the current that will flow in the event of an earth fault; i.e., Line to Earth.
It is important that we conduct the tests to make sure that the protective devices installed within a circuit are rated at the correct breaking capacity. Within a domestic installation, it is common to find 6000A (6kA) rated MCB’s installed within a circuit. If a domestic premises is situated particularly close to a supply sub-station and the measured external impedance (Ze) of the property was 0.03 Ohms, Ohm’s law calculates that 7,666A may flow in the event of a fault on a 230V supply. This may cause concern if the switch gear is only rated at 6000A.
The test result can be determined by calculation, ascertained by enquiry to the relevant electricity board, or measured using a Loop Tester. If you are using a Loop Tester, you would measure both PSC and PFC values and record the highest value. Due to the nature of different supply types, you would expect to find a PSC value higher than a PFC value on both TT and TN-S systems, however on a TNC-S system both the PFC and PSC value should be identical.
We supply a range of instruments that are capable of measuring both PFC and PSC values. These are commonly in the form of either a standalone loop tester, or a multifunction tester capable of measuring other electrical parameters. Most instruments today display the PFC/PSC value alongside the loop impedance value and as such, it is no longer necessary to conduct a separate PSC or PSC test.
Click here to see our range of multifunction testers, or here for our range of standalone loop testers.