Impact of proposed reductions in 'maximum Zs' figures?

[EFLImpudence]

Whilst your figures are obviously correct, I think you may be over concerned by numbers and W2 Towers.

Although adjustments will undoubtedly have to be made in very large houses, commercial and industrial installations, here in 'normal' domestic property land where a circuit's maximum Zs rarely gets above half the maximum figure of the BGB values, therefore there is nothing to worry about or calculate, I don't think it will make any difference.

Even in the case of 50A and a maximum Zs now of 0.7Ω where Ze is half of this the reduction is only 10% and still allows a 10mm² cable of over 50m.
The last shower I installed had a Zs of 0.27Ω.



I am still wondering why this has been brought up now or why it hasn't been before.

It may be sensible to allow for the minimum voltage in this example and the maximum voltage in others but it does negate the need or application ever of a nominal voltage.
I realise it may be optimistic to expect all electricians, especially these days, to be able to do all the calculations themselves but unless they can there may have to be a very large OSG in the future.

I am minded of the previous (15th) requirements for bonding everything regardless and then the presumed enlightenment of subsequent regulation writers.
After all the physics hasn't changed much in the last one hundred and fifty years, has it?
So, why do the regulations?

 

[JohnW2]

Whilst your figures are obviously correct, I think you may be over concerned by numbers and W2 Towers.

That may well be true. That's the very reason I raised the issue, asking electricians whether they thought it would have any significant impact. You clearly think not - you go on to say.....

Although adjustments will undoubtedly have to be made in very large houses, commercial and industrial installations, here in 'normal' domestic property land where a circuit's maximum Zs rarely gets above half the maximum figure of the BGB values, therefore there is nothing to worry about or calculate, I don't think it will make any difference.

That's interesting. Ze figures must mainly be much lower than I had envisaged.

Even in the case of 50A and a maximum Zs now of 0.7Ω where Ze is half of this the reduction is only 10% and still allows a 10mm² cable of over 50m.

That's true - but, as above, I had imagined that Ze was often appreciably greater than 0.35Ω, particularly with TN-S.

The last shower I installed had a Zs of 0.27Ω.

Fair enough - but same comment again. I had thought that in an appreciable proportion of TN-S installations, Ze alone would be higher than 0.27Ω

I am still wondering why this has been brought up now or why it hasn't been before. ...

Indeed. As I said somewhere in one of these threads, given how 'conservative' the regs tend to be, I'm very surprised that they have, until now, accepted a situation in which one can 'follow the regs and guidances' and still end up with non-compliant disconnection times if supply voltage happens to be (as it is allowed to be) below 230V.

...It may be sensible to allow for the minimum voltage in this example and the maximum voltage in others but it does negate the need or application ever of a nominal voltage

As I'm always saying, the concept of 'nominal voltage is really just arbitrary and bureaucratic/political. All that matters, electrically, is the minimum and maximum permitted supply voltage - and, as you say, if one wants to be safety-conservative (i.e. if one wants things to 'remain safe' over the full range of permitetd supply voltages), one needs to use either the maximum or minimum in calculations, depending upon what one is calculating. The 'nominal voltage' of 230V means nothing. One could express the UK permitted supply voltage range as "2xxV -y% +z%", where there are an infinite number of possible xx/y/z combinations. 230V is, AFAIAA, not even close to being the 'average' supply voltage in UK - which I presume is much closer to 240V than 230V. To use 230V for calculations (at least in the UK) therefore makes no electrical sense at all.

I really can't seen any justification, electrically, for ever using a 'political', arbitrary, 'nominal voltage' for any calculations!

After all the physics hasn't changed much in the last one hundred and fifty years, has it? ... So, why do the regulations?

Well, in this particular case, as above I would suggest that they have for all the years until now being overlooking a lack of 'safety-conservatism' in the regulations (which exists because of the never-changing laws of physics), and have only recently 'noticed'!

Kind Regards, John

 

[JohnW2]

Deleted - sorry, wrong thread