230V/240V Question

[JohnW2]

So PBC was wrong, then.

I wouldn't say so - he included the word 'standard' (pressure) in his statement.

Kind Regards, John

 

[stillp]

Thank you PBC. How would you ensure repeatable measurements if the pressure were not specified?

Repeatable in what sense? The water will boil at some specific temperature whatever the pressure, even though that temperature will vary with pressure. The fact that the pressure has altered doesn't affect the accuracy of the temperature measurement taken, it's just that the water is boiling at a different temperature.

In the same way, while a badly bogged down network which results in a severe drop in supply frequency might well also have the consequences of below-normal voltages, the change in frequency doesn't affect the accuracy with which those voltages may be determined.

But if you do not know the frequency then you have no way of knowing the validity of your voltage measurement, just as if you do not know the ambient pressure you cannot be certain that you have measured the boiling point of water correctly.

 

[PBC_1966]

This really has nothing to do with what we are discussing. By asking about the boiling point of water you are talking about something which is dependent on something else (pressure). The AC potential difference measured between two points (such as the two conductors of a supply) is in no way dependent on the frequency of that voltage - so it can be measured as accurately and 'repeatably' as one's equipment allows without any knowledge of, or reference to, the frequency - even if that frequency is varying.

Indeed, I'm not seeing the analogy which is trying to be drawn here.

Where I live now is about 500 ft. above sea level, which according to a quick table lookup to save calculations, means that water should boil around 211 deg. My last home in Shingletown was at about 3200 ft. up the mountain, which means water there will boil at a little under 206 deg.

The difference is not due to any sort of accuracy or repeatability problem with measurements taken at these two different locations; it's a consequence of the slightly different atmospheric pressure which means that the boiling point of water is actually different, due to the 2700 ft. or so difference in altitude.

 

[JohnW2]

But if you do not know the frequency then you have no way of knowing the validity of your voltage measurement, just as if you do not know the ambient pressure you cannot be certain that you have measured the boiling point of water correctly.

I really don't understand your point. Boiling point is dependent on pressure, so you obviously need to know and specify the pressure at which the boiling point has been measured. The voltage of an electrical supply is NOT dependent upon its frequency, so you have no need to know anything about frequency is order to measure, and report, the voltage correctly and accurately.

As a result of changes in supply/demand, supply voltage and frequency may well both change simultaneously, but that's irrelevant. One can still measure the voltage accurately, regardless of its frequency.

Kind Regards, John

 

[PBC_1966]

But if you do not know the frequency then you have no way of knowing the validity of your voltage measurement

How so? RMS voltage is RMS voltage regardless of frequency. The voltage measured is perfectly accurate and valid whether the grid is supplying exactly 50Hz, 49.5Hz, 50.5Hz, or indeed some frequency outside those limits.

just as if you do not know the ambient pressure you cannot be certain that you have measured the boiling point of water correctly.

You can be certain you've measured the boiling point of the water at whatever the prevailing pressure happens to be.

I really don't see what analogy you're trying to draw here.

 

[PBC_1966]

One can still measure the voltage accurately, regardless of its frequency.

And vice versa.

 

[stillp]

One can still measure the voltage accurately, regardless of its frequency.

Of course you can, but what should the permissible variation of the voltage be?

 

[PBC_1966]

Of course you can, but what should the permissible variation of the voltage be?

The same as at exactly 50Hz?

 

[JohnW2]

One can still measure the voltage accurately, regardless of its frequency.

Of course you can, but what should the permissible variation of the voltage be?

That depends upon their intent. If they intend that the voltage limits specified in the regulation always apply, regardless of frequency, then they should remove the phrase "at nominal frequency" from that regulation. If, on the other hand, they intend that there should be different voltage limits for different (ranges of) frequencies, then they simply must 'spell out' those frequency-dependent voltage limits - otherwise we are completely in the dark as to what is required.

Kind Regards, John

 

[stillp]

[QUOTE="JohnW2, post: 3547392, member: 158945" The voltage of an electrical supply is NOT dependent upon its frequency, so you have no need to know anything about frequency is order to measure, and report, the voltage correctly and accurately.
[/QUOTE]The frequency and the voltage are both closely related to the grid loading, and a few other parameters.

 

[stillp]

we are completely in the dark as to what is required.

But you need not be. You have a precise specification of the permissible variation in voltage at a given frequency, and of the permissible variation in frequency. Why would you benefit from a table of voltage tolerances at different frequencies?

 

[JohnW2]

The frequency and the voltage are both closely related to the grid loading, and a few other parameters.

I'm already agreed with that, but it doesn't alter the fact that either voltage or frequency can be measured separately, without any reference to, or knowledge about, the other.

As I've just written, IF they want to have different voltage limits at different frequencies, they simply must define what they are.

Kind Regards, John

 

[stillp]

I really don't see what analogy you're trying to draw here.

Stating that, for example, "The boiling temperature of water is 100° C" does not have a precise meaning unless the pressure is specified, just as "The supply voltage shall be X ±10%" does not have a precise meaning unless the frequency is specified.

 

[stillp]

The frequency and the voltage are both closely related to the grid loading, and a few other parameters.

I'm already agreed with that, but it doesn't alter the fact that either voltage or frequency can be measured separately, without any reference to, or knowledge about, the other.

As I've just written, IF they want to have different voltage limits at different frequencies, they simply must define what they are.

Kind Regards, John

And what useful purpose would that serve?

 

[JohnW2]

we are completely in the dark as to what is required.

But you need not be. You have a precise specification of the permissible variation in voltage at a given frequency, and of the permissible variation in frequency. Why would you benefit from a table of voltage tolerances at different frequencies?

... because, without such a table, we haven't got a clue as to what what voltage variation is permissible at frequencies which are not 'at nominal' (i.e. 'at', or, at least, very close to, 50 Hz), but which are still within the acceptable frequency range.

Kind Regards, John