mapj1:

But that factor of 2 in the denominator of your equation 8 should be more like 1.5 in the case of twin and earth. Experience in the rest of Euroland is not so relevant here - in the UK our final circuits do not have full sized CPCs,  (nor do quite a few in-building sub-mains either) and in built up areas we may have larger lower impedance transformers, indeed in parts of London 1MVA transformers are meshed and the LV network in the streets is not even fused (AKA "the solid system"). Then there are a great many tower blocks with a megawatt transformer in the basement, and then bigger office and mixed use buildings with HV going up to a transformer on every 5th floor or so (look at Canary Wharf for an example of that if you like)

In such cases your assumption that the supply droop has a dominant effect is not appropriate - it is mostly in the cables.

Note that just because something is written by a committee does not make it infallible - look at the number of times the regs get updated for proof of that. ( I've sat on telecoms standards meetings, I know how it works, and it has perhaps made me slightly cynical. )

I agree the incoming voltage will droop very significantly during fault for those rural sites fed by smaller (100kVA and down) pole-pig transformers, but they are more commonly earthed as TT anyway.

I'm not sure we should underestimate the touch voltages to city dwellers by assuming that all installations are like that.

There is an additional complication in a PME system, as the live voltage goes down the neutral comes up to meet it, so you have to be clear if you mean touch voltag relative to the CPC of the system, or to terra-firma earth voltage either at ground level as on incoming telephone cables etc.

M.

I agree, but remember, with MCBs you'll be hitting the instantaneous function even on your maximum permitted Zs. Only larger circuits (over 125 amps) will take time clearing, and those will cause at least some dip even on 1 MVA units. As such I still maintain the the source has a moderate to major effect on touch voltage.


London like France, New York, Chicago ect are exceptions though. These networks, in particular Con Edison's networks, can be regarded as being almost truly infinite. Evidenced by 500MCM (253mm2) cables burning clear in manholes with only mild, local dimming of lights. At the same time remote earth becomes far more scarce- sidewalks, poor conductive floors, bonded pipes and building steal all work to reduce touch voltage. The scenario of being outside on damp ground while holding a metal tool or electric grill in the back yard like out in the country become very slim. 


I will agree with you that technical committees get it wrong or in the case of today influenced by the manufacturers, however I still hold the belief that 5 seconds is unlikely to present a danger in most cases.   




 

mapj1:

But that factor of 2 in the denominator of your equation 8 should be more like 1.5 in the case of twin and earth. Experience in the rest of Euroland is not so relevant here - in the UK our final circuits do not have full sized CPCs,  (nor do quite a few in-building sub-mains either) and in built up areas we may have larger lower impedance transformers, indeed in parts of London 1MVA transformers are meshed and the LV network in the streets is not even fused (AKA "the solid system"). Then there are a great many tower blocks with a megawatt transformer in the basement, and then bigger office and mixed use buildings with HV going up to a transformer on every 5th floor or so (look at Canary Wharf for an example of that if you like)

In such cases your assumption that the supply droop has a dominant effect is not appropriate - it is mostly in the cables.

Note that just because something is written by a committee does not make it infallible - look at the number of times the regs get updated for proof of that. ( I've sat on telecoms standards meetings, I know how it works, and it has perhaps made me slightly cynical. )

I agree the incoming voltage will droop very significantly during fault for those rural sites fed by smaller (100kVA and down) pole-pig transformers, but they are more commonly earthed as TT anyway.

I'm not sure we should underestimate the touch voltages to city dwellers by assuming that all installations are like that.

There is an additional complication in a PME system, as the live voltage goes down the neutral comes up to meet it, so you have to be clear if you mean touch voltag relative to the CPC of the system, or to terra-firma earth voltage either at ground level as on incoming telephone cables etc.

M.

I agree, but remember, with MCBs you'll be hitting the instantaneous function even on your maximum permitted Zs. Only larger circuits (over 125 amps) will take time clearing, and those will cause at least some dip even on 1 MVA units. As such I still maintain the the source has a moderate to major effect on touch voltage.


London like France, New York, Chicago ect are exceptions though. These networks, in particular Con Edison's networks, can be regarded as being almost truly infinite. Evidenced by 500MCM (253mm2) cables burning clear in manholes with only mild, local dimming of lights. At the same time remote earth becomes far more scarce- sidewalks, poor conductive floors, bonded pipes and building steal all work to reduce touch voltage. The scenario of being outside on damp ground while holding a metal tool or electric grill in the back yard like out in the country become very slim. 


I will agree with you that technical committees get it wrong or in the case of today influenced by the manufacturers, however I still hold the belief that 5 seconds is unlikely to present a danger in most cases.