Mains voltage

[JohnW2]

My first thought would be that the resistance may not be uniformly spread along the supply to your house. I'd imagine that the main supply cable feeding multiple houses has a much bigger cross section and lower resistance than the cable running from it to each individual house.

That's true, but I don't think it's consequences are adequate to explain the apparent 'anomaly' (aka a lack of understanding on my part!) I mentioned, at least in cases like my house, and many others like it.

The run of (smaller) cable from my house to the (larger) distribution cable (which happens to be overhead, so I can see it) is quite short, very much shorter than the run of that distribution cable from where mine joins it to the transformer. I would therefore imagine that the majority of the L-N loop impedance I see from my house is attributable to the distribution cable - along which all current from installations downstream of mine (on the same phase) has to run.

When running your 10.5KW shower it could well be that most of the voltage drop occurs on the section of cable feeding your house from the main supply, and only a small part of the voltage drop is occurring on the larger main supply cable.

See above. Whilst I understand what you are saying, and agree that it is a 'factor', I don't think it's an adequate explanation for installations that have a short length of (admitted higher resistivity) connecting them to the distribution cable.

EDIT: Another thought is how the main supply cables from the transformer are run. Are they simple radials from the transformer with branches out to houses along its path ...

to the best of my knowledge, yes (but, of course, I know that there's 'something wrong with my knowledge'!).

, or maybe some kind of ring formation from the transformer which would also reduce resistance and the voltage drop on the main supply cable.

As above, I think (same caveat as above!) not ('in normal service') . In any event, again in relation to my particular supply, I can see the (overhead) 'main distribution cable' all the way to the end of the run, and it clearly is not a ring.

I know that there are junctions on supply cables that allow DNOs to rearrange supplies to bypass sections if needed, I believe there is one such junction in a manhole not far from my house.

Indeed, but my understanding (caveat again!) was that they were not left as rings when in normal service.

Does anyone know the CSA of a typical LV distribution cable. If we knew that, we could roughly estimate the contributions (to resistance/impedance) of the parts of the total cable run.

However, as per all the 'caveats' above, I know that there is something wrong with my reasoning, so some (maybe all!) of what I've written above may be wrong, maybe even nonsense

Kind Regards, John

 

[reds42]

Does anyone know the CSA of a typical LV distribution cable. If we knew that, we could roughly estimate the contributions (to resistance/impedance) of the parts of the total cable run.

Quick google found this document from electricity northwest on overhead supplies. Makes mention of 35mm2 and 95mm2 cables.

 

[Harry Bloomfield]

In many place the 25kV overhead traction power for new electrification schemes is "extracted" from the existing local supply network.

OK, I thought that the railways had there own entirely separate and independant supply network.

 

[Harry Bloomfield]

to the best of my knowledge, yes (but, of course, I know that there's 'something wrong with my knowledge'!).

I was always of the opinion that they are usually a ring, like a ring main 13amp socket circuit for many domestic services. Thus a cable fault in one section, leaves the remainder still in operation.

 

[JohnW2]

I was always of the opinion that they are usually a ring, like a ring main 13amp socket circuit for many domestic services. Thus a cable fault in one section, leaves the remainder still in operation.

That isn't my understanding (albeit my 'understanding' is often wrong!), and I'm not even sure that it would be considered 'safe' - since (like ring final circuits) it would carry the risk that a break in the ring could go unnoticed yet lead the cables overloaded - and their might also be issues in relation to 'safe isolation'. Furthermore, as I've said, in the case of overhead LV distribution networks, one only needs eyes to see that they are not rings.

My understanding (caveat again!) was that 'ring mains' are used in HV distribution networks, not LV ones.

I'm sure someone will know the answer - this is where we need late-lamented Westie!

Kind Regards, John.

 

[JohnW2]

Quick google found this document from electricity northwest on overhead supplies. Makes mention of 35mm2 and 95mm2 cables.

Many thanks. However, having said that we needed that information, I suddenly realised that we don't really need it, since (given a known L-N loop impedance) it's only the bit of cable from the distribution cable to the consumer which matters, and, for illustration, I'm going to assume it is 16mm² - which I think is common, and is certainly roughly what mine looks like.

However, for what it's worth, and since you've gone to the trouble of providing the information which I (unnecessarily!) asked for, looking at my 'main' distribution cable (which is, indeed, ABC), I find it very hard to believe that its 95mm², and I would probably have guessed that it was around 35mm², so perhaps that's what it is!

Pulling numbers out of the air for illustration, consider a 10 metre feed to an installation from a distribution cable that has a run of 100 metres back to the transformer - which, I would suggest, is far from an impossible, or even uncommon, scenario - and, indeed, may be a reasonably stab at what mine is.

10 metres of 16mm² has a resistance/impedance of about 0.028Ω, so if there is an L-N loop impedance of, say, 0.30Ω (like mine), that leaves 0.272Ω as the impedance of the distribution cable back to the tranny (from where my installation connects to that main cable). Onset of a current of 44A from a downstream consumer's shower therefore ought to result in an increase in voltage drop (as seen in my installation) of about 12V (which way exceeds any drop in voltage I've ever seen), shouldn’t it?

Kind Regards, John

 

[Harry Bloomfield]

and their might also be issues in relation to 'safe isolation'.

If they do need to isolate, they trip the breaker or pull the fuse, which feeds both ends of the ring, but they usually work live, even jointing or tapping into cables. I'm fairly sure we are on a ring, up one side of the street, back down the other, because over the years we have suffered several cable faults. The last one I reported as a regular brown out - it was a cable between us and the sub, which was festering. Lots of steam coming out of the pavement, I know they had to cut the cable and joint a section in, but at no time did we loose supply.

 

[reds42]

I wonder how much of the L N loop impedance occurs within the transformer windings?

 

[JohnW2]

If they do need to isolate, they trip the breaker or pull the fuse, which feeds both ends of the ring, ...

As I said, I'm certainly open to being corrected if necessary, but it has always been my belief ('understanding') that they do not use rings in their LV networks (only in HV ones - the ones that used to be called MV).

... The last one I reported as a regular brown out - it was a cable between us and the sub, which was festering. Lots of steam coming out of the pavement, I know they had to cut the cable and joint a section in, but at no time did we loose supply.

I agree that would seem to support what you're saying but, as I've said, if it's true it is contrary to whatever I have always believed.

We unfortunately cannot resurrect westie, but there must be someone out there wo knows?

Kind Regards, John

 

[JohnW2]

I wonder how much of the L N loop impedance occurs within the transformer windings?

Thanks! Now that's an interesting suggestion (which I don't think has ever occurred to me before), which may actually go a fair way, or maybe the whole way, to providing the answer to my question!

In fact, in terms of the figures I was quoting, and if it were 35mm² cable, ~100 metres of that would only have a resistance/impedance of about 0.125Ω - nowhere near enough to explain my ~0.30Ω loop impedance.

Mind you, I'm not sure that it would necessarily be the 'whole' answer, since even 0.125Ω would result in a 5.5V voltage drop when that 44A downstream shower was switched on, and I don't think I've personally noticed any sudden changes even that great (although Adrian's graph does show some). .. but, there again, maybe the cables I've experienced are all larger than 35mm² !

Thanks again.

Kind Regards, John

 

[reds42]

maybe the cables I've experienced are all larger than 35mm² !

I also found this document from Scottish Power which I found interesting.

When I had my house supply unlooped from next door the DNO dug up the main supply cable so that they could run a new supply to next door, and from looking at the pictures in this document I'm quite confident that the main cable I could see in the hole was paper insulated lead covered steel tape armoured (I have a paper insulated lead covered service cable which would be consistent with the times this would of been installed).

The document says this type of cable was available in 70-300mm2 with outer diameters of 38.4-66.9mm. I would say the cable I could see in the hole was at least 60mm diameter so I could imagine its CSA could be in the 200mm2 region.

 

[Harry Bloomfield]

The document says this type of cable was available in 70-300mm2 with outer diameters of 38.4-66.9mm. I would say the cable I could see in the hole was at least 60mm diameter so I could imagine its CSA could be in the 200mm2 region.

Except it would have been an Imperial cable size, if paper.

 

[reds42]

Except it would have been an Imperial cable size, if paper.

I assume they are applying metric conversion in the document.

 

[Harry Bloomfield]

I assume they are applying metric conversion in the document.

Yep. From memory the metric sizes would be 280mm 340mm.

 

[JohnW2]

I also found this document from Scottish Power which I found interesting.
When I had my house supply unlooped from next door the DNO dug up the main supply cable so that they could run a new supply to next door, and from looking at the pictures in this document I'm quite confident that the main cable I could see in the hole was paper insulated lead covered steel tape armoured (I have a paper insulated lead covered service cable which would be consistent with the times this would of been installed).
The document says this type of cable was available in 70-300mm2 with outer diameters of 38.4-66.9mm. I would say the cable I could see in the hole was at least 60mm diameter so I could imagine its CSA could be in the 200mm2 region.

Thanks again. I don't doubt that some of the underground LV network cables are pretty big - ones I have occasionally seen 'down holes' when DNOs have been working have appeared pretty meaty.

However, my overhead supply is clearly not using such massive cables (yet, as observed, I don't see very large fluctuations in supply voltage. The photo below shows the last (most downstream) pole for the cable that supplies me. For a start, for some who have been commenting, it's clearly not a 'ring' (but I suppose someone wanting to continue the argument might suggest that it could be 'a spur from a ring' )

However, the main point of the phot is that the incoming ABC cable (the one coming from bottom right of pic) is not vastly bigger than the ones feeding individual installations from it, which I imagine are at least 16mm². Maybe I'm wrong, but I find it rather hard to believe that the 'main' feed cable (which looks the same throughout it's length) is bigger than 35mm² (maybe 50mm² at the very most). What do you think?

Kind regards, John