Is moving a light switch in the kitchen allowed/notifiable?

[JohnW2]

Definitions ?

You mean Part 2 of BS7671? That defines 'design current' as "The magnitude of the current to be carried by the circuit in normal service". Am I right in assuming that we are now going to have a debate or argument about the meaning of 'normal service'?

As for my part of that debate, I have to say that if a designer knows that the supply voltage to a particular installation is 'normally' over 250V, that (s)he would (or should) regard 'normal service' as relating to that situation. You presumably disagree?

If the same voltage is used to calculate design current, PEFC, PSCC, CPD rating, maximum Zs allowed etc. it doesn't really matter what that voltage is - the results will be the same.

In terms of the point being discussed, this makes little sense to me. I am only talking about whether the In of the CPD/OPD is, or is not, greater than the design current of the circuit {i.e. whether the circuit complies with 433.1.1(i)}, so the only relevant one of the things you mention is "CPD rating". As you know at least as well as I do, that In is not something that one calculates, and nor does it in any way depend upon the supply voltage - indeed, a SP OPD doesn't have a clue about the supply voltage of the circuit in which it is included; it is purely a characteristic of the device. If In is 32A, that will remain the case whether it's in a circuit suppled with 12V, 230V, 240V, 600V or 10,000V.

Kind Regards, John.

 

[JohnW2]

If everyone could be totally trusted never to drive whilst drunk, there would be no point/need in it being made illegal, would there?

There would be no point but - There's never going to have been a Council/Parliament meeting where it was proposed that... "Everything is fine now - the people don't need us any more".

Of course not - but, as you know, I was merely responding to BAS's statement about Utopia:

we also frequently legislate against things where there would be no need if people were not stupid

...and, essentially, making the same point as you.

Kind Regards, John.

 

[EFLImpudence]

Definitions ?

You mean Part 2 of BS7671? That defines 'design current' as "The magnitude of the current to be carried by the circuit in normal service". Am I right in assuming that we are now going to have a debate or argument about the meaning of 'normal service'?

I did consider it.

As for my part of that debate, I have to say that if a designer knows that the supply voltage to a particular installation is 'normally' over 250V, that (s)he would (or should) regard 'normal service' as relating to that situation. You presumably disagree?

I suppose I would in a normal installation.

What then, in your view, is the reason for using 230V to calculate design current?
Do you think it should be 250 with a large drop in voltage allowable?
A drop (to the present minimum) which would be greater than the allowed voltage drop.

If so, then why is that not the regulation?

If the same voltage is used to calculate design current, PEFC, PSCC, CPD rating, maximum Zs allowed etc. it doesn't really matter what that voltage is - the results will be the same.

In terms of the point being discussed, this makes little sense to me. I am only talking about whether the In of the CPD/OPD is, or is not, greater than the design current of the circuit {i.e. whether the circuit complies with 433.1.1(i)}, so the only relevant one of the things you mention is "CPD rating". As you know at least as well as I do, that In is not something that one calculates, and nor does it in any way depend upon the supply voltage - indeed, a SP OPD doesn't have a clue about the supply voltage of the circuit in which it is included; it is purely a characteristic of the device. If In is 32A, that will remain the case whether it's in a circuit supplied with 12V, 230V, 240V, 600V or 10,000V.

All I can say is that the 'design current' is that determined by using the nominal voltage of 230.

 

[JohnW2]

As for my part of that debate, I have to say that if a designer knows that the supply voltage to a particular installation is 'normally' over 250V, that (s)he would (or should) regard 'normal service' as relating to that situation. You presumably disagree?

I suppose I would in a normal installation.

Sorry, but I have to ask 'you would what?' - you would take the same view as me, OR you would disagree with me? ... and, either way, what do you mean by a "normal installation"?

What then, in your view, is the reason for using 230V to calculate design current?

I think you're asking the wrong person since, as you will realise, I am yet to be convinced that there is a reason to use 230V, or that 230V is what one 'has to use'.

Do you think it should be 250 with a large drop in voltage allowable? A drop (to the present minimum) which would be greater than the allowed voltage drop.

Given that supply voltage can vary within the permitted limits, a rational (and 'conservative') engineering approach would probably be to take the worst case scenario of the maximum permitted supply voltage, hence the maximum possible current drawn by the load. One has to realise that it's not necessarily a case of supply voltages swinging around all over the place, on both sides of nominal (in which case it might be reasonable to work with nominal voltage as an 'average) - it's quite possible that some installations, close to transformer, may always have supply voltages of 250V or more.

All I can say is that the 'design current' is that determined by using the nominal voltage of 230.

You may well be right. If you can tell me what regulation specifies this, I will shut up and regard this as one of those arbitrary regulations with which we have to live

Kind Regards, John.

 

[RF Lighting]

I thought you were leaving?

 

[JohnW2]

I thought you were leaving?

That's the second time, isn't it? (or was it someone else?!). Anyway, as I said before (to you or someone else) .... not quite yet (if ever )

Kind Regards, John.

 

[RF Lighting]

It's the first time I've used that hilarious gag I think.

 

[EFLImpudence]

As for my part of that debate, I have to say that if a designer knows that the supply voltage to a particular installation is 'normally' over 250V, that (s)he would (or should) regard 'normal service' as relating to that situation. You presumably disagree?

I suppose I would in a normal installation.

Sorry, but I have to ask 'you would what?' - you would take the same view as me, OR you would disagree with me?

I meant I would disagree - in answer to the only question in the quote.

... and, either way, what do you mean by a "normal installation"?

A bit 'tongue-in-cheek' in relation to normal service.

All I can say is that the 'design current' is that determined by using the nominal voltage of 230.

You may well be right. If you can tell me what regulation specifies this, I will shut up and regard this as one of those arbitrary regulations with which we have to live

I am not sure what would satisfy you.

There are many tables in BGB which quote Uo of 230V so it would seem incongruous (wrong) to use one value in one calculation and a different value in related calculation.

 

[JohnW2]

It's the first time I've used that hilarious gag I think.

Yes, apologies - a case of mistaken identity (although it does render you post a little unoriginal ) ...

I thought you were retiring

Kind Regards, John.

 

[JohnW2]

I am not sure what would satisfy you.

Just a simple statement in the regs (or maybe even something else 'official') that a designer should calculatre 'design current' as the current that would usually flow in the circuit when the supply voltage was U0, regardless of the actual supply voltage of the installation.

There are many tables in BGB which quote Uo of 230V so it would seem incongruous (wrong) to use one value in one calculation and a different value in related calculation.

No-one is disputing that U0 is 230V. The question is whether U0 should be used when determining the design current of a circuit. The fact that the value of U0 is mentioned in many tables (none of which relate to the determination of design current) does not seem relevant.

How does your MFT (or dedicated loop impedance tester) calculate PFC or PSCC ?

Kind Regards, John.

 

[EFLImpudence]

How does your MFT (or dedicated loop impedance tester) calculate PFC or PSCC ?

Not sure if that is rhetorical or not.

It does it only by calculation.

To save me going out -

In the fluke 165X manual it shows a Loop of 0.23 and the related PFC of 1000A.

 

[JohnW2]

How does your MFT (or dedicated loop impedance tester) calculate PFC or PSCC ?

Not sure if that is rhetorical or not.

Essentially rhetorical.

It does it only by calculation.

Indeed so - I think/presume that they all do.

To save me going out - In the fluke 165X manual it shows a Loop of 0.23 and the related PFC of 1000A.

Indeed it does, I presume because that relates to familiar numbers and leads to easy arithmetic. However, if you read the manual, you will see (p61 in my 165X manual) that it says:

Prospective Earth Fault Current or PSC determined by dividing measured mains voltage by measured loop (L-PE) resistance or line (L-N) resistance.

Note that is says "measured mains voltage" - not 'nominal supply voltage' or '230V. My point? To illustrate that, even though "U0=230V" appears in many Tables in the BGB, that does not mean that 230V is used for all calculations of currents.

Kind Regards, John

 

[EFLImpudence]

I shall check tomorrow.

 

[JohnW2]

I shall check tomorrow.

Fair enough. I've just done a PSC measurement here (quite distant from origin of installation, through laziness) and get:

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• JohnW2
• 12 Jan 2012
• 2

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• JohnW2
• 12 Jan 2012
• 2

Note that 230/0.36=638.9 and that 246.8/0.36=685.6 ... so it appears the manual is telling the truth!

Kind Regards, John.

 

[EFLImpudence]

Right - very interesting.


Do you remember when you were asking similar questions about determining volt drop and whether to use actual voltage or nominal voltage?

I inadvertently found the answer in Appendix 12 BRB, now Appendix 4, 6.4 BGB., where it states that nominal voltage should be used.

Now you seem to be wanting the nominal voltage to alter so that it will be similar or equal to the actual voltage.

If nominal voltage in tables is stated as 230V., and you seem not to think this applies universally, are you suggesting that the tables (and other values) are adjusted to take account of different nominal voltages on a daily or installation (or whatever) basis?

How would you know if you had visited a premises on a particularly high or low voltage day?