Quick Ring Question

[wattsy]

Hi All,

Testing due to Consumer unit change

r1 = 0.26
rn = 0.26
r2 = 0.46
R1+R2 = 0.19

Two of the sockets close to the consumer unit had lower than expected R1+R2 readings of 0.07 & 0.14

I can see how this is possible can someone shed some light on this for me please.

Thanks

 

[Richard Burns]

If you have the two sockets as spurs off the ring but connected at the CU then the resistance will be lower.
Alternatively if you were testing the figure 8 test with the incoming cables connected together and the outgoing cables connected together rather than the incoming and outgoing crossed over then the results close to the CU would be lower than those at the mid point of the ring.

 

[wattsy]

They aren't two spurs.

I know the readings will be higher towards the mid point of the ring but one was 0.07!

r1 + r2 /4 = 0.18 ohms

the resistance at the board where the cross connections were made was 0.18 ohms

I've even be drawing picture to see if i can figure out what is going on!

 

[Richard Burns]

Did you connect the incoming line with the outgoing cpc and the incoming cpc with the outgoing line or
did you connect the incoming line with the incoming cpc and the outgoing line with the outgoing cpc

If you did the first one then the resistance will be the same at each socket outlet.
If you did the second then the resistance will be low close to the CU and higher as you go out along the ring.

 

[wattsy]

I know what your saying Richard but the test has been carried out to the book so to speak.

The resistance readings are as to be expected from all of the other socket outlets, 0.2 ohms at the furthest point away from the board 0.18 at the board. Except for the 2 in question of which one is half of what is expected from calculation.

 

[Richard Burns]

If the test has been done according to the prescribed methods and the resistance at every socket outlet is 0.18 ohms except for these two sockets and they are not spurs then you have discovered a new law of physics, well done.
Sorry but there must be something different here.
if the ring is a long one with two sockets close by and a few sockets far away then you could get these results with the wrong crossover since the far sockets would be about right but the near sockets would be low.
Other than that I cannot determine a reason unless the close sockets have an additional cpc back to the board.

 

[wattsy]

Sounds like i've fired you up chap, sorry if i've offended you.

It's not not 0.18 all the way around it's 0.2 at the furthest point as i stated in my last post.

How would an additional cpc back to the board effect an R1+R2 Test?

 

[Richard Burns]

If you had an additional cpc just to the first two sockets then this would give a low resistance path which would drop the overall measured resistance by adding a parallel resistor however you would have had to included this in your cross over so it is unlikely to be a reality that you missed.

If your result is 0.2 ohms at the midpoint of the ring this is not conclusive since it is close and within measurement limits.
0.02 ohms difference could easily be a slightly different probe connection resistance.

If you describe how the ring is laid out this may help to explain the readings.
I am still going with incorrect crossover at the moment.

 

[wattsy]

I have in the past incorrectly crossed over the conductors in a board that was dressed like a birds nest, and it became apparent immediately that they weren't cross connected because the readings were way out based on the end to end resistances.

It's a 2 bed bungalow with 2 rings one for the kitchen and one for the rest of the property.
Board above the front door in the hallway, cables drop down somewhere and sockets are fed from under the floor. There are sighs of alteration and DIY. Found a switched fuse under the floor for instance.

 

[Richard Burns]

OK so if there were bed 2 sockets going direct into the CU then someone put a junction box in and run a wire to the 0.07 socket and the 0.14 socket and then junctioned back into the ring as the other cable goes out to the lounge this might do it.
Haven't thought it through as I seem to remember cross connected rings read OK.

 

[dagrat]

I don't think you have any problems with those results. When testing a ring for continuity, due to the difference in CSA of the cpc and the line conductor differences up to 0.05ohms are to be expected. It is therefore only the 0.07 reading that appears too good to be true. I would worry more if the resistance readings were higher.

Incidentally, what was the reading at the joined ends at the consumer unit?

 

[Deleted member 26818]

To be honest, it sounds more like the 0.7Ω socket and the 0.14Ω socket are one and two of a spur.
It may be that you have a bridge or cross connection on the RFC, but I would then expect both sockets to have the same reading.

 

[wattsy]

Incidentally, what was the reading at the joined ends at the consumer unit?

0.18 ohms

 

[wattsy]

If it was a interconnected RFC then the r1 r2 & rn readings would be of the path of the interconnection would they not?

 

[SYKRAPS]

I suppose if you split the ring to add the 2 sockets in question in the sub floor using 2 JB's and then also run a t+e cable between the 2 JB's this would account for the reading differences, maybe 2 spliced spurrs with a link between the 2 spurred sockets aswell???