Zs reading help?

[liddley]

Hi guys,
I had a job this morning to change a faulty shower unit in a flat with a direct replacment. when I had fitted the replacment unit I started the minor works cert. Tested the insulation resistance on the 10mm T+E cable all fine. But when I did then Zs I had a reading of 595 ohm? Having messed about checking the conections on the probes on the meter and what not I just thought I would test another circuit to compare,and sure enough same round about reading. Did a Ze on the board to make sure the incoming earthing was up to scratch and got a reading of 0.12 ohm (TNS Supply). What could cause this kind of reading on the Zs??:thinking:

 

[Guest123]

A very poor connection along the shower circuit somewhere, DB, Pullswitch, Shower unit etc etc.

Where di you measure Zs from???

 

[spark 68]

Hi,

thought I would test another circuit to compare

RCD's can sometimes give high readings, there are a few threads on here discussing this situation

Obviously check all connections for tightness etc., including the RCD for that circuit/side of board

 

[liddley]

I measured the Zs from the shower end of the circuit to start with, but then I took a reading dirct from the RCBO and got same similar reading.
I took a Zs reading on both ring circuits using the plug lead on my tester and got really high readings also?

 

[tony mc]

What was your R1 + R2 reading ?

 

[spark 68]

Hi,

but then I took a reading dirct from the RCBO and got same similar reading

.

that is what I said, ie. the RCD/RCBO can introduce a resistance, so if your measured Zs/Ze is ok before the RCD etc. and high after, it may be the RCD itself (or it's connection), providing your R1+R2 is ok for the circuit in question.

 

[tony mc]

You should have a good idea what your live ZS is going to be in relation to your dead tests and ZE.

Try testing the circuit on a high current loop test bypassing the RCD!

 

[liddley]

Whats the best way to combat this problem?
Sorry for all the questions quite new to testing not got much experiance with all the snags and solutions yet.

 

[telectrix]

i posted a similar scenario. Ze of 0.2 ish and a Zs of 350 every circuit. it was the crappy RCD.

 

[liddley]

High current loop test?

 

[spark 68]

You should have a good idea what your live ZS is going to be in relation to your dead tests and ZE

.

This how I usually do this, ie. calculate my results for the EIC, and then do a quick measurement reading to satisfy myself that the 'real world' results tally up with my expected/calculated results for my own piece of mind, sometimes the no trip results can be a little bit higher, but not usually that far out.

 

[Octopus]

Did you take a R1 + R2 reading of the afore mentioned circuit? Did you try the Zs on any other circuit?

 

[spark 68]

Hi Liddley,

Zs = Ze + R1 + R2

so if your Ze is ok and your R1 + R2 is ok, the only things that can cause such a high result, is the RCD + MCB, RCBO or the connections to the afore mentioned devices, if you are having bad readings on other circuits, then providing the other results are ok (R1 + R2 and Ze), then it has to be something common to the other circuits as well, namely an RCD or the connections loose to the CU busbars etc.

 

[telectrix]

either live with it , enter 1667ohms as the max. or replace the RCD. the choice is yours (obviously once you have made sure there is not a poor connection)

 

[spark 68]

Hi Tel,

I would want to investigate this further, as the OP was asking about TNS supplies (Max Ze) in another thread.

High resistance joints lead to overheating and fires, so I would want to be doubly sure it was a crappy RCD, and then I would likely replace this item, more so as he was getting iffy readings on other circuits, his earth fault (RCD) trip times may or may not be OK