Shed straight concentric supply query

[EFLImpudence]

Am I correct in thinking that you (both) have (only) calculated what the thickness of the shielding foil would have to be to be adequate for a CPC?

Does this bear any resemblance to what the thickness actually is or are you assuming the manufacturer will have made it adequate for a CPC when that isn't really its purpose?

 

[JohnW2]

Am I correct in thinking that you (both) have (only) calculated what the thickness of the shielding foil would have to be to be adequate for a CPC?

I have simply calculated, to illustrate for you, what thickness a 12mm diameter 'shielding foil' would have to be to provide a 1.5mm², 2.5mm² or 4mm² CSA (those bing CSAs commonly seen as CPCs in cables).

I have not made any comment on it's "adequacy as a CPC" - but I think one can conclude that if a core of 1.5mm², 2.5mm² or 4mm² would be adequate as the CPC of a particular circuit, then a 'shielding foil' of the same CSA would also be adequate, couldn't one?

Does this bear any resemblance to what the thickness actually is ...

Not at all, without doing some research, I haven't got a clue as to how thick these 'foils' actually are.

.... or are you assuming the manufacturer will have made it adequate for a CPC when that isn't really its purpose?

I am making no assumptions about what is manufactured. As I said, I was just illustrating for you that pretty thin 'foils' could have as large a CSA as conventional CPC cores (e.g. a 12mm diameter foil the thickness of a hair from my head would provide about 1.5mm²).

The point is that BS7671 requires that a buried cable, if not 'mechanically protected', should have an outer earthed covering that would be "adequate as a CPC". Hence, for whatever circuit you're interested in, you have to determine what CSA (of the material in question) would be required (either 'by tradition' or by adiabatic calculation) and then ascertain whether the 'foil' in question would provide at least that CSA.

Kind Regards, John

 

[EFLImpudence]

Although no thickeness is stated, it would seem to be adequate:

http://www.batt.co.uk/upload/files/guardianali-tube-lszhcablebs8436300-500v_1362743208.pdf

 

[ban-all-sheds]

{Details of a method which seems an awful lot of work for the sake of improved accuracy to the values of fractions of microns when compared to 30 seconds work with a calculator}

 

[ban-all-sheds]

I have simply calculated, to illustrate for you, what thickness a 12mm diameter 'shielding foil' would have to be to provide a 1.5mm², 2.5mm² or 4mm² CSA (those bing CSAs commonly seen as CPCs in cables).

I have not made any comment on it's "adequacy as a CPC" - but I think one can conclude that if a core of 1.5mm², 2.5mm² or 4mm² would be adequate as the CPC of a particular circuit, then a 'shielding foil' of the same CSA would also be adequate, couldn't one?

One could.

If it were copper.

 

[JohnW2]

One could. If it were copper.

Indeed. As I went on to write (with a bit of added emboldening) ...

The point is that BS7671 requires that a buried cable, if not 'mechanically protected', should have an outer earthed covering that would be "adequate as a CPC". Hence, for whatever circuit you're interested in, you have to determine what CSA (of the material in question) would be required (either 'by tradition' or by adiabatic calculation) and then ascertain whether the 'foil' in question would provide at least that CSA.

Kind Regards, John

 

[JohnW2]

{Details of a method which seems an awful lot of work for the sake of improved accuracy to the values of fractions of microns when compared to 30 seconds work with a calculator}

Indeed. I suppose due to 'habit' and my inability to think laterally late at night! I'm accustomed to having to do such calculations with shapes much more complicated than circles, in which case 'subtracting the inner area from the outer one' is the natural way to do it - and, of course, probably the only sensible way when the difference in areas is substantial and the 'distance' between the two perimeters variable.

So, yes, I probably could have done it (your method) in 30 seconds, but what I did with the spreadsheet last night probably took no more than a couple of minutes, and writing the code today to 'check' the results not much longer.

Kind Regards, John

 

[JohnW2]

Although no thickeness is stated, it would seem to be adequate: ...

... for ali-tube cable, that is. However, the "ability to withstand fault currents" (i.e. the adiabatic part of it) is only part of "adequacy as a CPC". To be so "adequate", it obviously has to also have a low enough impedance for the Ze to be low enough for operation of the OPD - aluminium having an appreciably higher resistivity than copper means that one would expect that, for a given circuit, the CSA of aluminium required for that would be appreciably greater than the required CSA of a copper CPC.

Kind Regards, John

 

[EFLImpudence]

... for ali-tube cable, that is. However, the "ability to withstand fault currents" (i.e. the adiabatic part of it) is only part of "adequacy as a CPC". To be so "adequate", it obviously has to also have a low enough impedance for the Ze to be low enough for operation of the OPD

Yes, obviously - but you're doing it again. We were talking about the shielding and whether cables with it would be suitable for burying.

- aluminium having an appreciably higher resistivity than copper means that one would expect that, for a given circuit, the CSA of aluminium required for that would be appreciably greater than the required CSA of a copper CPC.

Yes, and so would a copper one several times longer than another.

 

[JohnW2]

Yes, obviously - but you're doing it again. We were talking about the shielding and whether cables with it would be suitable for burying.

Doing what again? Yes, we're talking about the suitability of cables for burying, and BS7671 seems to say that that requires that the outer earthed <whatever> be "adequate as a CPC". I can but assume that it means "adequate as a CPC for the cable/circuit concerned" and that will depend upon the resultant Ze as well as the conductor's ability to withstand the PFC during the period before the OPD operates.

Yes, and so would a copper one several times longer than another.

Indeed and, as we know, if the cable is several times longer, we might well have to use a higher CSA cable in order to achieve a low enough Ze. Admittedly, in practice, we would literally do that (use a higher CSA cable), so that the R1 (if not also R2) would be lower - but if the R1+R2 were not too high, we could theoretically achieve the required Ze by just increasing the effective CSA of the CPC (i.e. by adding an additional one in parallel with the one in the cable) - not that we would normally do that (not the least because R1+Rn might also be too high).

Kind Regards, John

 

[EFLImpudence]

Doing what again?

Introducing side issues in addition to the argument.

Yes, we're talking about the suitability of cables for burying, and BS7671 seems to say that that requires that the outer earthed <whatever> be "adequate as a CPC". I can but assume that it means "adequate as a CPC for the cable/circuit concerned" and that will depend upon the resultant Ze as well as the conductor's ability to withstand the PFC during the period before the OPD operates.

Well, of course it will (depend on Zs) but that is nothing to do with whether the cable is suitable for burying.
The same can be said for s.w.a. - or any cable anywhere.

As a point of interest, how does one determine if the foil covering is "adquate for a cpc"; it is in contact throughout its length with an actual cpc.?
If it were not, how would the foil be terminated?

Does that therefore mean it is NOT suitable for burying (without RCD coverage)?

Or, as I postulated, does it just mean that as long as it is adequate to cause ADS (when penetrated and live contact), it does not have to offer complete earth fault protection to the items at the load end?

 

[JohnW2]

Introducing side issues in addition to the argument.

It's not really a 'side issue'. It's a direct consequence of what BS7671 says about what cable can be buried.

Well, of course it will (depend on Zs) but that is nothing to do with whether the cable is suitable for burying.

As above, BS7671 says that it is only suitable for burying if the outher earthed <whateber> is "suitable as a CPC" (presumably a CPC of the cable/circuit concerned) - which will depend, amongst other things, on whether its resistance is low enough for Zs to be compliant.

As a point of interest, how does one determine if the foil covering is "adquate for a cpc";

I have taken it to mean something like "would be suitable as the sole CPC of the cable/circuit". As such, it has to have an adequately low resistance and be able to tolerate the PFC.

... it is in contact throughout its length with an actual cpc.? If it were not, how would the foil be terminated?

I don't really understand that.

... Or, as I postulated, does it just mean that as long as it is adequate to cause ADS (when penetrated and live contact), it does not have to offer complete earth fault protection to the items at the load end?

It could be penetrated anywhere along its length - including very close to "the items at the load end", so the requirements would presumably be the same as those required "at the load end" of the cable, wouldn't they?

Kind Regards, John

 

[EFLImpudence]

It's not really a 'side issue'. It's a direct consequence of what BS7671 says about what cable can be buried.

No it isn't.

As above, BS7671 says that it is only suitable for burying if the outher earthed <whateber> is "suitable as a CPC" (presumably a CPC of the cable/circuit concerned) - which will depend, amongst other things, on whether its resistance is low enough for Zs to be compliant.

Yes, I know. That's obvious. It applies to s.w.a. as well but s.w.a. is suitable for burying.

I have taken it to mean something like "would be suitable as the sole CPC of the cable/circuit".

I know you have but is that correct?

As such, it has to have an adequately low resistance and be able to tolerate the PFC.

Don't keep on about the resistance. That applies to every cable.

I don't really understand that.

The foil in question is in contact with an actual wire c.p.c throughought its length, if the foil was to be the sole c.p.c. how would it be terminated - being just flimsy foil.

It could be penetrated anywhere along its length - including very close to "the items at the load end", so the requirements would presumably be the same as those required "at the load end" of the cable, wouldn't they?

The foil in question is in contact with an actual wire c.p.c throughought its length, so only has to connect a penetrating nail to that very nearby c.p.c.

 

[JohnW2]

The foil in question is in contact with an actual wire c.p.c throughought its length, if the foil was to be the sole c.p.c. how would it be terminated - being just flimsy foil. .... The foil in question is in contact with an actual wire c.p.c throughought its length, so only has to connect a penetrating nail to that very nearby c.p.c.

You appear to have shifted this discussion from a very general one (about what cables can be buried) to one that relates specifically so some particular cable.

What particular cable do you have in mind? SY, for example has an insulated CPC, within another 'insulating' sheath - so two layers of plastic between the CPC core and the outer "wire braid" which one earths.

Kind Regards, John

 

[ban-all-sheds]

It's not really a 'side issue'. It's a direct consequence of what BS7671 says about what cable can be buried.

Do you intend to ignore the fact that as well as suitability as a cpc, it also says that the cable must be armoured or have a metal sheath?

Ali-tube and SY cables are/have neither, and therefore no matter in what qualitative or quantitative ways their screens or braids are suitable as a cpc, they may not be directly buried.