Mft zs and pfc

Discussion in 'Electricians' Talk' started by spen123, Aug 13, 2022.

  1. spen123

    spen123 Active Member

    I'm currently working with a few testers in a large site that are recording the pfc when doing the ze.
    I don't see why they are recording this as pfc was taught to me to be done at the origin of supply.
    Is there a reason they are recording it or is it them doing what they are told by the powers above.

    Its 3 phase if it makes a.difference.
    They also seem to be unplugging and plugging in the test Leeds when they get a high zs reading with the impression it will gain them a lower result.

    I'm not sure if it its just them not understanding testing
     
  2. Teki

    Teki Screwfix Select

    The PFC is used to determine the correct breaking capacity of overcurrent protective devices in an installation. It is usually measured at the origin of the supply however, if you have sub-distribution it may be recorded at those points. In an industrial installation, you may find overcurrent protective devices with a breaking capacity of >6000A (standard domestic breakers).
     
    The Happy Builder likes this.
  3. spen123

    spen123 Active Member

    I'm fully aware of where it should be taken and yes this is a sub supply however this should be at panel fed from mains.
    This panel then distributed to commando sockets
    My question is why record at the plug.
    I didn't write the question very well
    They are doing a ze at each socket.
    They are also recording the pfc
     
  4. Coloumb

    Coloumb Screwfix Select

    Presuming radials they should record the Zs at the further point on the ctt then it's a forgone for each commando. Sounds like they don't know how to test properly. Did you ask?
     
  5. Ind spark

    Ind spark Screwfix Select

    Where do they record the pfc on the cert?
    No box for it per circuit.
     
  6. spen123

    spen123 Active Member

    They are individual radial commando sockets for various supply's.
    I did ask but no real.ansqwr other than they are recording all results.
    They arnt proper sheets they are from the company for live testing to give to the qs so he can fill out all results properly on a computer.
    I did mention pfc should only be recorded at origin or sup origin and its to determine breaker breaking capacity but again no real answer
     
  7. Ind spark

    Ind spark Screwfix Select

    If it was me and I was being asked for it I'd just wrote it down, if your not responsible for the test sheets dont worry about it.
     
  8. chesterw

    chesterw Well-Known Member

    I don't understand this discussion -

    643.7.3.201 Prospective fault current
    The prospective short-circuit current and prospective earth fault current shall be measured, calculated or determined
    by another method, at the origin and at other relevant points in the installation.

    I've worked on sites where the power supplies are bigger than the surrounding towns, hence the pfc is also greater all over the site.
     
    The Happy Builder likes this.
  9. adgjl

    adgjl Active Member

    What is a “proper sheet”? BS7671 sets out the minimum requirements. If a company chooses to make additional measurements, why not? When the QS is doing their paperwork it is far better if they have more results than they need, rather than having to revisit site, to make extra measurements.
     
  10. Bob Rathbone

    Bob Rathbone Screwfix Select

    The PFC should be known at every point of the installation where overcurrent or earth leakage breakers are fitted. This is to ensure that the PFC rating of the device is equal to or greater than the PFC at the point of installation of the device.
     
    The Happy Builder and Teki like this.
  11. The Happy Builder

    The Happy Builder Screwfix Select

    If they are doing this at each distribution board then I would say they are perfectly correct in doing so.

    Rather than Ze I would be referring to Zdb as it is being measured at the distribution board rather than at the origin of supply and it being the Z external to the installation, the PSCC needs to be confirmed to check the circuit protective devices within that particular distribution board are adequate.
     
  12. spen123

    spen123 Active Member

    No its at each commando plug so zs rather than ze my mistake.
    Would be like taking a zs at each accessory you tested and also recording the pfc.
    Seems pointless
     
  13. Bazza-spark

    Bazza-spark Screwfix Select

    If they are radials then efli and pfc at each socket seems correct to me.
     
  14. The Happy Builder

    The Happy Builder Screwfix Select

    [​IMG]
    Megger MFT1741+ & ECVA210 - Express Instrument Hire (expresshire.net)

    First off, neither the Prospective Short Circuit Current (PSSC) nor the Prospective Fault Current (PFC) tests are not required at the end of a circuit such as at sockets. They are to find out how big the bang will be if things go wrong. Generally, the biggest current that will flow in a domestic installation during a fault will be between live and neutral where the DNO cables enter the building, not live and earth. This live to neutral fault current is typically around 1500 amps (1.5 kA) in a domestic installation but can be significantly more I have recorded over 6000 amps (6.0 kA) in somebodies' kitchen in a house next to a transformer. This test is carried out to ensure that the consumer unit or fuse board protective devices, MCBs or fuses can operate safely and not just disappear in a ball of flames if there is a short circuit possibly taking the whole fuse board or CU with it.

    Secondly, you have to understand how testers "do a PSCC test". In the picture above the megger has completed a Z (loop impedance) test and is displaying the test result, which here is shown as 0.55 ohms, it has then calculated the amount of current that would flow if there were a fault which in this example is 416 amps. From the tables in the Wiring Regulations and supplied by circuit protective device (fuse and MCB) manufacturers an electrician can tell if the protective device will work and within what timescale, for example the fuse will blow in less than 0.4 seconds (400 milliseconds).
    But and it is a big but, that meter probably isn't displaying the PSCC, it is displaying the PFC. The first rotary knob is set to do a Z test between L-PE (Live and Protective earth) the Prospective Fault Current (PFC), but the highest fault current is most likely to be between L-L (Live to Live in a two or three phase installation) or L-N (Live to Neutral in a single-phase installation), so to do the Z test L-PE between live and earth then you have to turn the knob to the second position L-L + L-N and repeat the test between phases or between live and neutral and record the highest test result, so you know what the biggest fault current will be.

    So, if the guys doing the testing are merely doing a Zs test at the sockets to confirm that the protective devices are going to work and within the required time, but then recording both test results displayed on the screen the "PSCC test result" displayed is meaningless as it is not the highest PSCC or PFC in the circuit or installation and does not need to be recorded.

    Other testers require you to alter the settings on the meter to test the PSCC and displays them separately, but the process is the same with the meter doing a Z test then calculating the PSCC, they don't risk firing thousands of amps through the installation, meter and test leads then try to measure it as it passes through.

    Maybe I should point out that anyone "bang testing" an electrical installation by turning things on to see if a fuse blows or a MCB trips is potentially firing thousands of amps through the installation to see what happens, typically one and a half thousand amps in a domestic installation, which is why it is not a very good idea, particularly as old rewireable fuse boards are often only rated at 1000 amps (1kA) and generally never more than 3000 amps (3kA).

    Generally, in a single phase the most current will flow between live and neutral of the DNO cable, but it is possible that if there is metal water and gas pipes or other earthed metalwork the earth path back to the DNO transformer could actually have a lower resistance (impedance) than the neutral of the supply cable, so you could end up with the L-PE fault current being higher than the L-N short circuit current. There tends to be some confusion between PFC and PSCC, in a TT earthed installation the fault current recorded L-PE may be as low as 2 amps, and I have seen electricians actually record this as the PSCC, but if they actually test the PSCC between L-N it will be a huge current in comparison.

    Just to add a twist, DNO guys do actually use the fault currents and think that recording Z is "a thing that electricians do".

    Ze is the loop impedance external to the installation and measured at the consumer unit, Zs is the total measured at the extremities of the installation, the ends of the circuit so the total of the impedance in the circuit plus the Ze.

    Edited to try and clarify PSSC versus PFC, it is easiest to just remember you want to know where the biggest bang will be and how big it will be if things go wrong.
     
    Last edited: Aug 15, 2022
  15. chesterw

    chesterw Well-Known Member

    zz
     
    Last edited: Aug 15, 2022
  16. MGW

    MGW Screwfix Select

    The prospective fault current, and the loop impedance where the voltage is fixed are the same thing really, simple ohms law is loop impedance is 1.44 Ω and the voltage is 230 then the PFC is 160 amp. Selected the figures as a type B MCB magnetic trip will trip at between 3 and 5 times the rated current, so we need for a 32 amp trip 5 x 32 = 160 amp to flow so the loop impedance needs to be less than 1.44 Ω today we allow a 5% volt drop, so 1.37 Ω or 168 amp. But really only needs one recording as the can be calculated from it.

    We measure the loop impedance or prospective short circuit current for three reasons.
    1) To ensure the protective device will not be overloaded.
    2) To ensure the protective device will operate within the allowed time.
    3) To work out the volt drop.
    Since the earth wires are not always the same resistance to neutral wires, we measure both, and should record the worst reading. Unless using a RCD, as with an RCD the line - earth loop impedance can raise to 200 Ω with some earthing systems, above that considered unstable.

    With a 100 amp supply to keep within the DNO volt drop limit the line - neutral loop impedance is around the 0.35 Ω mark, and with that the line - neutral impedance at the centre of a ring final would need to be 0.94 Ω maximum to be within the volt drop limit, but the line - earth can be 1.37 Ω so the easy way with a plug in tester is to record the line - neutral as current and line - earth as resistance, so testers automatic do this.

    The problem I found with a three core test lead was some like the Robin would auto change between earth and neutral for the two tests, but not all. Seeing test results which using ohms law and 230 volt always matched pointed to the tester not auto changing between the two.

    Also used one tester where they always matched at 240 volt. Clearly the tester does a calculation to give both readings, but is the current measured or the resistance? Which reading is correct?

    I do remember in my younger days measuring the readings and writing them down, and thinking do these readings pass? And thinking in the future could some one look at the readings and say this never complied when installed, come back and correct it free of charge please. Renewing a 100 meter run of 6 mm² for 10 mm² would be quite costly, so I needed to work out the limits on site, not after returning to an office, so first using VB then java script I wrote a problem to check volt drop etc.

    However by time one looks at the accuracy of the meters, measure the loop impedance three times and likely three different figures, the volt drop would need to be way out before you could show it had been exceeded.

    I can only remember one problem with volt drop, a shrink wrap machine, clearly volt drop as worked OK in one location but not in another, plus did measure. But with so many switch mode power supplies volt drop is now not the problem it once was.

    Have had it on a caravan site where transmitted mains hum at certain times of the day, but this was before you could get a switched mode power supply which would work on a transceiver, and the cure was easy, use a battery.
     
  17. The Happy Builder

    The Happy Builder Screwfix Select

     
  18. chesterw

    chesterw Well-Known Member

    #The Happy Builder - I deleted a post and replaced by "zz", its not a cryptic message about anything
     
  19. The Happy Builder

    The Happy Builder Screwfix Select

    :)
     
  20. chesterw

    chesterw Well-Known Member

    I don't understand
     

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