earth electrode testing

snowplough · Mar 17, 2011

Hi Guys, Wonder if you can help ,im a little confused,
When you do a live earth electrode test i understand that your measuring Ze, so you use the earth loop impeadance tester to get reading.

Its the dead test earth electrode test that i dont understand, does this test measure Ze as well because from the diagrams it looks as tho its measuring its own resistance between the other two electrodes and not the earthed transformer .

Many Thanks

Locket
LOCKET

Engineer54 · Mar 17, 2011

locket4241 said:
Cheers IQ,
So if your testing the rod and the near ground around it only what good is that if you need the fault current to find its way back to the source to complete the loop like in the tt system .
Sorry for sounding stupid

Many Thanks

Locket

I have no idea what you want to know now, as it has been explained to you by several respected posters now!!

Ra is not Ze, you are testing the rod(s) field of resistance in the surrounding ground. All of which depends very much on the type of soil the rod(s) have been driven in. Ra works the same way as Ze, so if you measure the Ra of a rod(s) and get say 10ohms your looking at a fault current in the region of 24 Amps. This is why you only need a 2.5mm cable (mechanically protected) to connect your ground rod to the MET etc. So what more do you want to know, and what is it that you still don't understand??

snowplough · Mar 18, 2011

Cheers Engineer 55,

Ok ,what i dont understand is, when you have a tt system say, your earth fault path is the eath electrode consumer end via general mass of earth back to earth electrode at transformer , hence rcd is used because low resistance path cannot be maintained to ensure disconnection of circuit breaker etc.
So i understand you use a earth fault loop impeadance tester same test as for Ze to find the external resistance.

What i am trying to understand is why would you need to just measure the rod resistance and just the bit of earth its stuck in ,where would the fault current be going as in the tt system .

Thanks for your patience

Locket

Deleted member 26818 · Mar 18, 2011

You wouldn't need to test the earth electrode separately when for instance conducting a PIR, or making an adition or alteration to an existing installation.
You would normally test the earth electrode on a new build where for instance the installation is not connected to a supply.

snowplough · Mar 18, 2011

Cheers Spinlondon,

So if you are testing the rod on the tt system say (doing the dead test) if your just testing the ground around it and that reading is low, but what about the rest of the ground from end of say the 30m measured area to the other rod connecting to the suppliers transformer ,if that ground resistance is high then surely it gives a high reading any way regardless of the initial 30m dead test if im making sense, as when you do your live electrode or Ze test.

Many Thanks

Locket

nickblake · Mar 18, 2011

Have a look at this it may well help

snowplough · Mar 18, 2011

Cheers nickblake,

I have actually seen this diagram with the 10 x the rod length, current and potential rod etc, im just trying understand if the ground around the rod is of pretty low resistance , what good would that be if the rest of it leading upto say the suppliers earth tansformers rod is of high resistanc, hence high reading i think.

Many Thanks

Locket

Deleted member 26818 · Mar 18, 2011

There will very likely be other earth paths available between the installation electrode and the TX electrode.
The majority of TXs provide various earthing systems. TT is usually used for installations which are so distant that the cost is prohibitive to supply either TN-S or TN-C-S.
Other installations supplied by the same TX may well be on TN-S or TN-C-S, and if any of them are between the TT installation and the TX, their earth paths will be utilised.
On a new installation, you would test the electrode to ensure that you have a sound and reliable electrode/soil interface.
Then upon connection to the supply, you would test the Ze to ensure that the earth fault path is within limits. If it isn't, then you would consider your alternatives, such as extending the depth of a rod, or the installation of supplementary rods.
Remember that although it is recommended that the earth electrode resistance should be below 200Ω for stabillity and reliability, the Zs of the final circuits in the installation can be up to 1667Ω.