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

 

[telectrix]

yeah. i'd make sure of the connections. as its affecting all circuits, any fault has to be at or close to the CU, then replace the RCD and see what effect that had on the Zs.

 

[spark 68]

I suppose a quick way of determining if it was the RCD at fault, would be to open the RCD/RCBO then join the CPC and L at the far end and then loop test across the open RCD/RCBO.

 

[Deleted member 9648]

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

I dont get this,and have never experienced it myself.....Surely there is a direct CPC path to the MET,so there can be no excessive resistance there,and if you had a resistance of hundreds of ohms through the RCD contacts any loads connected to it would not work??
Perhaps the OP's shower was not faulty but it was the RCD's high resistance contacts preventing it from working properly??

 

[telectrix]

I dont get this,and have never experienced it myself.....Surely there is a direct CPC path to the MET,so there can be no excessive resistance there,and if you had a resistance of hundreds of ohms through the RCD contacts any loads connected to it would not work??
Perhaps the OP's shower was not faulty but it was the RCD's high resistance contacts preventing it from working properly??

the one i came across had Zs of 351 on all circuits. measuring on the outgoing side of the RCD gave a reading of 349.odd. but everything had been working fine for years. Ze was 0.2

 

[liddley]

I have never experienced this high readings before now. I work for a local authority and we use the same setup in every property. Hager disboard with hager Bs 61009 RCBO`s .I just found it strange how all the circuits gave me high readings. My metral tester has never failed testing circuits fed by RCBO`s before and I thouht It would be highly unlikely all 8 of the RCBO`s would be faulty.
I questioned the max Ze in my last post just to make sure I was right in thinking it is 0.8 ohm for a TNS system, as most of the dwellings I work in have been upgraded to a TNCS supply.
The shower unit did not have an electrical fault on the unit it was damaged by frost in the cold weather last year an never got replaced.
All connections are tight within the consumer unit as this is the first thing I checked when I got the high readings. Was not sure on how to combat this w ith my limited testing experience.