High ze readings. Even higher zs?

[Bsharp]

testing a TNS installation today, found a high incoming ze of 1.14ohms, in my head putbthis down to incoming earth issue as all terminations tight, but when I test any subsequent circuit I’m getting much greater zs readings approx 4.0 ohms. I was expecting high earth loops as my incomer was high but not this great, it does seem lower when linking out rcds and using two lead test, but using three leads I’m getting these high result?? Has anybody had similar or have any suggestions. The DNO have been informed so I’ll see what happens.
Any advice appreciated.
Cheers.

 

[westward10]

Did you recheck the Ze.

 

[telectrix]

try your tester at home (hopefully you should know what readings you should get). tester may be playing up. my 1553 was earlier this year. was just a dirty 4mm socket. a quick rub with a 3.5mm re-threader sorted.

 

[Strima]

Three lead test will often give higher results as it has a lower output to avoid tripping.

 

[Bsharp]

Hi yer measured external loop which is high at 1.14 ohms, but with an R1+R2 on ring of 0.35 wouldn’t expect a ZS of nearly 4.0 ohms. I have a very similar impedance on all circuits 3.8-4.0.
DNO have been out and are going to fit a front end RCD as they agree the ZE is high, but I’m sill confused as to the very high ZS.

 

[KEV 1 N]

Yes I've had this the other day, the ze at origin was 0.44ohms tested on hi setting (I tested it on lo no trip and got 1.90ohms) so then doing zs at lighting circuit I got 4.6ohms (rcbo) I then did a r1r2 test and got 2.90ohms. In my experience where ze at origin is high, it has a knock on effect with zs (particularly small cable sizes and testing through rcbos)

 

[davesparks]

Using a three lead Zs test on a poor earth connection will give high results.

Carrying out these tests is a little pointless whilst the incoming earth is in poor condition, it will give you a rough idea of whether the circuit is ok or not, but until the external earth is fixed you can't rely on the Zs readings to assess full compliance or complete a schedule of test results.

 

[KEV 1 N]

However 1.14ohms is very high for TNS and the DNOs Earth sheath has failed at some point, what is their cable type? Is it the old paper lead type?

 

[Bsharp]

Yes I agree with the knock on effect small cable diameters, Rcbos etc.
Yes correct it is a lead sheathed paper insulation cable.
It looks like the DNO have decided not to investigate further and are just fitting an RCD whereas the fault will almost definitely be a cracked/snapped lead sheath. which as Davesparks says makes the completion of the test certification in some ways a slightly pointless exercise, which is pretty annoying as the job was going to be for our annual inspection as it’s one the few available and within the “40 min radius” typical!! Best laid plans and all that.

 

[westward10]

If it is a breakdown of external earth continuity, fitting an upfront rcd does not resolve the issue.

 

[Bsharp]

I agree I’d like them to dig a lovely big hole and repair but the powers that be are not allowing this to happen.

 

[KEV 1 N]

I'm sure there's maximum limit for TNS, can't remember off the top of my head but I would think 1.14ohms is well over

 

[Bsharp]

Correct TNS 0.8.

 

[westward10]

Have they told you why it is so high.

 

[Bsharp]

Their explanation as is normal with the DNO in my experience is either a damaged joint to the property or a damaged joint further down which could mean entire street etc. With ZE’s which don’t comply but there only course of action is an RCD which is pointless as consumers circuits are all rcd protected anyway (17th edition board) so disconnection times achieved by RCd, but as you say doesn’t resolve the supply earth issue.