Weird readings!

hoppy · Sep 4, 2014

Hi I was testing a ring circuit and got some weird r1 r2 readings and just wondered if any one could shed a bit of light as to why as I am a bit puzzled. It is a periodic test!

13 sockets on ring one of which is spur, 2.5 / 1.0 twin and earth!
end to ends done at socket as board a bit messy!
r1 = 0.65
Rn = 0.65
r2 = 1.58

end to ends are ok.
Was expecting. 0.56 r1r2.

but the r1r2 readings I was getting were quote varied, one socket I got 0.50 and some others 0.53, others 0.55 and others 0.57 and that is to the terminals with the socket off the wall!

Yes I had the correct legs linked out!
could this be a ring within a ring! Also do you think this should be a code as all results are acceptable but just seem to vary a bit more than expected!

the socket I got 0.5 at is defo part of the ring!
what do u think!

oh possibility of joints under floor were kitchen sockets have been taken from
general ring, and put on their own circuit.

davesparks · Sep 4, 2014

What exactly do you think is wrong with those readings? A variation of 0.07 ohms around an existing circuit doesn't strike me as all that odd.

PEG · Sep 4, 2014

As above,unless it's a 2' square wendy house,sleep well :stooge_curly: ...and if you want to get those end to end readings perfect,shorten the cpc a bit...

hoppy · Sep 5, 2014

Thanks guys for the replies, just seemed a little odd, most rings I test are pretty spot on, I.e within a couple of ohms, just my inexperience I suppose, thanks for putting me at ease!

davesparks · Sep 5, 2014

Within a couple of ohms is bad, you should be within a few 1/100 of an ohm!

ipf · Sep 5, 2014

hoppy said:
Hi I was testing a ring circuit and got some weird r1 r2 readings and just wondered if any one could shed a bit of light as to why as I am a bit puzzled. It is a periodic test!

13 sockets on ring one of which is spur, 2.5 / 1.0 twin and earth!
end to ends done at socket as board a bit messy!
r1 = 0.65
Rn = 0.65
r2 = 1.58

end to ends are ok.
Was expecting. 0.56 r1r2.

but the r1r2 readings I was getting were quote varied, one socket I got 0.50 and some others 0.53, others 0.55 and others 0.57 and that is to the terminals with the socket off the wall!

Yes I had the correct legs linked out!
could this be a ring within a ring! Also do you think this should be a code as all results are acceptable but just seem to vary a bit more than expected!

the socket I got 0.5 at is defo part of the ring!
what do u think!

oh possibility of joints under floor were kitchen sockets have been taken from
general ring, and put on their own circuit.

Getting your rrRR's mixed up.........could get hung, drawn and Quartered for that.

SJD · Sep 5, 2014

For an RFC in older 2.5mm2 cable, where the CPC is only 1.0mm2 (instead of 1.5mm2), which is what you appear to have, the variation in (R1+R2) is greater (than if standard 2.5/1.5 cable).

My own house is like this, and I calculated (from r1, rN, r2) a few years back (in Excel) that the (R1+R2) could vary from as little as 0.603 (for a socket more or less at the origin) to as much as 0.733 Ohm (socket mid-RFC). In practice, the results at the time varied from 0.64 Ohm to 0.75 Ohm, discounting a few spurs.

I just put your r1, rN, r2 values into the Excel sheet, it gave results from 0.46 Ohm at origin, 0.49 Ohm at 10% from the origin, to max 0.56 Ohm at mid-point.

The simplistic +/- 0.05 Ohm doesn't take into account these older circuits, where CPC is much smaller than L/N conductors.

Octopus · Sep 5, 2014

I would applaud you for doing it properly and questioning a minor variation.

Your readings, using a socket adaptor, could easily vary a lot more!

happysteve · Sep 5, 2014

Here's a pretty graph showing you the variations in (r1+r2)/4 for different ratios of line conductor and CPC. 2.5+1.0mm is the dashed red and cyan line (being identical to 10+4mm).

[ElectriciansForums.net] Weird readings!

HappyHippyDad · Sep 5, 2014

Thats interesting Steve. I have to admit I thought R1+R2 pretty much always = (r1+r2)/4. This diagram shows that one could be up to 20% lower than the other!

Richard Burns · Sep 5, 2014

When you measure the figure of eight testing then you are effectively measuring the worst case scenario at every point in the circuit because of the interconnections; the fact that each measurement should be the same (allowing for variations in contact resistance), due to the cable connections you have made, allows you to assess if there is anything wrong at any point in the circuit.

However in practice the actual R1+R2 you would get at sockets at varying distances from the CU is as in the graph above, the R1+R2 = (r1+r2)/4 is only found at the mid point of the circuit.
For testing purposes this is the highest value (apart from spurs) that you will get and so the circuit must comply with that value as the worst case.
This is the same as when measuring a radial circuit where your R1+R2 increases with distance away from the CU and the highest value is the one recorded.