The DNO network system categorised as Hot site or Cold site (430V or 650V ~ in case operating voltage of 33kV with sensitive earth faults cleared within 200ms). My submission is the standard always talks about the voltage (i.e pressure) or 30mA ELCB limits in case of housing earth leakage protection(i.e current). I presume either voltage or current are not alone cause for safety hazard. It is the product i.e voltage*current or V*I*t which causes burns on human body. Hence may i suggest in this forum to review this and consider the subject resistivity+ time aspects while calculating the "Step & touch power / energy" rather than "Step & touch voltage" as is being mentioned in the earthing standards. The additional variables of resistivity& time would help us to choose the appropriate insulating materials and relay settings and enhance human safety. Happy to discuss further, to bring this idea to shape and benefit wider community.
In DNO slang a hot site has to have the HV and LV neutral and earth electrodes spaced far enough apart that a fault from HV windings to the core or tank of the TX will not cause excessive LV side star point and neutral voltage rise relative to earth potential far away, or if it does, not for long enough to kill someone.
A cold site has a low enough ground impedance that HV and LV can share earth electrodes without this being an issue.
Historically the threshold for linked or separated was an HV electrode resistance of below or above about 1 ohm respectively, and in reality still is in most common cases, but the exact value depends on HV fault level and how the HV side earth fault protection is arranged (NER at the source end for example comes into that sum as well as if the HV cable is overhead singles or brings an HV earthed armour with it) and a bit of local geology knowledge is also needed.
Small pole pig transformers are nearly all "hot" for example, so the LV earth may run down the next wooden post along to the pole with the HV earth, while city centre MW and half MW units are usually ground mounted on large slabs and given a big buried grid and can be "cold".
In DNO slang a hot site has to have the HV and LV neutral and earth electrodes spaced far enough apart that a fault from HV windings to the core or tank of the TX will not cause excessive LV side star point and neutral voltage rise relative to earth potential far away, or if it does, not for long enough to kill someone.
A cold site has a low enough ground impedance that HV and LV can share earth electrodes without this being an issue.
Historically the threshold for linked or separated was an HV electrode resistance of below or above about 1 ohm respectively, and in reality still is in most common cases, but the exact value depends on HV fault level and how the HV side earth fault protection is arranged (NER at the source end for example comes into that sum as well as if the HV cable is overhead singles or brings an HV earthed armour with it) and a bit of local geology knowledge is also needed.
Small pole pig transformers are nearly all "hot" for example, so the LV earth may run down the next wooden post along to the pole with the HV earth, while city centre MW and half MW units are usually ground mounted on large slabs and given a big buried grid and can be "cold".
