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PFC Testing in Single-Phase Installations – PEFC, PSCC and Recording the Correct Ipf

Mohamad Alabdallah

One of the most common questions from electricians preparing for inspection and testing is:

"Why do we perform three different fault current tests when only one PFC value is finally recorded on the Electrical Installation Certificate?"

Although most multifunction testers can display fault current immediately, understanding why each measurement is taken is far more important than simply reading the number.

This article explains the reasoning behind each test and why comparing the results is essential for determining the correct Prospective Fault Current (PFC). The accompanying presentation illustrates each step with photographs and real test results.

What is Prospective Fault Current?

Prospective Fault Current (PFC or Ipf) is the maximum fault current that could flow at a particular point within an electrical installation.

In a single-phase installation, this value is obtained by comparing:

  • Prospective Earth Fault Current (PEFC)
  • Prospective Short-Circuit Current (PSCC)

The higher of these two values is recorded as the installation PFC.

This requirement is referenced in BS 7671 Appendix 14 and Regulation 643.7.3.201, which require the prospective fault current to be determined at the origin and other relevant points of the installation.

Why Perform Three Tests?

The presentation demonstrates the following sequence.

Test 1 – PEFC with the Main Earthing Conductor Disconnected

The first measurement is taken between Line and Earth after temporarily disconnecting the main earthing conductor.

This removes parallel earth paths, allowing the tester to verify the integrity of the main earthing conductor and determine the available earth fault current through the primary earth path only.

If the main earth were broken, the tester would identify an open circuit, and the test would not proceed.

Test 2 – PEFC with the Main Earthing Conductor Connected

The main earthing conductor is then reconnected and the test repeated.

This measurement now includes any parallel earth paths, such as bonded metal pipework or structural steelwork, which may reduce the overall earth loop impedance and increase the available earth fault current.

This represents the earth fault conditions that exist during normal operation.

Test 3 – PSCC Between Line and Neutral

Finally, a measurement is taken between Line and Neutral.

This determines the Prospective Short-Circuit Current (PSCC).

Depending on conductor sizes and circuit impedance, the line-to-neutral fault current may be greater than the earth fault current.

For this reason, both PEFC and PSCC must be compared before selecting the final PFC value.

Warning️ Practical Consideration (UK Practice)

Image courtesy of   (IET EngX discussion).

Although measuring PSCC forms part of the general method for determining the Prospective Fault Current (PFC), direct Line-to-Neutral testing at the origin is not always necessary or recommended.

For many typical 100 A domestic installations fitted with a BS EN 61439-3 consumer unit with a 16 kA conditional short-circuit rating, BS 7671 Appendix 14 and IET Guidance Note 3 recognise that the prospective fault current may be determined by enquiry, calculation, previous verified data, or other suitable methods, avoiding unnecessary live testing.

Where direct measurement is required, it should be carried out downstream of a suitably rated protective device, not directly on an unprotected supply.

Image courtesy of   (IET EngX discussion).

Key Learning:

Understanding when a test is required is just as important as knowing how to perform it safely.

Recording the Correct Value

In the practical example shown in the presentation:

The correct value entered on the Electrical Installation Certificate is therefore:

PFC (Ipf) = 2.095 kA

The Ze value is taken from the earth fault loop impedance measurement, which in this example is 0.12 Ω.

Where Are These Values Recorded on the EIC/EICR?

Safety Reminder

These are live tests.

Safe isolation procedures must be completed before disconnecting the main earthing conductor.

Before restoring the installation:

  • reconnect the main earthing conductor,
  • confirm the earthing terminal is secure,
  • never re-energise the installation with the main earth disconnected.

The presentation highlights these precautions using practical site photographs to reinforce good inspection and testing practice.

Final Thoughts

Understanding why each test is performed is more valuable than simply memorising the testing sequence.

By understanding the purpose of PEFC, PSCC and the comparison process, electricians can confidently determine the correct Prospective Fault Current and accurately complete the Electrical Installation Certificate while complying with BS 7671.

Discussion

How do you normally explain PFC testing to apprentices or electricians who are new to Inspection & Testing? Have you found that understanding the reasoning behind the three measurements improves confidence compared with simply following the tester's instructions?

Reference:

BS 7671:2022+A2:2026 – Appendix 14 (Informative): Determination of Prospective Fault Current

Regulation 643.7.3.201

IET Guidance Note 3 – Inspection & Testing, Section 2.6.16 (Prospective Fault Current)

 

  • in reply to Philip Oakley

    The principal is the same, a very basic polarity test will confirm if an earth is connected or not, as it says in IET GN3 regards testing the earth terminal provided by the DNO.

    However, at the main intake you have to confirm it’s not a parallel earth path and the DNO earth terminal is actually connected to the DNO network.

  • Something to consider.

    What should be remembered is that in a TN earthing system you can confirm the the earth terminal the DNO have supplied for use by their customer is connected to the PEN conductor in a TN-C-S system or the network earth conductor in a TNS system, but when you test Ze on either TN system you don’t actually confirm that the DNO network LV earthing system is connected, so you have to trust that the DNO are keeping the network Low Voltage earthing system in good condition.

    Even if the DNO network earthing is in good condition, the resistance on the DNO earthing system tying the PEN/earth conductor down to true earth could be up 20 ohms, despite the electrician measuring less than 0.35 ohms on the provided earth terminal.

    commercial.nationalgrid.co.uk/.../361558

  • in reply to Sparkingchip

    Being pedantic, a voltage test doesn't need a well connected non-live side. A proper lamp (no neons thank you;-), taking proper current, maybe..

    Too many skinned cats out there!

  • in reply to Philip Oakley

    The electrician provided an installation certificate after the event stating he had confirmed polarity was correct and there was a satisfactory Zs earth loop impedance, which was impossible to do as there was not an earth connection.

    On its own a polarity test with a test lamp would have been enough to determine that there was not an earth connection.


    “The board found that Mr Burton failed to adequately test the installation to confirm that the work was electrically safe to use and issued a false and misleading Certificate of Compliance.”

    www.1news.co.nz/.../

  • in reply to Philip Oakley

    Thank you, Philip.

    I really appreciate your constructive feedback, and I'd also like to thank  Sparkingchip for raising the practical aspects of PFC testing.

    My goal with this presentation is to help learners understand the principles behind the measurements, not just how to perform them. Your comments have highlighted some important practical and safety considerations that deserve to be included.

    I'll update the presentation to cover when certain measurements may not be necessary in typical 100 A domestic installations, together with the relevant guidance and safety recommendations.

    Thank you both for taking the time to share your knowledge. It's greatly appreciated.

  • Hi Mohamad. In the diagrams for tests 1&2, you have blue probes connected to the neutral terminals. Should they be brown probes connected to the live terminals?

  • in reply to OlympusMons

    Thank you for noticing that.

    You're absolutely right. It was an unintended editing mistake after I revised the article a couple of times.

    I've now corrected the diagrams. Thank you very much for your careful review and for helping improve the article.

  • Why Perform Three Tests?

    For PFC I'm not sure you do need three tests. Unless you can imagine a situation where connecting a parallel path will increase the loop impedance (reduce current flow), the PEFC test with the earthing conductor disconnected cannot yield a worst (higher) PFC figure than with it connected. So from a PFC point of view the 1st test is redundant.

    We do have to perform a test with the earthing conductor disconnected, but that's to look for worst case (highest) Ze, not worst case PFC. On many meters loop impedance is a different setting to PFC.

    Ze and PFC are related, but worst cases of each are not under the same conditions. Hence the otherwise apparently contradictory conventional design limits of 16kA and 0.35Ω - at 230V 16kA would imply a loop impedance of around 0.015Ω while 0.35Ω implies a fault current of less than 658A.

      - Andy.


  • When you test at the installation lives, neutral and PME earthing terminal the underground DNO earthing conductor and rods or mat are not part of the circuit you are testing, so you have absolutely no idea if it is connected or not.

    The groundworkers could have ripped through buried earth conductor trenching for a pipe or it could have failed for some other reason, so although you may have obtained a good Ze test result the whole earthing system could be floating and not connected to true earth.

    Indeed, if you decided to convert the installation earthing system to TT and installed a rod, then used your installation tester to do a Ze loop test to test the rod, you may not get any readings at all if the DNO earth has failed.

    You are trusting the DNO to ensure that their installation and earthing is correctly installed and maintained.

  • in reply to Sparkingchip

    The same applies when testing Ze with a TNS earthing arrangement, the DNO transformer earth connection to true earth is not in circuit and can have a resistance to true earth of 20 ohms.

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