ZS measured higher than zs calculation

[lee1002]

I was wondering if anyone can help me out with this conundrum.

I was testing a domestic property and tested a security alarm which was a 6A MCB not supplied by any RCD.

Ze was 0.12 Ohms
R1+R2 was 0.10 Ohms

Now using the calculation I assumed the reading would be no greater than 0.22 Ohms

When carrying out the ZS test I recorded a value of 0.23 Ohms. I thought this was not possible?

I ensured during R1+R2 the leads were nulled and the battery was recently changed.

I was wondering how this could occur?

 

[telectrix]

all sorts of things will affect your Zs. a finite resistance of the MCB for example.

 

[lee1002]

Thank you I was just worrying because I have never encountered this before and always worked to the logic That the Zs was never higher than the calculated result

 

[Flanders]

As Tele says MCB,s will have some resistance if you do the Ze on the out going side of the MCB you may find a higher reading than at the main switch , I have found in the past merlin gerin 2 amp MCB's seem to have high resistance, also check for any loose connections within the CU that could be coursing the higher resistance

 

[lee1002]

I checked for loose connections and ensured all connections were sound and tight. I worked through lots of different things in my mind and just couldn't work it out why that would be the case.

 

[telectrix]

a worst case scenario i came across was BG condemned an installation for having a Zs of 350 ohms on a TNC_S. the Ze was 0.2 on the incomer, but 340 ohms on the outgoing side of the up front RCD. replaced RCD and problem solved.

 

[lee1002]

Thank you for the heads up. I will take on board these comments and keep a look out when i'm testing other installations

 

[Flanders]

You could do a resistance from main switch to the out going MCB's to see if the deference is within the CU. But in the great scheme of things 0.01 ohms is nothing to worry about the tolerances of you meter are probable higher that that.

 

[Badged01]

.... I assumed the reading would be no greater than 0.22 Ohms.

When carrying out the ZS test I recorded a value of 0.23 Ohms

I would say thats a very good corrolation between the two! Would be more than happy if all my actual readings came that close to what I was expecting ...... dont worry about how it occurred, just take it as being lost in the tolerances!!

Edit: Think GN3 advises that if reading are within 0.05 of each other then its acceptable