High Zs

[telectrix]

Interesting one yesterday...... my remit was just to install a outdoor socket direct throuh wall (back to back) from existing double socket inside. existing socket on a 32A RFC , RCD protected on recently fitted dual RCD board. testing time and Zs is 1.59 ohms, obviously > tabulated value even without correction factor/s. sharpie scrawl on TN-S earth in meter box says 0.32. this in itself seems high . the RFC is not excessively long either. Noted on MWC and explained to customer that's it's not dangerous, but he's awaiting the return of the spark who has done a fair bit of work in the house last year and still has outstanding works to be done.

 

[timbobelfast]

It does seem unusually high. Did you give it the ole R2 test as well from the MET?

 

[buzzlightyear]

i would ask to see the test certs before hand if a new c/u was fitted to see what values there was before .

 

[Ian1981]

Did you actually measure the Ze (zdb as written on the minor works form) at the CU as required on the minor works certificate or assume the sharpie scrawled 0.32 ohms?
Depending on where the TX is , I don’t think 0.32 for a TNS is high.

 

[telectrix]

to be honest. for what the job of fitting an outside socket was worth, i wasn't prepared to climb up steps, reach over a huge freezer and disconnect circuits in the high level CU for dead tests.

 

[Ian1981]

If you wanted it easy, you should have been a joiner ?

 

[loz2754]

It's possible that a break in the RFC continuity may be contributing to the high reading.

 

[buzzlightyear]

i wasn't prepared to climb up steps,

If you wanted it easy, you should have been a joiner

to make some steps .

 

[telectrix]

It's possible that a break in the RFC continuity may be contributing to the high reading.

agreed. but basically, before the customer knew of me, last year he's had a spark from Rochdale (40+miles) doing the major works. he phoned me for the outside socket as it's not worthwhile the other guy doing the distance for a £70 job. the Rochdale spark has further work in hand, so it's buck passed to him.

 

[westward10]

to be honest. for what the job of fitting an outside socket was worth, i wasn't prepared to climb up steps, reach over a huge freezer and disconnect circuits in the high level CU for dead tests.

But shouldn't you have done the fundamental tests to enter on the Minor Works?

 

[telectrix]

generally with a small job like this I don't bother with dead tests as the installation is already energised. obviously if I saw a dangerous or potentially dangerous situation, I would investigate further. in this instance, I didn't want to interfere with another sparks's work as this could lead to confrontation as to who is responsible....... "it was OK till you interfered with it". etc. so I've reported on what is a minor departure ( high Zs) on another guy's work. it's up to him to deal with it.

 

[DefyG]

Possible reason!
Had some RCD's that have been as much as 0.7 ohms after the RCD than before when measuring Zs which produces a much higher result than expected and sometimes higher than permitted max Zs.
I usually correct the certificate entry and make a note of doing so.

This was discussed during our annual NICEIC inspection, and the inspector said he had never come across this before but advised to put the corrected figure on the cert.

 

[loz2754]

Possible reason!
Had some RCD's that have been as much as 0.7 ohms after the RCD than before when measuring Zs which produces a much higher result than expected and sometimes higher than permitted max Zs.
I usually correct the certificate entry and make a note of doing so.

This was discussed during our annual NICEIC inspection, and the inspector said he had never come across this before but advised to put the corrected figure on the cert.

Yes, RCD uplift is a common problem and could well be the reason for the higher than expected Zs in this case.

 

[pc1966]

Some of the MFT (e.g. Megger MFT1741) claim not to be too sensitive to that 'uplift' factor.

I guess plugging the MFT in to another socket elsewhere would allow a quick check of the Zs and to see if it was similar circuit-wide (so uplift likely) or varying a lot (so dodgy terminals or open ring likely).

 

[telectrix]

possible RCD uplift causing this high reading, but as said before. CU recently fitted by another spark, so it's up to him. with hindsight, maybe i should have asked to see his EIC (if there is one). see if his readings tally.