[rather lengthy, so only really for interested and dedicated 'techies' 
]
In another thread, in response to comments from winston1 ....
It would clearly be inappropriate and unworkable to undertake calculations on the basis of the actual supply voltage of a particular installation at some random point in time, not the least because supply voltages can (and are permitted to) vary appreciably. It is therefore necessary that we use some fixed value for such calculations, with current regulations nearly always requiring that fixed value to be U0.
One can argue (as winston1, unnecessarily and annoyingly, does repeatedly) that it is not appropriate in the UK to calculate on the basis of an arbitrary U0 of 230V when the average UK supply voltage is much closer to 240V. Similarly, if we changed U0 to 240V, one imagines that some foreign manifestation of winston1 would argue that, in their country, it would be inappropriate to calculate on the basis of 240V when the average supply voltage in their country was much closer to 230V or 220V. What these viewpoints don’t usually acknowledge is that it is probably not really logical to use any arbitrarily-selected “nominal voltage”, somewhere between the minimum and maximum permitted values (which I will call Umin and Umax).
The regulations are primarily about electrical safety and hence generally tend to work on the basis of ‘worst possible scenarios’. When one is undertaking calculations which involve, or are dependent upon, supply voltage, the ‘worst possible scenario’ will nearly always arise when the supply voltage is Umin or Umax (depending on what one is calculating), so one might have expected the regs to recognise this - but, up until now, they generally have not done this.
However, maybe IET/BSI (specifically JPEL/64) have at last come to this way of thinking, since in a few months’ time we are going to see one of the first manifestations of this way of thinking in Amendment 3 of BS7671:2008, even though the way of implementing this (as proposed in the latest draft of the Amendment) seems somewhat illogical and unnecessarily confusing.
As most people know, the change relates to the calculation of the maximum permitted Zs required to achieve required disconnection times with OPDs of varying type and In. Currently, these calculations (the results of which are tabulated in the regs and OSG) are based on the assumption that supply voltage is U0 (currently 230V). This means that a circuit which only just satisfies the disconnection time requirements per those calculations/tabulations (which assume that supply voltage is U0) will fail to satisfy the requirements if supply voltage is actually lower than U0 - so the logical conservative (‘worst case’) approach would be to require the calculations to be undertaken on the assumption of a supply voltage of Umin (currently 230V - 6%, namely 216.2V).
However, rather than simply requiring that Zs be low enough to satisfy the disconnection requirements even if supply voltage was as low as “Umin”,they have decided to stick with a calculation based on U0, but have added a correction factor (“Minimum Voltage Factor”, Cmin) by which one multiplies U0 (hence the factor by which one multiplies the calculated Zs). Had they chosen a value for Cmin of 0.94, this correction would have the same effect as the ‘logical’ (IMO) approach of calculating using “Umin” but, for reasons better known to themselves, the latest draft proposed Cmin=0.95, hence not quite representing the ‘worst possible scenario’ (U0-5%, rather than U0-6%). I and others have made representations about this apparent illogicality - but, whether they take notice of that or not, it would seem that come next year, maximum Zs values will be calculated on the basis of either “Umin” or something fairly close to that, rather than the arbitrary U0.
If we want regs which are conservative (i.e. which ‘err on the side of safety’) (do we?), this seems logical to me. Do people think that, as the years go by, we will see this ‘worst case scenario’ approach extended to other calculations - e.g. determination of ‘worst case Ib on the basis of loads being supplied with a voltage of “Umax” (currently 253V)? If we do, then, apart from the obvious, we might eventually see the end of rants and arguments about the (clearly not ideal) use of an arbitrary “nominal voltage” for calculations. The fact that the proposed Amd3 includes provision for definition of Cmax ("Currently Not Used"), as well as Cmin, does suggests that they may well have thoughts of extending this concept (effectively of using Umin or Umax, as appropriate, rather than U0 for calculations) in the future.
Kind Regards, John
]In another thread, in response to comments from winston1 ....
It is IMO a great pity that winston1 will not stop confusing/spoiling so many threads with his repetitive rants about the use of a “nominal supply voltage” (U0) for calculations since not only does it irritate many of us and destroy many discussions but it also detracts from the fact that there are some legitimate issues to discuss and that many of us probably actually share many of his feelings (but accept that, in general, the regulations dictate what we must do).
It would clearly be inappropriate and unworkable to undertake calculations on the basis of the actual supply voltage of a particular installation at some random point in time, not the least because supply voltages can (and are permitted to) vary appreciably. It is therefore necessary that we use some fixed value for such calculations, with current regulations nearly always requiring that fixed value to be U0.
One can argue (as winston1, unnecessarily and annoyingly, does repeatedly) that it is not appropriate in the UK to calculate on the basis of an arbitrary U0 of 230V when the average UK supply voltage is much closer to 240V. Similarly, if we changed U0 to 240V, one imagines that some foreign manifestation of winston1 would argue that, in their country, it would be inappropriate to calculate on the basis of 240V when the average supply voltage in their country was much closer to 230V or 220V. What these viewpoints don’t usually acknowledge is that it is probably not really logical to use any arbitrarily-selected “nominal voltage”, somewhere between the minimum and maximum permitted values (which I will call Umin and Umax).
The regulations are primarily about electrical safety and hence generally tend to work on the basis of ‘worst possible scenarios’. When one is undertaking calculations which involve, or are dependent upon, supply voltage, the ‘worst possible scenario’ will nearly always arise when the supply voltage is Umin or Umax (depending on what one is calculating), so one might have expected the regs to recognise this - but, up until now, they generally have not done this.
However, maybe IET/BSI (specifically JPEL/64) have at last come to this way of thinking, since in a few months’ time we are going to see one of the first manifestations of this way of thinking in Amendment 3 of BS7671:2008, even though the way of implementing this (as proposed in the latest draft of the Amendment) seems somewhat illogical and unnecessarily confusing.
As most people know, the change relates to the calculation of the maximum permitted Zs required to achieve required disconnection times with OPDs of varying type and In. Currently, these calculations (the results of which are tabulated in the regs and OSG) are based on the assumption that supply voltage is U0 (currently 230V). This means that a circuit which only just satisfies the disconnection time requirements per those calculations/tabulations (which assume that supply voltage is U0) will fail to satisfy the requirements if supply voltage is actually lower than U0 - so the logical conservative (‘worst case’) approach would be to require the calculations to be undertaken on the assumption of a supply voltage of Umin (currently 230V - 6%, namely 216.2V).
However, rather than simply requiring that Zs be low enough to satisfy the disconnection requirements even if supply voltage was as low as “Umin”,they have decided to stick with a calculation based on U0, but have added a correction factor (“Minimum Voltage Factor”, Cmin) by which one multiplies U0 (hence the factor by which one multiplies the calculated Zs). Had they chosen a value for Cmin of 0.94, this correction would have the same effect as the ‘logical’ (IMO) approach of calculating using “Umin” but, for reasons better known to themselves, the latest draft proposed Cmin=0.95, hence not quite representing the ‘worst possible scenario’ (U0-5%, rather than U0-6%). I and others have made representations about this apparent illogicality - but, whether they take notice of that or not, it would seem that come next year, maximum Zs values will be calculated on the basis of either “Umin” or something fairly close to that, rather than the arbitrary U0.
If we want regs which are conservative (i.e. which ‘err on the side of safety’) (do we?), this seems logical to me. Do people think that, as the years go by, we will see this ‘worst case scenario’ approach extended to other calculations - e.g. determination of ‘worst case Ib on the basis of loads being supplied with a voltage of “Umax” (currently 253V)? If we do, then, apart from the obvious, we might eventually see the end of rants and arguments about the (clearly not ideal) use of an arbitrary “nominal voltage” for calculations. The fact that the proposed Amd3 includes provision for definition of Cmax ("Currently Not Used"), as well as Cmin, does suggests that they may well have thoughts of extending this concept (effectively of using Umin or Umax, as appropriate, rather than U0 for calculations) in the future.
Kind Regards, John
