high r1 reading

[ukpablo]

ok

doing a CU change yesterday, only one ring main which supplys about 12 sockets in house including kitchen.........................the rest of the house is all small radial ccts.
anyway, reading are

r1 = 0.78
rn = 0.34
r2 = 0.21 r2 x 1.67 = 0.35

so r1 is way out, r n is spot on as seen above. ok i joined r1 and r2 at board and proceeded to check skts around house to try and find high reading, (r1+r2/4 = 0.24) highest reading was 1.21 which was a spur.(this is gonig to be too high a reading as the max zs for a 32A is 1.16 and the ZE is 0.17) most other sockets came in at between 0.24 and 0.30.
Firstly i know the spur needs investiating
so where from here?...................there is obviously a jb somewhere, perhaps a loose terminal?
any ideas on how i can find it? could be anywhere i know.
this is only my 4th CU change and my testing is not bad too, but i still only have limited experience, so any pointers would be good?
cheers anyway

 

[Sintra]

Disconnect both of the offending r1 legs at cu. Then connect a wander lead to one of them and test to each point on the ring until the reading jumps. This will show you were the offending connection is. It could be that a cable is terminated onto the insulation.

 

[GStueyXR]

As Sintra said.

Or split the ring at the CU, connect each legs L&E, then split the ring in the middle and test R1+R2 of each leg (effectively Radials) to determine the leg with the fault. Then work back to the CU on the offending leg.

 

[malcolmsanford]

Are you sure you are right. If the cable is standard T+E of 2.5/1.5 then your r2 should be 1.67 times more than your r1 or rn. If it the old 2.5/1.0 then it would be even more.

I would have expected your rs to have been around 0.34*1.67 = 0.57.

 

[Octopus]

Is the circuit live? I was doing a PIR last week and could see from the R2 value on the wander lead circuit wasn't too great. I then did the earth loop impedance test on each socket, in turn and hey presto, the value climbed up, and up, then dropped suddenly - so I looked at the point with the highest value first and there I found the offending "BAD" connection.

Obviously I was lucky to be dealing with the R2 issue but the logical "approach" saved me time.

Good luck!

 

[ukpablo]

i will check again on the weekend as it was getting dark last night and was running out of time. i see ur point malcomsford regarding calcs. will re-check and let u all know when i re-visit. thanks all.

 

[dixon9]

Are you sure you are right. If the cable is standard T+E of 2.5/1.5 then your r2 should be 1.67 times more than your r1 or rn. If it the old 2.5/1.0 then it would be even more.

I would have expected your rs to have been around 0.34*1.67 = 0.57.

What he said...

(it´s your R2 that´s out)

...and Rn should be approx the same as R1


So, you should have:

R1 0.78
Rn 0.78
R2 1.3

OR (if R1 is out aswell)

R1 0.34
Rn 0.34
R2 0.57

 

[Deleted member 9648]

ok

doing a CU change yesterday, only one ring main which supplys about 12 sockets in house including kitchen.........................the rest of the house is all small radial ccts.
anyway, reading are

r1 = 0.78
rn = 0.34
r2 = 0.21 r2 x 1.67 = 0.35

so r1 is way out, r n is spot on as seen above. ok i joined r1 and r2 at board and proceeded to check skts around house to try and find high reading, (r1+r2/4 = 0.24) highest reading was 1.21 which was a spur.(this is gonig to be too high a reading as the max zs for a 32A is 1.16 and the ZE is 0.17) most other sockets came in at between 0.24 and 0.30.
Firstly i know the spur needs investiating
so where from here?...................there is obviously a jb somewhere, perhaps a loose terminal?
any ideas on how i can find it? could be anywhere i know.
this is only my 4th CU change and my testing is not bad too, but i still only have limited experience, so any pointers would be good?
cheers anyway

Presumably you have provided RCD protection in your new CU in which case the max Zs for the circuit is now 1667 ohms,so other tests being satisfactory you can ignore a Zs reading on the spur which slightly exceeds the maximum allowable for the MCB ,as the RCD will provide comliance with the required disconnection time.

 

[ukpablo]

thanks all........having a brain fart with this cct?! one of those days. Im re-visiting over weekend with fresh mind and will get to the bottom of it. I will let u know what i find and thanks all once again. Cheers wirepuller too, it is split rcd board, all up and running, just half of tests done so agree with comments about ZS.

cheers dixon9, i can see what he is saying. I have just been nominated for the burk awards 2010................again!

 

[GStueyXR]

So, you should have:

R1 0.78
Rn 0.78
R2 1.3

Or even,
r1 0.78
rn 0.78
r2 1.3