Maths and Java Script any one tried it?

[ericmark]

Sorry my point is a ring with 106 meters of cable will have a Zs of 1.50 Ω

Doesn't 106m of 2.5/1.5 have an r1+r2 of 19.51x106/1000 = 2.068 / 4 = R1+R2 of 0.52Ω x 1.2Ct = 0.62Ω.

Even with a Ze of 0.8 (unlikely) that only results in a Zs of 1.42Ω.

I will revisit and check. It does seem there is an error which is why I posted to see if any errors.

I think I have forgot to divide the Ω by 2 before adding line and earth together as tables give feed and return Ω combined.

 

[ericmark]

It would seem with second calculator the original the Zs was not calculated but just a figure entered as a start point with the modified version of calculator I have added a calculation to give Zs.

I have added standard values for a ring final so just clicking on calculate should give a result but user can change the values.

With the first calculator I had made an error with ring divided by 2 instead of 4 in one part now corrected. It would seem my Excel version also incorrect same fault but since not shared no one had found it.

My thanks to EFLImpudence for finding error and JohnW2 for confirming error.

 

[JohnW2]

It would seem with second calculator the original the Zs was not calculated but just a figure entered as a start point with the modified version of calculator I have added a calculation to give Zs.

Ah yes, I should have remembered that - I previously pointed out that the calculator, as it was, did not seem very useful for design purposes, since one had to enter a Zs (as well as Ze) value for the circuit one hadn’t yet designed!

With the first calculator I had made an error with ring divided by 2 instead of 4 in one part now corrected. It would seem my Excel version also incorrect same fault but since not shared no one had found it.

As I suspected. Thanks for correcting it.

I’ll make a few more comments about your revised calculator in a separate post.

Kind Regards, John

 

[ericmark]

As I suspected. Thanks for correcting it.

No it's thank you for pointing out the error. I did put the prospective Zs reading as well as the prospective short circuit current at circuit end which use Ze as start for Zs and prospective short circuit current at circuit start for prospective short circuit current at circuit end.

I have seldom sat down and worked out the figures for a circuit I do remember one where I said to foreman I thought 4mm was too small for distance involved but did not work it out and after problems with shrink wrap machine only in that location it was re-wired with 10mm which cured the problem. I really think that is the only time where I can say volt drop caused the problem.

OK there have been temps with florescent lighting but to work out with a lamp every 6 foot using 25 lights on a 16A supply what the volt drop would be just seemed pointless it was done by trial and error all lamps set to 127 volt tapping and if they did not work swapped to 110 volt tapping.

As I look back I remember a job my son did wiring up new houses. At the start the builder used beams with pre-made knock out holes so wiring did not go to centre of room. However seems these were too expensive and it was changed to using standard wood beams so 1/3 rule came in so far more cable was required. Not until 3 houses had been completed was it realised the cable used was too much and future houses had a kitchen ring to reduce the cable used.

I had assumed it was ELI but looking now maybe it was volt drop which was problem.

Although one can work it out I wonder how many do and is it really a problem. On a caravan site 1990's I have problems with volt drop when down to 190 volts at times but this was the days of power supply using simple transformer rectifier and regulator and volt drop caused hum. But today with switch mode power supplies likely even at 180 volt everything would still work OK?

My daughter had the fuel tank changed FOC on her 10 year old car because of a design fault and a re-call. I wonder with house wiring could we see in the future builders or electricians brought back to correct design faults? Years ago there was no record of what had been done but today with all the installation certificates being sent to LABC I suppose one could like a car years latter demand faults are corrected?

 

[JohnW2]

It would seem with second calculator the original the Zs was not calculated but just a figure entered as a start point with the modified version of calculator I have added a calculation to give Zs.

In accordance with what I said earlier, think that the revised format is much more useful. One can now enter Ze (or PSCC at CU, as you have now done it), cable details and circuit length and get calculated figures for max circuit VD and Zs (and PSCC). It’s no longer possible to calculate max circuit length from other inputs, but I never really saw much practical point in that, anyway.

You have added a note indicating that the inputted Ze figure is “No longer used for anything” but, as the calculator currently is, that is not true. The inputted Ze is currently a crucial part of the calculation of the circuit Zs. You need to input Ze (or, if you prefer, PEFR at CU - what some call Zdb)

Now that it is an ‘output’ (only), Zs would probably be best moved to the bottom, with the other ‘outputs’ - and, similarly, ‘cable length’ (now only an input) should probably be ‘moved up’. More generally, I would be inclined to put all the inputs first, then the outputs. Some of the displayed figures do not seem very useful (e.g. ‘Impedance of Live Conductors’, ‘mV/A/m’ figures etc.), and could perhaps be removed.

As I wrote last night, for a TN installation, I don’t really see any point in having “PSCC at circuit end” as an output. What you do need as an output is Zs (or PEFC) ‘at circuit end’ - and, as I said, if that satisfies the disconnection time requirements (either in terms of Zs or PEFC) for the circuit’s protective device, then so, inevitably, also will the PSCC. However, it’s probably worth retaining that facility just for TT installations, where Zs requirements will not usually be satisfied (but the PSCC ones still need to be). As for confirming that the PSCC is below the 6 kA, 10 kA or whatever limit of the device, PSCC will obviously be at it’s maximum at the CU, and you don’t need to calculate that, since it is one of the figures you have to input.

The ‘tabulated CCCs’ displayed for a radial circuit appear to be those for Method 100 - it should probably say so. I’m not sure how you have calculated the ‘tabulated CCCs’ in the case of radial circuits, and I wonder if it wouldn’t be best to suppress that output, to avoid confusing people? In fact, you could simply display the ‘tabulated CCC of (one instance of) the cable in use’ - i.e. as per the current figures for radials.

AFAICS, it appears that you, EFLI and I are all using slightly different methods of arriving at conductor resistivities at 70°C, which leads to us all getting slightly different answers (for Zs etc.). I’m not sure which is the most appropriate/correct, but it might be worth looking into that.

As mentioned last night, you probably should point out that the ‘design current’ being asked for is not Ib in the case of a ring - but, rather, a VD-specific ‘effective design current’. If you wanted to get more clever, you could ask about the circuit’s OPD and then work it out for itself. You probably should point out (if you retain them) that the displayed ‘mV/A/m’ figures relate to the live conductors, not CPC.

For radials, you appear to be calculating VD on the basis of all the (‘design current’) load being at the far end. As I wrote yesterday, is that correct (i.e. no ‘convention’ about load spreading, as per rings)? Do I take it that the intended ‘design current’ for a radial is the In of the OPD?

Hope some of that might be helpful. If I think of more, I’ll let you know.

Kind Regards, John

 

[JohnW2]

I have seldom sat down and worked out the figures for a circuit I do remember one where I said to foreman I thought 4mm was too small for distance involved but did not work it out and after problems with shrink wrap machine only in that location it was re-wired with 10mm which cured the problem. I really think that is the only time where I can say volt drop caused the problem.

Quite so - I think that (within reason!), this VD business is little more than academic, at least for domestic installations. Don't forget that (despite the 'nominal 230V') the great majority of UK domestic installations have a supply of at least 240V, which means that their 'starting point' is at least 6% above 'what it could be' - which certainly makes one wonder about the 5%/3% suggested 'limits' for VD. I suppose that rules and recommendations have to cater for the possibility that the supply could theoretically be as low as 216.2V, but I think there is probably a limit to how far one can/should sensibly go.

Kind Regards, John

 

[EFLImpudence]

Re: different values being used:

Unless you are designing an OSG calculator, i.e. worst case, no thought, then I would prefer the 'basic' values to be used.
That is the absolute maximum (BGB) values for Zs and
the 20° values for conductor resistance.

To correct these values, Zs / 1.2 or R1+R2 x 1.2, for temperature in every case is to take no account of the actual conditions that will be encountered.

Much better, in my opinion, to allow individual adjustments as required.

 

[JohnW2]

Re: different values being used: Unless you are designing an OSG calculator, i.e. worst case, no though ....

I'm not sure I understand in what sense the OSG approach is 'worst case, no thought' - I would have thought it was the opposite. All of the 'maximum Zs' figures given in Appendix 2 fthe OSG are, for reasons best known to the authors, based on an assumption of conductor temperature of 10°C. That means that if one designs a circuit which just satisfies the OSG Zs requirement, then Zs is likely to be too high (to satisfy required disconnection times) at most ambient (let alone 'running') temperatures likely to be experienced in many/most UK domestic situations for much of the time. ... so it seems more like 'best case' than 'worst case' to me - am I missing something? Edit: Yes, I was missing something. Most of this is wrong (the OSG IS being very conservative) - see subsequent post from me

... then I would prefer the 'basic' values to be used. .... That is the absolute maximum (BGB) values for Zs and the 20° values for conductor resistance.

Is that not a bit not inconsistent? I presume that by "absolute maximum (BGB) values for Zs" you are referring to those given in Tables 41.2-41.4 of the BGB. If so, and given that those tables assume an operating temperature of 70°C, why use 20°C resistance figures to calculate whether the circuit satisfies the 'maximum Zs' requirements (which assume a 70°C conductor temperature) specified in those Tables?

To correct these values, Zs / 1.2 or R1+R2 x 1.2, for temperature in every case is to take no account of the actual conditions that will be encountered.

That is essentially true - but since one is often not going to know, with any certainty, what the conductor temperature will be, is the not 'safest' ('most conservative') approach to assume that the conductors might be running at their 'maximum operating temperature? [the OSG, of course, uses a 1.24 correction factor, since they are going from 70°C figures {in Tables 41.2-41.4} to 10°C ones]

Having said all this, I think that the resistivity temperature corrections undertaken by eric's calculator are even more complicated - perhaps based on Equation 6 in 6.1 of Appendix 4 of the BGB.

Kind Regards, John

 

[ericmark]

I have used 13, 16, 21, 27, 34, and 45 amp for 1, 1.5, 2.5, 4, 6, 10, and 16 mm² I suppose I could extend to allow one to select a few different installation methods now up and running. I am open to suggestions.

I have as suggested moved things around inputs first then separated by calculate button the outputs.

It's been a good exercise ups missed one I'll alter that before posting. Correction factor Ct is an output only. I have included only really to help fault find.

And yes I used the calculation out of BS7671:2008 page 258 under 6.1 to correct. At time I did my C&G 2382 the OSG had not been printed so never bought one.

 

[EFLImpudence]

I'm not sure I understand in what sense the OSG approach is 'worst case, no thought' - I would have thought it was the opposite. All of the 'maximum Zs' figures given in Appendix 2 fthe OSG are, for reasons best known to the authors, based on an assumption of conductor temperature of 10°C.

Yes, but the OSG maximum Zs figures are 'corrected'.

Is that not a bit not inconsistent? I presume that by "absolute maximum (BGB) values for Zs" you are referring to those given in Tables 41.2-41.4 of the BGB. If so, and given that those tables assume an operating temperature of 70°C,

No, they are 20° aren't they? The same as the resistance values I use.

That is essentially true - but since one is often not going to know, with any certainty, what the conductor temperature will be, is the not 'safest'

Well, in that case you can correct to 70°.

My point is that in many cases it will not be necessary at all - lighting circuits - or to a lesser degree - 45A shower on 10mm² cable, immersion on 2.5mm² etc.

 

[ericmark]

You can write in your own figures with anything above the calculation button. If I was going to offer a second installation method what would you want?

 

[ericmark]

Maybe I can add on the bottom now designed with the assistance of EFLImpudence and JohnW2 since you have put in so much time and effort in honing it to it's present state?

 

[JohnW2]

I'm not sure I understand in what sense the OSG approach is 'worst case, no thought' - I would have thought it was the opposite. All of the 'maximum Zs' figures given in Appendix 2 fthe OSG are, for reasons best known to the authors, based on an assumption of conductor temperature of 10°C.

Yes, but the OSG maximum Zs figures are 'corrected'.

Maybe I (or perhaps most of us!) are getting confused. My understanding is that the OSG 'maximum OSG' figures are 'corrected' to an assumption of 10 degree conductor temperature.

Is that not a bit not inconsistent? I presume that by "absolute maximum (BGB) values for Zs" you are referring to those given in Tables 41.2-41.4 of the BGB. If so, and given that those tables assume an operating temperature of 70°C,

No, they are 20° aren't they?

I guess that depends upon interpretation. The footnote to Table 41.3 (the one relating to MCBs) reads "The circuit loop impedances given in the table should not be exceeded when the conductors are at their normal operating temperature". I suppose that there can be some debate as to what 'normal operating temperature' means, but, given the uncertainties, I would think the only safe figure to use would be the maximum permitted operating temperature. The OSG certainly believes, and states, that BGB Table 41.3 relates to 70° conductor temperature, which is why they have used a correction factor of 1.24 to get to the 10° figures included in their Table.

The same as the resistance values I use.

As above, if my (and the OSG's) interpretation were correct, then it would be different, not the same - hence my comment about 'inconsistency'.

That is essentially true - but since one is often not going to know, with any certainty, what the conductor temperature will be, is the not 'safest'

Well, in that case you can correct to 70°. My point is that in many cases it will not be necessary at all - lighting circuits - or to a lesser degree - 45A shower on 10mm² cable, immersion on 2.5mm² etc.

You seem to be assuming that the current through the conductors is the only thing which determines their temperature. The temperature in my roofspace, where much lighting, as well as some shower and immersion cables run, has been known to be well over 50° even before any current starts flowing through cables. As I said, given the uncertainties, I think the only safe/conservative approach in many/most cases is to assume that (for whatever reason) the conductor temperature could be as high as the maximum permissible for the cable.

Kind Regards, John

 

[JohnW2]

I'm not sure I understand in what sense the OSG approach is 'worst case, no thought' - I would have thought it was the opposite. All of the 'maximum Zs' figures given in Appendix 2 fthe OSG are, for reasons best known to the authors, based on an assumption of conductor temperature of 10°C.

Yes, but the OSG maximum Zs figures are 'corrected'.

I've pondered some more, and on reflection, I think you are right - and that the OSG is being pretty conservative.

In contrast to what I previously wrote (some of which was plain wrong) .... the OSG is saying that if you measure Zs when the conductor temperature is 10°C (or higher), then if the value is below their tabulated 'maximum Zs' figure (which is lower than the BGB one by a factor of 1.24), then that will guarantee that the Zs will be below the BGB figure (Tables 41.2-41.4) even if the conductor temp rises to 70°C.

Apologies for any confusion caused by what I previously wrote (I have added an edit to my original post accordingly)

Kind Regards, John

 

[ericmark]

How about this version where you can select installation method? Not sure really helps and as yet only done for live cables not done earth so will likely be slightly out just an idea.