Measuring Ze value??

[Spark2be]

Hi can anyone give me a procedure to measure the Ze value of a house? I am currently studying to become an electrician and I am very much a rookie so please be patient with the baby step questions

 

[Spark2be]

I think they are lower due to parallel earth paths and it should be isolated because Ze is a live test.

 

[GLENNSPARK]

I think they are lower due to parallel earth paths and it should be isolated because Ze is a live test.

ok wyllis....parrallel paths is correct..and to the second question the answer is yes...both Ze and Zs are live tests but in order to eliminate parrallel paths the main earthing conductor is dropped out of the C/Us MET...under energised conditions this would be a dangerous practice as it would render the installation without a means of earthing (apart from any earth paths through bonding)....and so a safe isolation is performed first before a measured value of Ze can be obtained....

 

[Spark2be]

Thanks Glennspark that is very helpful. Would you mind explaining the theory behind the lower readings due to the parallel earth paths please?

 

[GLENNSPARK]

as you have more that one conductor that the fault current can take....due to extranious-conductive-parts....such as metalwork bonded back to the MET....and so you have a parrallel path/s...which will reduce the impedence to any fault current....but you cant substitute bonded metalwork for a reliable means of earthing.....and likewise gas and water services that were once used as a means of earthing an install is no longer allowed......what a Ze test does is verefy that you have a reliable means of earthing of a low enough impedence to ensure that protective devices will operate within times..further more it tells you what the condition of the earthing at star is as well.....

 

[Spark2be]

Ah I understand, so a direct reading is a more accurate measurement as appose to the calculation due to it taking into account parallel earth paths. Thanks for taking the time to answer my questions.

 

[J&M Electrical]

as you have more that one conductor that the fault current can take....due to extranious-conductive-parts....such as metalwork bonded back to the MET....and so you have a parrallel path/s...which will reduce the impedence to any fault current....but you cant substitute bonded metalwork for a reliable means of earthing.....and likewise gas and water services that were once used as a means of earthing an install is no longer allowed......what a Ze test does is verefy that you have a reliable means of earthing of a low enough impedence to ensure that protective devices will operate within times..further more it tells you what the condition of the earthing at star is as well.....

You should re-write the regs in a way that we can understand. Your explanations are brilliant.

 

[GLENNSPARK]

well i dont think i have had any of my explanations described as `brilliant` before..lol....but thanks all the same....lol......

 

[GLENNSPARK]

dont forget though wyllis...that you can obtain a Ze by enquiry to the DNO as well......but you will get the standard spiel....0.35 ohm for a TN-C-S....or...0.8 ohm for a TNS....which you can add to an R1 R2.....to get a Zs value for a schedule of tests.....better to get a direct reading though.....