How big does a 3 phase in-balance need to be to mask an open pen fault

Question

This is looking at the previous project I was discussing with two EVCP's on two phases of a 3 phase supply.
Just realised that I may have issues relying on single phase voltage based open pen detection when connecting to a 3 phase supply.

AI says Regulation 722.411.4.1(iv) of BS 7671:2018+A1:2020 prohibits the use of a protection device for open-PEN (Protective Earth and Neutral) detection in single-phase EV charging points supplied by a three-phase installation. This is because this specific type of open-PEN protection is designed for single-phase installations only, and a three-phase supply has different electrical characteristics that are not compatible with this protective measure.

Looking at amendment 2 I can't see this wording, am I just missing it, or has it been removed?

gkenyon · Answer

Alan B said: Looking at amendment 2 I can't see this wording, am I just missing it, or has it been removed? 
 It was true for Amendment 1:2020, the wording of 722.411.4.1(iv) changed for Amendment 2:2022. 
 Amendment 1 wording: 
 (iv) Protection against electric shock in a single-phase installation is provided by a device which electrically disconnects the vehicle from the live conductors of the supply and from protective earth in accordance with Regulation 543.3.3.101(ii) within 5 s in the event of the utilisation voltage at the charging point, between the line and neutral conductors, being greater than 253 V rms or less than 207 V rms. The device shall provide isolation and be selected in accordance with Table 537.4. Equivalent means of functionality could be included within the charging equipment. Closing or resetting of the device shall be possible only if the voltage between line and neutral conductors is in the range 207 to 253 V rms. 
 Amendment 2 wording 
 (iv) Protection against electric shock is provided by a device which electrically disconnects the vehicle from the live conductors of the supply and from protective earth in accordance with Regulation 543.3.3.101(ii) within 5 s in the event of the utilisation voltage at the charging point, between the line and neutral conductors, being greater than 253 V rms or less than 207 V rms. The device shall provide isolation and be selected in accordance with Table 537.4. Equivalent means of functionality could be included within the charging equipment. Closing or resetting of the device shall be possible only if the voltage between line and neutral conductors is in the range 207 to 253 V rms. See Note 1. 
 Alan B said: Just realised that I may have issues relying on single phase voltage based open pen detection when connecting to a 3 phase supply. 
 Most single-phase installations on PME systems are derived from a three-phase supply. 
 In three-phase installations, OPDDs that work along the lines of 722.411.4.1 (iii) ... M2a and M2b in IET 01:2024 ... are much less susceptible to non-detection on phase unbalance, and also unwanted operation due to voltage variation, then 722.411.4.1 (iv) devices ... M1a and M1b in IET 01:2024. Hence, it's better to use the (iii) devices in a three-phase installation if you can, although this may mean protecting a three-phase distribution circuit rather than single-phase final circuits. 
 Alan B said: How big does a 3 phase in-balance need to be to mask an open pen fault 
 In a real open-PEN fault, the phase unbalance changes over time ... every few minutes if not more quickly ... as loading changes. The issue is that we want to avoid, is OPDDs on more than one phase not seeing the fault in certain ratios of unbalance in certain conditions (because if OPDDs on at least one phase operate, phase unbalance will change, and it's highly likely any that didn't detect will operate). Those conditions were carefully explored in between Amendment 1:2020 and Amendment 2:2022, and it was discovered that you would need serious unbalance, plus very unusual conditions of power factor, for two phases at once to fail to see the conditions.

How big does a 3 phase in-balance need to be to mask an open pen fault

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