New Office Computer Circuits - High Integrity Earth?

[JohnW2]

I am not in the slightest interested in what the nominal voltage is, and the question of whether it is 'right' or 'wrong' is plain silly - since it is essentially arbitrary, hence 'right' by a self-fulfilling definition.
What I do know is that safety-critical calculations should not be undertaken with nominal or 'average' values, but with the 'worst case scenarios' - which will be the minimum or maximum permitted supply voltages, depending upon what calculation is being undertaken. In the case of calculation of the maximum Zs of a circuit to achieve the required disconnection times by ADS, the minimum permissible supply voltage is the figure which should be used.

 

[JohnW2]

I don't think maximum Zs is a very good example to use. Surely the result will be the same for all voltages - IF the calculation is done correctly.

I don't understand. The maximum Zs which will result in a high enough fault current to produce the required disconnection times by ADS is totally dependent upon what voltage one uses for the calculation.

The maximum Zs that will result in a high enough fault current to achieve the required disconnection time (e.g. 5In in the case of a Type B MCB) is simply the voltage divided by that current.

Consider a B32, hence a required fault current of 160A. If one calculates with 240V, one will conclude that the maximum pwrmissible Zs is 1.50Ω. If one calculates with 230V, one will get 1.44Ω. If (as the regs currently require) one calculates with 218.5V (230V*Cmin), one will get 1.37Ω - and if winston calculated with 228V (240V*Cmin), he would get 1.43Ω.

What makes you think that the maximum permissible Zs (for compliant ADS) does not depend upon the voltage one uses for the calculation?

Kind Regards, John

 

[EFLImpudence]

Yes, but now we have Cmin the virtual voltage is 218.5 (should be 216.2) so if someone IS using the actual voltage then they will have to calculate the Cmin percentage for that value.

A bit pointless as they may as well start with 218.5/5In (or 216.2/5In)

I realise that is likely not what the nominal voltage detractors had in mind but to calculate anything for the actual voltage, that is what they would have to do.

 

[JohnW2]

Yes, but now we have Cmin the virtual voltage is 218.5 (should be 216.2) so if someone IS using the actual voltage then they will have to calculate the Cmin percentage for that value.

Yes, but my point is that if things were sensible and properly safety-conscious, one would not use (a) the actual voltage (at the time of measurement), or (b) a nominal voltage (any nominal voltage) or (c) some sort of 'average' voltage.

(b) and (c) are just plain silly/wrong in terms of safety, but even with (a) no matter what voltage one measures at a particular point in time, there is no guarantee that it will remain unchanged, so the only safety-sensible approach is to calculate on the basis of the 'worst case scenario' - i.e. the lowest that the supply voltage would be allowed to become in the future (i.e. 216.2V).

One would not calculate whether or not some ambient-temperature-dependent component of an aircraft was adequately safe on the basis of a measurement of ambient temperature at some point in time, let alone some arbitrary 'nominal' ambient temperature or the average ambient temp to which the aircraft would be subjected - one would consider the worst-case-scenario - either the minimum and/or maximum ambient temp to which the aircraft might be subjected (depending upon which extreme(s) of temp were a potential problem).

In terms of maximum Zs, the introduction of Cmin, decades after people first started doing these calculations, went a long way towards addressing this issue. If Cmin were changed to 0.94 then it would be totally addressed.

Kind Regards, John

 

[EFLImpudence]

Yes, but my point is that if things were sensible and properly safety-conscious, one would not use (a) the actual voltage (at the time of measurement), or (b) a nominal voltage (any nominal voltage) or (c) some sort of 'average' voltage.

No, but that's not what those who don't accept nominal voltage are advocating.

(b) and (c) are just plain silly/wrong in terms of safety, but even with (a) no matter what voltage one measures at a particular point in time, there is no guarantee that it will remain unchanged, so the only safety-sensible approach is to calculate on the basis of the 'worst case scenario' - i.e. the lowest that the supply voltage would be allowed to become in the future (i.e. 216.2V).

Yes, I know but that's not the argument.

A couple are saying that they use the actual voltage.
If they do that then they must recalculate the tables in BS7671 and other publications.

In terms of maximum Zs, the introduction of Cmin, decades after people first started doing these calculations, went a long way towards addressing this issue.

That's why I said that using the maximum Zs tables was not a good example - because now it is reduced to 218.5V so instead of faffing about with 95% of 230/5In, just start with 218.5V.

If Cmin were changed to 0.94 then it would be totally addressed.

I agree, so start with 216.2V, then

 

[JohnW2]

No, but that's not what those who don't accept nominal voltage are advocating.

You seem to be trying to move me into a discussion which I'm not engaging in. As I wrote earlier today, I'm really not interested in nominal voltage, what value that nominal voltage is given, or discussions about what it 'should' be or whether/why it should have changed.

Those who moan about the change in nominal voltage seem to do so primarily because, at least in the UK, the change moved the nominal voltage away from the 'average' UK supply voltage. However, as I've said, even if the nominal voltage were exactly equal to the UK average, it would still be totally inappropriate as a basis for safety-critical calculations.

A couple are saying that they use the actual voltage.

If they are doing that, in safety terms they are making just as much of a mistake as if they used a nominal or average value.

I agree, so start with 216.2V, then

That's what I've always done in terms of my own house. Even before Cmin, when BS7671 allowed/'required' calculations to be done using 230V, I would not have accepted a Zs which would not be low enough if supply voltage fell to 216.2V.

I really don't understand how this situation arose and was allowed to persist for so long. It must surely be just about the first principle of any safety-related calculations that they should be done in relation to 'worst-case-scenarios' - yet countless groups of seemingly intelligent and trained people seemed to ignore that principle. In the days when the nominal voltage was 240V, hence probably fairly close to the UK average (and obviously before Cmin), something approaching half of installations were probably 'at risk' of being 'unsafe' despite regs-compliant calculations.

Kind Regards, John

 

[EFLImpudence]

I really don't understand how this situation arose and was allowed to persist for so long. It must surely be just about the first principle of any safety-related calculations that they should be done in relation to 'worst-case-scenarios' - yet countless groups of seemingly intelligent and trained people seemed to ignore that principle.

Like bonding window frames.

 

[ban-all-sheds]

the question of whether it is 'right' or 'wrong' is plain silly - since it is essentially arbitrary, hence 'right' by a self-fulfilling definition.

Try explaining that to the benighted ones who will not stop going on about measuring voltage whenever the topic is raised, preferably in a way which they understand, and therefore stop going on about measurements, and averages, etc, whenever the topic is raised.

 

[JohnW2]

Like bonding window frames.

I realise that there was a 'bond virtually everything' era, but was there actually ever an explicit regulation which required bonding of metal window frames?

Kind Regards, John

 

[JohnW2]

The question of whether any chosen nominal value is 'right' is, as I said, plain silly, but whether or not having a nominal value (any nominal value) serves any useful purpose in relation to the sort of issues we are discussing is, in my opinion, much more questionable.

 

[ban-all-sheds]

A nominal value would be perfectly fine if there was a built-in tolerance for excursions in either direction in the CCC of cables, the tripping current of devices etc.

 

[EFLImpudence]

but whether or not having a nominal value (any nominal value) serves any useful purpose in relation to the sort of issues we are discussing is,

If not, you presumably are discussing the wrong issues.

 

[winston1]

BAS. You said, "You are required to use a nominal voltage of 230 in your calculations.

Whether you like it or not, it is mandatory."

I replied, "Required and mandatory by whom. Please post the relevant laws and penalties for breaking them."


Are you going to answer this, even if it is to apologise and say you were wrong?

 

[winston1]

Ah - that will be when I pointed out that our supply is 230/400, which you know full well is it's official nominal value, you suggested I measure it.

We all know what the official nominal value is. And it has been pointed out that it exists in name only.

But please measure it and you will see that it does indeed exist in name only.

 

[EFLImpudence]

We all know what the official nominal value is.

And it has been pointed out that it exists in name only.

That's what 'nominal' means.

But please measure it and you will see that it does indeed exist in name only.

Hence - nominal.