High Zs reading!

[justuk68]

Guys, request advice.

I was recently carrying out remedial work on 2x Ltg cts reported on EICR as having very high Zs value.

When I retested the measured Zs was 44.8 and 42.6. These are on a TNC-S system and Crabtree C50 5a MCBs. These would appear to be readings expected of a TT System!

I repaired CPC connections and carried out good R2 tests at all the fittings. There was also good 240v at the fittings L-CPC.

My tester is in Date for cal and I have no reason tho suspect it to be at fault. Check test on x2 other circuits on same DB were OK. I did not use the Trip Lock function on the tester. I do not expect the Zs at the DB to be high as nothing else was reported to be high on the EICR.

Could an old MCB be the cause, loose MCB or connections??

Your thoughts please............

 

[Plonker 3]

What was your Ze?

 

[telectrix]

could be the MCB. if your R1+R2 readings are sensible.

 

[ChrisElectrical88]

What was the r2 reading? Did this satisfy the table in GN3?

 

[Sintra]

Step 1 test Zdb
Step 2 test R1 + R2, R1 + Rn and R2 + Rn
Step 3 calculate expected Zs
Step 4 measure Zs
Step 5 if everything above apart from step 4 is ok measure resistance / impedance across mcb.

 

[justuk68]

Gents Thanks,

Highest R2 was .84
Did not measure Ze as I did't suspect it to be high (EICR no mention, and my check tests)

The DB is 25yrs ish and like a birds nest. ID of 'N' and 'CPC' very difficult. Given the tests I've carried out just wondered what you Guys think???........
thanks

 

[ChrisElectrical88]

Gents Thanks,

Highest R2 was .84
Did not measure Ze as I did't suspect it to be high (EICR no mention, and my check tests)

The DB is 25yrs isn and like a birds nest. ID of 'N' and 'CPC' very difficult. Given the tests I've carried out just wondered what you Guys think???........
thanks

I would say it's defo worth getting a Ze and some other Zs reading off local circuits and go from there ?

 

[Plonker 3]

Well if you results for r1+r2 are correct there really are only 2 other possible causes for your readings.

Never assume anything, always check the obvious even if others have done so before you.

 

[soulman]

I would have started with Ze or Zdb. I had a similar situation & the circuit breaker was at fault, I put the line in a different c.b & measured Zs all o.k.

 

[justuk68]

Gents,

Totally get the not taking things for granted comment.

Im fairly competent testing but not seen this type of fault given all my CPC checks etc.

Think when I return I will initially check the MCB connections retest, then try the Alt MCB!

Thanks again.....will let you know result........

 

[nickblake]

One of the first things that should be done is to carry out a Ze as soul man has said you could have a high Ze which will effect the zs , to carry out the Ze you should isolate the supply and disconnect the main earth and test connected to the disconnected earth and the incoming L and N , this eliminates parallel earth paths from the circuits , you could have a high Zs due to a fault on the incoming earth or the earth connected to an un connected earth block on the side of the cut out and its using parallel earth paths ,

 

[kingeri]

I thought everyone always did a Ze anyway........just to make sure, or is that just me???

 

[Plonker 3]

Disconnecting all parallel paths wont make any difference if its the cause of the high Zs reading.As has already been said do Zdb.

 

[Aindowsimon]

Well if you results for r1+r2 are correct there really are only 2 other possible causes for your readings.

Never assume anything, always check the obvious even if others have done so before you.

Tithe 2nd bit I agree entirely !

 

[Engineer54]

Methodical testing will always send you in the right direction to isolating or solving a problem. In your case, good start would be to follow Sintra's steps in post #5...