What voltage to use

[Fender]

My first question, so be nice. I'm very sensitive.

We use 230 volts as the go to figure when doing calculations on single phase.

However on page 362 of the regs, they squeeze in another voltage to use on the bottom of the page. Namely 218.5 volts (95%).

It's when calculating Zs. I can't find this voltage (218.5 V) anywhere else. I might be wrong. If I'm not, why is it only used for Zs calculations?

 

[timhoward]

Welcome! Does this help:

Max Zs Values 18th Edition BS 7671 - Pro Certs Software

BS 7671 18th Edtion Max Zs Values - How to calculate max Zs, maximum zs value tables. Maximum values of earth loop impedance (Zs)

www.procertssoftware.com

 

[Fender]

Welcome! Does this help:

Max Zs Values 18th Edition BS 7671 - Pro Certs Software

BS 7671 18th Edtion Max Zs Values - How to calculate max Zs, maximum zs value tables. Maximum values of earth loop impedance (Zs)

www.procertssoftware.com

Thanks for that, but I don't think I made myself clear.
What I should have said is why don't we use 218.5 volts for all calculations, not just Zs?

 

[timhoward]

Ah I see. It's an interesting question.
It's funny but I've never linked the two before you got me thinking - but I suppose it correlates with the max 5% volt drop rule, so in theory the voltage at the end of a longer run would still be within the parameters used in the formula to prove the disconnection times.

So maybe one answer is that having run the numbers for ADS with the worst possible permitted voltage, it's fine to use the correct nominal voltage in other calculations which might be up to 5% out if the volt drop regs are adhered to?

 

[Julie.]

Thanks for that, but I don't think I made myself clear.
What I should have said is why don't we use 218.5 volts for all calculations, not just Zs?

When you have a supply it isn't always 230V - in practice it has a +/- figure that it can be , so the supply may be 240V or 220V and it is still well within the acceptable tolerance.

So let's say we determine that a 32A MCB needs to see 160A in order to disconnect in the correct time, well at 230V this would be 230/160 = 1.4375 ohm - great, so we have a circuit that has 1.43 ohm - all well and good, but what happens if the supply voltage drops to it's lowest limit of 218.5V?

well 218.5/1.43 = 153A - below that needed to trip within the time - in fact due to the curve it could be around 13 seconds! - so if the voltage was at it's lowest this installation would be pretty unsafe!

Hence when calculating the ability to trip in the correct time, you must use the worst condition to ensure it operates in the correct time FOR ALL CONDITIONS. - using 218.5V/160A = 1.37 ohm - which matches the figure in the book.

So this raises another problem, this would be when the system is hot - when the resistance has gone up, typically we use 80% as a correction factor for when it is cold - so 1.37 X 0.8 = 1.1 ohm - which is the figure for the onsite guide! If you have a system that gives 1.1 ohm when measured cold, when it runs hot it will end up around 1.27 ohm - which means it is safe for minimum voltage with hot running - the worst case!

For some other calculations this isn't necessary.

 

[Fender]

When you have a supply it isn't always 230V - in practice it has a +/- figure that it can be , so the supply may be 240V or 220V and it is still well within the acceptable tolerance.

So let's say we determine that a 32A MCB needs to see 160A in order to disconnect in the correct time, well at 230V this would be 230/160 = 1.4375 ohm - great, so we have a circuit that has 1.43 ohm - all well and good, but what happens if the supply voltage drops to it's lowest limit of 218.5V?

well 218.5/1.43 = 153A - below that needed to trip within the time - in fact due to the curve it could be around 13 seconds! - so if the voltage was at it's lowest this installation would be pretty unsafe!

Hence when calculating the ability to trip in the correct time, you must use the worst condition to ensure it operates in the correct time FOR ALL CONDITIONS. - using 218.5V/160A = 1.37 ohm - which matches the figure in the book.

So this raises another problem, this would be when the system is hot - when the resistance has gone up, typically we use 80% as a correction factor for when it is cold - so 1.37 X 0.8 = 1.1 ohm - which is the figure for the onsite guide! If you have a system that gives 1.1 ohm when measured cold, when it runs hot it will end up around 1.27 ohm - which means it is safe for minimum voltage with hot running - the worst case!
For some other calculations this isn't necessary

Cheers Julie. But it's the last sentence of your post that is the essence of my question.
.
'For some other calculations this isn't necessary.'

Why is it not necessary?

 

[Fender]

Ah I see. It's an interesting question.
It's funny but I've never linked the two before you got me thinking - but I suppose it correlates with the max 5% volt drop rule, so in theory the voltage at the end of a longer run would still be within the parameters used in the formula to prove the disconnection times.

So maybe one answer is that having run the numbers for ADS with the worst possible permitted voltage, it's fine to use the correct nominal voltage in other calculations which might be up to 5% out if the volt drop regs are adhered to?

Ooops. Sorry. I missed your answer. I'm new, and that will be my excuse for years to come.
I think you could be on to something Thanks

 

[Julie.]

Cheers Julie. But it's the last sentence of your post that is the essence of my question.
.
'For some other calculations this isn't necessary.'

Why is it not necessary?

It depends on the calculation, in order to check the worst condition.

So for example say you want to check the cable can withstand the maximum fault current (cables can burst at high currents, or overheat and be damaged) - in this case you want to use the maximum voltage and minimum resistance to determine the maximum current it could be in order to check it is suitable.

So
Minimum voltage - maximum resistance to check you will have sufficient current to operate protection

Maximum voltage - minimum resistance to check the equipment can withstand the highest current that could occur.

In other cases the rated voltage is most suitable, for example if a heater is 4kw @220v then the 220v would be the correct voltage to use to determine the heater's resistance. Not minimum, maximum or nominal

Etc.

Each calculation will need to be evaluated to see what is the most suitable figures to use.

 

[pc1966]

Just to add that computing current from voltage and power is not always easy/correct.

• For a simple resistive load then I = V/R and that is fine.
• For a heater then often R increases with temperature so going +10% in V won't give you quite 10% more I, most dramatic for filament lamps (but not likely these days) or high temperature ovens like kilns.
• But for switching power supplies the opposite can be true, that in fact P is constant so as you increase V then I drops due to the adjusting of the switching conversion process.

But as above, for protection calculations the UK practice is to use (almost) worst case for determining Zs so 80% for cable heating, 95% for supply voltage (though actually spec is 94%...) and the upper limit of the "instant" magnetic trip for a MCB, for example a B-curve is 3-5 * In so we use 5*In (but Aus/NZ uses 4*In).