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.
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.
It is not just body burns that we are concerned about. The heart and breathing can be affected too. Chapter 41 of B.S. 7671 deals with this matter, shock protection of humans and livestock. Much research has already been undertaken and published about electric shocks and their affects on the human body. This is a question that will cause debate on the I.E.T's B.S. 7671 Wiring and Regulations Forum. You should get much response. Not many souls pass through here in Netherworld it is a bit quiet. Good for a sleep though.
I think 50mv is enough to stop the heart and RCD protection is 50mA , so touch voltages must be well below this , at the other end of the scale a lightening strike is millions of volts, and the burns are the resistance of the tissue to electrical voltage flow, these burns are in turn sufficient to damage tissue and the body may fail due to the damage caused , there is a great picture of the collar of lighteningstrike injury (they survived pretty well) and its just like a still from a Tesla coil , like a madlrbrot plot , lots of fractal flames from one central point and completely geometric , however most skin patterns found on lightenstrike injuries are not as neat as this very lucky person.
There is a resistivity figure for human tissue (works for animal tissue too I think)
I think 50mv is enough to stop the heart and RCD protection is 50mA , so touch voltages must be well below this , at the other end of the scale a lightening strike is millions of volts, and the burns are the resistance of the tissue to electrical voltage flow, these burns are in turn sufficient to damage tissue and the body may fail due to the damage caused , there is a great picture of the collar of lighteningstrike injury (they survived pretty well) and its just like a still from a Tesla coil , like a madlrbrot plot , lots of fractal flames from one central point and completely geometric , however most skin patterns found on lightenstrike injuries are not as neat as this very lucky person.
There is a resistivity figure for human tissue (works for animal tissue too I think)
Most U.K. R.C.D.s are rated at 30mA and operate within 40mS. They will normally prevent death of a human.
The many types of shock occurrences from mains' electrical supplies in the home or industry vary so much due to circumstances, that solid test results are difficult to achieve. Many variations exist due to individual circumstances such as body parts involved, path of shock current, whether the body is dry or wet, whether the person is wearing insulating footwear or gloves, standing on an insulating carpet or on the soil etc.
The greatest unknown is always the resistance of the particular path through the human body - this defines the current that flows for a given shock voltage, in turn the resistance is defined by the area of contact and the thickness of skin on that part of the body that makes the contact, and other factors such as water, sweat or blood that may lower the resistance. Where skin is removed a far lower voltage will drive a lethal shock current. As this thread shows, how disconnection times are decided is an involved topic.
For a given current, the disconnection times for given survival likelihoods relate to the period of the human heartbeat - a far higher current can be withstood if the shock is very small fraction of a heartbeat long, while a shock that is much more than a heartbeat long may as well be forever.
a) hot and cold are telecoms standards, concerned with potential transferred away from the substation on metallic circuits, so they're not directly related to safe touch/step potential
b) as more learned commentators than I have noted, the issue is about the low currents which cause fibrillation. The limits for what touch/step voltages are deemed to be safe can be higher than the hot/cold levels, because the safe limits make assumptions about the impedance of the unfortunate person involved, their footwear, and the surface they're standing on. That's why there are different permissible voltages for different surfaces: there's also a completely different (and more stringent) set of limits for railways, because they've made different assumptions about those impedances
There are IEC standards for all of this, so I suggest that we avoid re-inventing the wheel
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".
