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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)

 

Parents

  • I’m sure that long standing members of this forum will raise a smile when I say I had a discussion about PSCC testing whilst having a cup of coffee earlier this year with the former forum member Perspicacious aka BOD.

    The consumer unit shown in the original post has a conditional short circuit rating of 16 kA, so it is highly unlikely that you find a problem.

    https://www.beama.org.uk/static/uploaded/992ea5f3-2044-46b2-9db965480b99d7c8.pdf

    PSCC testing is dangerous, as you are shorting out the incoming supply and the IET Guidance Note Three’s, both the brown and orange editions, advise that you should not rely on fused test leads for your safety and you should test downstream of a suitably rated protective device, and if there isn’t a suitable device in the consumer unit then you need to go and get one out of your van and temporarily install it to give you the additional protection you require.

  • in reply to Sparkingchip

    Thank you, that's a very helpful explanation.

    I agree that Guidance Note 3 emphasises carrying out prospective fault current measurements safely, and where direct measurement is necessary it should be taken downstream of a suitably rated protective device.

    My intention in the post was mainly to explain the principle that the recorded Ipf is the greater of the PEFC and PSCC, rather than to recommend carrying out direct PSCC testing in every domestic installation.

    I appreciate you highlighting the practical and safety considerations. That's a useful distinction for anyone reading this discussion.

  • in reply to Sparkingchip
    Sparkingchip said:
    PSCC testing is dangerous, as you are shorting out the incoming supply and the IET Guidance Note Three’s, both the brown and orange editions, advise that you should not rely on fused test leads for your safety and you should test downstream of a suitably rated protective device, and if there isn’t a suitable device in the consumer unit then you need to go and get one out of your van and temporarily install it to give you the additional protection you require.

    What about the DNO's 100 A (could be less) fuse?

  • in reply to Chris Pearson

    We have all done it, but is it really safe?

  • in reply to Sparkingchip

    Any of the live tests are potentially unsafe. I must say that I feel much happier measuring Zs at a socket-outlet than at a ceiling rose.

    I agree that there is no need to know PFC in an ordinary dwelling. 16 kA would only flow at 230 V if the impedance were less than about 14 mΩ which represents less than 10 m of 25 mm² copper, or 35 mm² aluminium (OSG Table I1), so might just be achived if the transformer is next door.

    The reason that I mention Ze is that I want to be assured that it is low enough to blow the fuse in the service head should something drastic happen.

    When I measure Ze, my MFT also gives me the supply voltage and calculates the PEFC. If the supply is TN-C-S, PSCC = PEFC, so job done.

  • in reply to Mohamad Alabdallah

    I feel that what's needed here is to say if the presentation is/will be updated to include this part, as part of the original purpose of

    "understanding why each measurement is taken is far more important than simply reading the number"

    to say why some measurements might not actually be done in some cases (e.g. domestic 100A..), Plus extra caveats at the various places in the slide deck.

    I'm 100% with you on ensuring the UKU (underpinning knowledge and understanding) provides the foundations, and while noting that blinkered rote actions can be bad.

  • 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.

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  • 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.

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