Lo Volts

[The Wanderer]

I would, especially if the properties were planned for gas CH.

As someone pointed out, the (ESI) planning standard when installing new services on an estate was R-Y-B-B-Y-R-R-Y-B and so on. I can't remember the theory now, but this gives better load distribution than R-Y-B-R-Y-B-R-Y-B.

Dunno if it's still the case but 4 way service joints were generally the maximum, due to the physical limitations of making the connections inside the joint.

If the houses were gas CH then an individual service could well be looped to the next property, especially if the properties were mirrored so the meter positions of adjoining properties are close together. Distribution of phases along the main then becomes R-R-Y-Y-B-B-B-B-Y-Y-R-R-R-R-Y-Y etc.

This would be most prevalent in high density housing developments. Lower density, the dwellings are obviously spread out more, and it becomes less easy to maintain the sequence depending on mains cable routing.

[ARWadsworth]

Looking at the lights that go off in a powercut on our street it seems to be R-R-Y-Y-B-B-R-R-Y-Y-B-B etc The incoming supply runs down the garden between a pair of semis, feeds one cut out and then feeds back across the garden to the next semis cutout.

Adam

[The Wanderer]

May well be, the sequence I set out was how it was done in Southern.

[Dave Plowman (News)]

Is the looping normally visible - ie at the riser - or underground?

[ARWadsworth]

On my street, 50 year old TNS supplies the looping is done at the riser/cutout next to the meter. So in my house I have the incoming supply and then the outgoing one to next door.

Two streets away and slightly older semi detatched houses they use a PME supply. The supply is run externally and feed 4 houses. The joints on this are external.

The link below shows one of the joints and the cable linking between 2 semis if you zoom in. You can see the joint on the left hand semi and also the cable that runs around the back to feed the other neighbour.

[The Wanderer]

Usually from the main fuse.

[Dave Plowman (News)]

Right - this one wasn't - only the one cable to the riser. But was the end of the block.

[Andy Burns]

bunglalows these days?

[Andy Burns]

[tony sayer]

Near here a bit out in the sticks a man lost his supply, well almost lost it the neutral had fallen off the distribution line and was laying on the damp grass and he had some rather dim lighting the live sort of returning to the transformer that way....

He thought if it was a bit wetter then it might be brighter as well.. water sorta conducts. So it was watered and the lights did indeed get a lot brighter and it was like this several days till I saw him and almost shat myself with what had happened.. And so did the blokes from the power company when they turned up;!....

[James Salisbury]

Hi, When I was on holiday in Canada, I saw lots of installations with only one HV line providing power out in the sticks between Quebec and Montreal... must have been earth return single phase.

[tony sayer]

Yes I did read about that somewhere, seems the earth at that sort of voltage and DC was quite suitable for that purpose....

[James Salisbury]

Hi, This was AC..

[James Salisbury]

This was AC.....

[Dave Liquorice]

The impedance of The Earth is effectively zero, making good connection to it is the tricky bit. In the days of trams and traction power return via the tracks they initially weren't very bothered about making sure the tracks had decent electrical continuity. The up shot being that the traction power would find the path of least resistance, sometimes down the lead sheaths of telephone cables buried near by until the GPO started making a fuss.

[Bob Eager]

the bit under 'The Tram'...!

[James Salisbury]

It is still a modern problem, the CERN tunnels do suffer from surface traction current ingress....

James

[Andy Burns]

[Bob Eager]

Oops, thanks.

People used to go bathing near a running one because they heard the electricity was good for them...

[John Rumm]

But who is guilty of the stoopthink here I wonder? There is more to be concerned about than just catching fire.

As an example, take a class one appliance plugged into a spur from a ring circuit with an earth loop impedance of 1.1 ohms. Circuit protection is a 32A type B MCB.

Prospective fault current at normal voltage is 230/1.1 = 209A, or well over the 160A required to open the MCB in 0.1 secs.

Let's say we have a serious undervolt with the supply at 100V. Our prospective fault current now falls to only 90A. According to Fig 3.3B in the 17th edtn of BS7671, that gives a fault clearance time of about

45 seconds. That alone should give cause for concern, since the fault clearance time is now well out of either the 0.4 or 5 sec limits allowed.

However, it gets worse....

With normal voltage, in the even of an earth fault, we can calculate the minimum CSA of CPC required to clear the fault without failing:

min CSA CPC = sqrt( 209^2 x 0.1 ) / 115 = 0.6mm^2

Which is well under the 1.5mm^2 available on a spur connected by a single run of 2.5mm^2 T&E with 1.5mm^2 CPC.

However if we repeat the above calculation for the undervolt situation we now get:

min CSA CPC = sqrt( 90^2 x 45 ) / 115 = 5.25mm^2

Well in excess of even the combined 3mm^2 available on the ring, never mind the spur.

So not only do we have a dangerous touch voltage present for an extended period, the chances are the CPC in the cable will fail before it gets cleared. Resulting in electrocution risk, cable damage, and fire risk.