High Zs value with 2 sockets

[TheCodboy]

Hi all, Doing some pre board change testing and ive found 2 sockets with high Zs readings. Everything else is fine, done all my end to ends and the Zs's are what should be expected. A couple of the sockets have Zs lower than expected but 2 are higher. They should be about 0.8 ohms but ive got one at 1.20 and one at 1.40. Taken the highest value socket off, re terminated, still high. Tested at the terminals of the socket just in case the socket was duff, but still high. Its a PME system with a good low ZE (0.15). Any thoughts people??

 

[TheCodboy]

Don't know the 1.0mm resistance of

1mm 18.10 Table B1 GN3

 

[bigspark17]

I think a return to the property on monday and get some socket fronts off....

Is the customer paying for all these pre test.?

 

[TheCodboy]

Theres 2 rings in the bungalow. One for the kitchen and one for the rest of the house Kitchen readings were r1 0.49 rN 0.49 r2 0.83. So even these figures seem a bit high then? MFT has been calibrated recently and checked it against my Cal card the other day.

 

[TheCodboy]

Is the customer paying for all these pre test.?

I put the pre test on the price but told him any faults would need rectified and would be extra.

 

[Wilko]

Theres 2 rings in the bungalow. One for the kitchen and one for the rest of the house Kitchen readings were r1 0.49 rN 0.49 r2 0.83. So even these figures seem a bit high then? MFT has been calibrated recently and checked it against my Cal card the other day.

If you are measuring the rfc loop e2e and it's made from 2.5 /1.5 then these seem ok to me as 0.49x2.5/1.5 = 0.817, so practical reading of 0.83 gets a tick
This would indicate an rfc about 65m length end to end and added to your Ze 0.14 should give Zs of around 0.47 ish which is fine for 32A. Is that roughly the Zs you've measured?

 

[TheCodboy]

If you are measuring the rfc loop e2e and it's made from 2.5 /1.5 then these seem ok to me as 0.49x2.5/1.5 = 0.817, so practical reading of 0.83 gets a tick
This would indicate an rfc about 65m length end to end and added to your Ze 0.14 should give Zs of around 0.47 ish which is fine for 32A. Is that roughly the Zs you've measured?

Yep, 0.46 was my highest value in the kitchen. My calcs work out for the other ring (apart Zs sockets) work out, but r1 r2 & rN values are high if you put them against cable resistance table in GN3. See post 12 for other values.

 

[Midwest]

Is the customer paying for all these pre test.?

No but the electrician needs to do some, or he/she will pay for them in the long run

 

[bigspark17]

No but the electrician needs to do some, or he/she will pay for them in the long run

Not zs every socket...

 

[Toneyz]

The socket with the highest reading had 2 x 2.5 T&E. Took L-L, N-N, E-E reading at the consumer unit. r1 + r2/4 + Ze = 0.82 Ze was measured at 0.14. The calculated Zs of 0.82 was very near what i was getting all around the ring. 2 sockets were a bit lower (0.45 & 0.55) and apart from the 2 that were high, the rest were virtually spot on. ( My end to end values were L=0.92 N=0.92 E=1.83)

Td to end is a bit higher than expected calculation of 1.55 this may be down to loose/bad connection or 1.0mm cpc(or in part of) that aside did you do a R1 +R2 dead test this will prove that these two sockets are spurs. Midwest posted a good point in post #21 put the ends together at the C.U. and do a ring test at the high reading socket will prove it. You posted that there was still two cables at the highest reading socket my feeling is that there is another socket/outlet from this you have not located maybe in loft just a thought. Let us know what you find out on Monday

 

[TheCodboy]

Td to end is a bit higher than expected calculation of 1.55 this may be down to loose/bad connection or 1.0mm cpc(or in part of) that aside did you do a R1 +R2 dead test this will prove that these two sockets are spurs. Midwest posted a good point in post #21 put the ends together at the C.U. and do a ring test at the high reading socket will prove it. You posted that there was still two cables at the highest reading socket my feeling is that there is another socket/outlet from this you have not located maybe in loft just a thought. Let us know what you find out on Monday

I hope its not in the loft, its covered in insulation. Two layers, And there's hardly any space to move....

 

[TheCodboy]

Might just pretend i didn't see the high value socket...... (I'm joking!)

 

[Midwest]

As it's a bungalow, you'll probably be able to stand upright. It'll be just moving the odd upright piano, travel chest and rolls of old carpets.

 

[TheCodboy]

Im still scratching my head a bit as to why my r1 rn r2 values on the offending ring are as high as they are. The circuit cant be over 120m, so why so high?? ultimately if the Zs ok then its not a problem. Unfortunately my brain doesnt work like that and i always what to know why things are what they are!! Bit OCD!

 

[Ib≤In≤Iz]

I hope its not in the loft, its covered in insulation. Two layers, And there's hardly any space to move....

The story of my life.

 

[Ib≤In≤Iz]

Might be a junction box somewhere??