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Details of BS7671:2018 Amendment 1 are here.

Alan Capon
Details of Amendment 1 of BS7671:2018 is available here: https://electrical.theiet.org/bs-7671/updates/


Regards,


Alan.
  • 0

    AJJewsbury:




    Paul Judd:

    Is the formula for calculating the maximum neutral current (Im) in Annex A722; A722.1 Neutral current of a three-phase installation, actually correct?


    There's an L missing (IL3) and only half of the final multiplication is shown. 


    Probably!

     




    Good question - it's also nothing like the formula in BS 7671:2018 yet doesn't have any change bars...


      - Andy.


     




    The correct expression is:

    bea1b3f366016bc756977985ec71ea38-huge-a722.1.jpg


    Mathematically, it is identical to the expression in BS 7671:2018 ... if you consider that 0.866 = SQRT(3)/2 and do a bit of algebra.


    The replacement is more likely to have been encountered by those who have come across three-phase calculations in text books.

  • 0

    AJJewsbury:




    I note and concur with the 25V maximum exposed voltage instead of 50 instead of 70



    The thinking behind the 25V was that that earth leakage currents can raise a TT earthing system significantly above true Earth potential - so in theory an ordinary TT system could be hovering at anything up to 50V from Earth without the RCD tripping. An adjacent charge point might possibly also be at 50V - but there's no guarantee that the leakage currents are in phase - so you could have 50V x √3 between two adjacent TT systems on different phases - or even 50V x 2 = 100V if they're supplied from different sides of a split-phase supply. Ensuring the RCD trips above 25V was just intended to ensure the touch voltage between any two adjacent point couldn't exceed 50V rather than looking for a lower limit as such.


    Of course with a typical 30mA RCD and a 200 Ohms electrode we'd be looking at about 6V per system or 12V max between adjacent points - so in practice it's a lot safer even than that.


      - Andy.

     




    Interesting discussion.


    The 70 V rms is derived, as discussed in earlier post, from IEC TR 60479-5, considering two hands to feet in wet conditions.


    In terms of the effects of TT systems, we don't need to consider simultaneous contact with exposed-conductive-parts of other earthing systems because:

    (a) The TT should have adequate separation below ground from the PME earthing system, and


    (b) Regulation 411.3.1.1 requires simultaneously accessible exposed-conductive-parts to be connnected to the same earthing system.


    In any event, disconnection times in TN systems can be up to 5 s to comply with Chapter 41 (Reg 411.3.2.3), and the touch voltage there may well be over 70 V! Consider further that this could be the disconnection time required at larger charging equipment. Only the charging point (i.e. vehicle connector or socket-outlet) requires RCD protection unless it's required for other reasons in the General Rules (Parts 4 & 5) ! So, is 70 V really a concern?

  • 0

    Somewhere on the planet it is daytime, somewhere it is night time, somewhere it is raining, somewhere it is snowing, somewhere it is sunny, somewhere it is winter, somewhere it is summer.




    Made me think of the Seekers - Morning Town Ride (starting at the bit I time-linked to ... the following verse at 1:24 starts "Somewhere it is night time, somewhere it is day ...").


    Sad, I know ... something my Mum used to sing to me when I was a toddler, and we sang to our kids when they were toddlers ?


  • 0

    gkenyon:




    Somewhere on the planet it is daytime, somewhere it is night time, somewhere it is raining, somewhere it is snowing, somewhere it is sunny, somewhere it is winter, somewhere it is summer.




    Made me think of the Seekers - Morning Town Ride (starting at the bit I time-linked to ... the following verse at 1:24 starts "Somewhere it is night time, somewhere it is day ...").


    Sad, I know ... something my Mum used to sing to me when I was a toddler, and we sang to our kids when they were toddlers ?




    Splendid - not even the New Seekers!


    Back on topic, all the above confirms my belief that the situation has become, or is being made too complicated.


    And what of 3-phase? Let's say that you have 3-phase at home and do your best to balance the phases. Whilst the car is charging, the phases will be sufficiently balanced to keep the neutral voltage down; but when you plug in or disconnect, the same does not necessarily hold true.

  • 0

    Chris Pearson:




    gkenyon:




    Somewhere on the planet it is daytime, somewhere it is night time, somewhere it is raining, somewhere it is snowing, somewhere it is sunny, somewhere it is winter, somewhere it is summer.




    Made me think of the Seekers - Morning Town Ride (starting at the bit I time-linked to ... the following verse at 1:24 starts "Somewhere it is night time, somewhere it is day ...").


    Sad, I know ... something my Mum used to sing to me when I was a toddler, and we sang to our kids when they were toddlers ?




    Splendid - not even the New Seekers!


    Back on topic, all the above confirms my belief that the situation has become, or is being made too complicated.


    EV charging installations were never that easy, but no fault of BS 7671 per say




    And what of 3-phase? Let's say that you have 3-phase at home and do your best to balance the phases. Whilst the car is charging, the phases will be sufficiently balanced to keep the neutral voltage down; but when you plug in or disconnect, the same does not necessarily hold true.


     




    In that case, 722.411.4.1 (i) clearly does not hold apply.


    For larger installations, it may, however, be practicable. There will be additional guidance, and a rule of thumb, to help make that decision, in the 4th Edition of the IET CoP.

  • 0
    Back on the 70V

    With a 5 second breaking time you need to be holding something attached to the circuit at the instant the fault occurs, or at least within 4 and a half seconds of it, or the overvoltage dissappears, or is not there long enough to kill you.

    The 70V might just be there all day until the fault is cleared, as the equipment will run earth return instead of neutral, the chances of touching it are much higher.


    It may a 'safe' voltage for continuous exposure,  based on hands to feet in the test conditions, but the contact area is small, and I'm not sure it is the right model for a car in a front garden where children may play - I detect a desire to justify the highest possible rise of earth potential to make installation easier, rather than good science to make the installation safe.

  • 0

    gkenyon:




    Chris Pearson:

    And what of 3-phase? Let's say that you have 3-phase at home and do your best to balance the phases. Whilst the car is charging, the phases will be sufficiently balanced to keep the neutral voltage down; but when you plug in or disconnect, the same does not necessarily hold true.




    In that case, 722.411.4.1 (i) clearly does not hold apply.


    For larger installations, it may, however, be practicable. There will be additional guidance, and a rule of thumb, to help make that decision, in the 4th Edition of the IET CoP.



    That to some extent is my gripe. The amendment aims to deal with the risk in single phase installations, but not 3-phase ones. There are spare ways in my fuse box just waiting for a solution. I suppose that one will arrive within the next 15 years. ?

  • 0

    Chris Pearson:




    gkenyon:




    Chris Pearson:

    And what of 3-phase? Let's say that you have 3-phase at home and do your best to balance the phases. Whilst the car is charging, the phases will be sufficiently balanced to keep the neutral voltage down; but when you plug in or disconnect, the same does not necessarily hold true.




    In that case, 722.411.4.1 (i) clearly does not hold apply.


    For larger installations, it may, however, be practicable. There will be additional guidance, and a rule of thumb, to help make that decision, in the 4th Edition of the IET CoP.



    That to some extent is my gripe. The amendment aims to deal with the risk in single phase installations, but not 3-phase ones. There are spare ways in my fuse box just waiting for a solution. I suppose that one will arrive within the next 15 years. ?


     




    How does Amendment No. 1 not serve three-phase installations?


    If you have a three-phase PME (or TN-S from a  public supply without a dedicated transformer) installation that cannot be guaranteed to meet 722.411.4.1 (i), then you have the following options:


    • Install additional earth electrodes in accordance with 722.411.4.1 (ii), if you can satisfy the three-phase requirement in A722.3. Even if your worst-case unbalance doesn't meet 722.411.4.1 (i), you might get away with an earth electrode at 10's of Ohms, rather than single figures or sub-One Ohm.

    • Use an open-PEN detection device that meets the requirements of 722.411.4.1 (iii). There are definitely products on the market for three-phase installations that do not require a measurement earth electrode, although equally there are now devices with measurement earth electrode available. If it's single-phase charging equipment, the device can be fitted upstream in the three-phase part of the network, and in some cases operate a single-phase tripping device or contactor. To accommodate innovation, 722.411.4.1 (v) would permit a device offering equivalent safety  to 722.411.4.1 (iii) to be used.

    • Use an isolating transformer as described in A722.5. Previously, people appeared to be concerned about this approach - granted it can be expensive, and you need space for additional (perhaps heavy) equipment.

    • Least-preferred, would be considering TT for either the charge point, or the whole installation. This is not prohibited by Amendment 1, but Note 6 to 722.411.4.1 outlines some issues to look out for, and in some cases, installations have been carried out where separation has not been achieved, or the PME touch voltage can be returned via other means. Some DNOs have stringent requirements for separation. Further guidance will be available in the 4th Edition of the IET CoP for EV charging equipment installation which should be available soon. As Note 6 to 722.411.4.1 says, converting to TT is not without its problems and risks, and I personally would recommend caution in small curtilage properties, or in suburban/urban areas, as separation from the PME system is tricky, simultaneous contact can be an issue, and if converting the whole property, when you apply main bonding (particularly in urban/suburban areas) are you really separating from the PME?



     

  • 0
    This thread shows an interesting perception of risk. Traveling by car is one of the most dangerous activities most of us do. There are around 5 deaths per day.


    What is the risk of death or serious injury due to earthing problems in the car charging system. It was quoted that there is possibly one broken PME event per day but this in itself does not result in death or injury, several other concurrent events are also required. Someone must be attempting to plug in or unplug the charging cable while the PME failure is present. They must have contact with two separate conductive objects. The contact resistances must be low enough that a harmful current flows. How often will this happen? The UK seems to have around 1 death per year due to earthing problems. How much will this increase with a large roll out of vehicle charging systems? Is 3 phase supplies that much of a problem. Most properties on the mainland have 3 phase supplies without apparent increase in risk.


    Best regards


    Roger
  • 0

    Roger Bryant:

    This thread shows an interesting perception of risk.




    I am not sure that I would agree. What we are looking at is not so much the risk of a fault causing a death but rather the (relatively simple) mitigations that can be put in place to prevent such a death. If it cost £1 billion to prevent each death it can be argued it is not practical. If it cost £10 to prevent each death then it is £10 well spent. Of course the real cost is in between but saying that there are more deaths on the road than would be caused be EV chargers and therefore we are not going to do anything to prevent them is not an acceptable argument (and I am sure that is not what you meant, Roger, but I  want to make sure nobody reads it that way).

    Alasdair

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