Lo Volts

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

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

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

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.

Usually from the main fuse.

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

bunglalows these days?

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;!....

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.

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

Hi, This was AC..

This was AC.....

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.

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

James

Oops, thanks.

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

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

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.