Strange earth reading on ring main

[Proper Job]

Hi guys.
I have just tested a ring main. The readings end yo end on the 3 conductors were all fine. When I cross connected the live to neutral and tested around the sockets all of the readings were correct ie R1 + rn÷4. But when I tested with live earth cross conceded the readings got higher towards the middle. I deffinetly have the right ends crossed in the db as the cables are twin and earth.

 

[Proper Job]

I should have mentioned that it is a ring with no radials off of it.

 

[Pete999]

Twin and earth the cpc is somewhat smaller than the live conductors, hence your higher reading.
You test r1 r2 and rn end to end, cross connect r1 and r2

 

[Ian1981]

Hi guys.
I have just tested a ring main. The readings end yo end on the 3 conductors were all fine. When I cross connected the live to neutral and tested around the sockets all of the readings were correct ie R1 + rn÷4. But when I tested with live earth cross conceded the readings got higher towards the middle. I deffinetly have the right ends crossed in the db as the cables are twin and earth.

When cross connecting your ends to do a R1+R2 test on a ring final if you find that your readings start low near the source but increase the further away you go then decrease as you go back round towards the source again means you've got the ends mixed and are in fact testing the ring as 2 separate radials.
For a ring with 2.5mm and a 1.5mm cpc the readings for R1+R2 should be in the region of the live end to end added to cpc end to end /4.
The cpc end to end should be 1.67 times higher than the end to end of the live and neutrals.

Or just watch petes video post ha

 

[Proper Job]

But the thing is the I have traced the cablessings and am sure that I have the correct ends cross connected.
In a domestic environment yes tge cpc would be 1.67 times greater but when you take into account the parralel paths you get sometimes when you earth the back box which is connected to conduit which is attached to earth basked your readings do come down quiet a bit. ( but the circuit in question is not installed in this way)

 

[Ian1981]

But the thing is the I have traced the cablessings and am sure that I have the correct ends cross connected.
In a domestic environment yes tge cpc would be 1.67 times greater but when you take into account the parralel paths you get sometimes when you earth the back box which is connected to conduit which is attached to earth basked your readings do come down quiet a bit. ( but the circuit in question is not installed in this way)

Parallel paths shouldn't be an issue as the cpcs should be disconnected from the MET during continuity and when cross connecting with the Live conductors for R1+R2 as you state the wiring isn't installed in containment which might affect results.

Are the cpcs the same size as the live conductors then?
Is it installed in singles if not domestic?

 

[davesparks]

Parallel paths shouldn't be an issue as the cpcs should be disconnected from the MET during continuity and when cross connecting with the Live conductors for R1+R2 as you state the wiring isn't installed in containment which might affect results.

Are the cpcs the same size as the live conductors then?
Is it installed in singles if not domestic?

Parallel paths can still affect it as the parallel path could be between two or more sockets.
With the mass amount of bonding under the 15th/16th edition this could be anywhere.

 

[Ian1981]

Parallel paths can still affect it as the parallel path could be between two or more sockets.
With the mass amount of bonding under the 15th/16th edition this could be anywhere.

What bonding would be connected between sockets that increases your R1+R2 test results.
Op states the ends are definitely correct but I'm not so sure as the results he's getting don't make sense unless they are cross connected wrong or are mixed with another circuit

 

[westward10]

Does it supply the boiler also.

 

[Wilko]

Hi - if you've got your readings post em up
Maths wise, if L and CPC were the same size the value is the same at all places around the cross connected ring (as you perhaps have found with L and N testing? ). With the smaller cpc the equation is accurate at the centre of the ring and will decrease a little on either side of that point ... IIRC