EICR - r1 very high but R1 + R2 low.

Talk Electrician Forum

Help Support Talk Electrician Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

pkbristol

Member
Joined
Dec 14, 2011
Messages
37
Reaction score
0
I did a periodic inspection on a property yesterday, 3 bed semi, and got a strange result on one of the rings.

When doing the end to end (i call them little r's), r2 was a normal(ish) reading, 1.6 ohms, rn wasn't a ring, and r1 was 20 k ohm, (yes 20,000 ohms), however when i cross connected and did R1 + R2 i got a normal readings all round the ring, highest was 1.01, and that was a spur, all the non spurs were lower than that.

So i investigate and find that one of the sockets has loose connections, retighten them and the r1 comes down to 1.6 ohms and rn is now a ring, 0.9 ohms.

My question is how can r1 be so high yet R1 + R2 be so low, after all i'm still testing the live wire so the resistance is still there, it's just being tested with the earth now as well.

Paul.

 
The answer to the question will jump out at you if you try the tests out on paper;

The r1 r2 cross connect test makes a big loop out of both the phase and cpc, and if you picture this as a circle, when you measure between L and cpc on the socket you are measuring between two points which are diagnally across from each other on the circle, in effect you are measuring the reistance of 2x semi-circular bits, in parralell... giving you your R1+R2 = (r1+r2)/4, [its half the length, so half resistance, and 2 in parralel, so half again... hence divide 4]

When the phase is open circuit, you have still got a path through the other side of the semi circle, this path is equal to the length of one of the conductors, but it will be made up of a mixture of phase and cpc depending on the position of the fault and the postion you are testing at, so the R1+R2 you measure will be (for your case) in the range of 0.9 to 1.6

 
Hmm, i sort of get that.

So next question, Zs = Ze (in this case 0.12) added to the highest R1 + R2, (1.01) so before i found the fault and tightened the connections my Zs was 1.13, below the max Zs for the 32a breaker, so would this be a pass ? I couldn't let that go as a pass knowing that the r1 was still 20 k ohms.

Thanks, Paul.

 
Sides

he goes on to add the R1 + R2

PK

Code 3 and then go back to fix it as a remedial

dunno how you can pass it knowing rn has no continuity

 
Last edited by a moderator:
Oh sorry, i tried to word it so it was easily understandable, to cut a long story short, my Ze was 0.12, R1 + R2 was 1.01, so Zs = 0.12 + 1.01 = 1.13.

I had it in my head but maybe didn't write it quite the same.

Paul.

 
PK

Code 3 and then go back to fix it as a remedial

dunno how you can pass it knowning rn has no continuity
It's all ok now, like i said tightened screws on the offending socket and all was good. r1 was good and rn became a ring again.

If i couldn't have made the rn back into a ring i would've classed the circuit as a radial and downrated the breaker. Correct ?

Paul.

 
As Phoenix says with one leg of the cross connected ring conductors open circuit, you still have one long length of copper conductor comprising the length of the CPC (r2) and r1 but with a break somewhere in the r1 conductor..

No mater where you put your meter onto the r1 and r2 there has got to be a path that cannot be any greater then the total value of the r1 & r2 lengths combined.

In reality because of the way a ring is wired this would be a maximum of about half of r1 + half of r2...

The more worrying point that needs further investigation is your new readings are way out of step with each other...

i.e.

r2 (CPC) = 1.6ohms that is either 88m of cable @ 1.0mm2 CSA or 132m of cable @ 1.5mm2 CSA.

rn (neutral) = 0.9ohms that is 121m of 2.5mm2 CSA,

so possible CPC is 1.5mm2 with a high res joint bangin on another 10m worth of copper resistance value..

BUT

if r1 is now 1.6ohms...?

that is equal to 216m of 2.5mm2 CSA

216m on the Line r1 leg and only 121m on the rn neutral leg????

I wouldn't be happy leaving a ring like that without further investigation...?

and I cant see how you will get a realistic Zs if the r1 figure is so high compared to the rn & r2

I would guess you have a few more bad joints around that circuit..

:(

 
Last edited by a moderator:
It's all ok now, like i said tightened screws on the offending socket and all was good. r1 was good and rn became a ring again.If i couldn't have made the rn back into a ring i would've classed the circuit as a radial and downrated the breaker. Correct ?

Paul.
So you now have a r1 of 1.6ohms and a rn of 0.9ohms and you think that everything is ok????

 

Latest posts

Top