earth electrode testing

[snowplough]

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

 

[Deleted member 26818]

In certain circumstances, an earth electrode will require accurate testing to determine it's resistance.
Such circumstances would be when the earth electrode is being used for the earthed star point on a TX or generator, or as an earth point on a PME system.
As such the resistance will be required to be bellow 21Ω, and no it will not be measuring Ze.
Other circumstances where accurate testing of an earth electrode could be when on a TT installation, an RCD for earth fault protection is not desired.
Where you test an installation that incorporates both an earth electrode and an RCD for earth fault protection. Accuracy is not that important.
As such the use of an EFLI tester is acceptable to test Ze, or as it is usually termed RA.

 

[snowplough]

Cheers Spin,
Im still trying to understand what your saying,

When you do a earth electrode test , say the dead test, are you just trying to measure the rsistance of the actual metal rod, not the earth its sunk into. If this is the case when you do the live test with the E.F.L.I tester isnt that measuring the phase,earth and transformer resistance as well as the metal earth electrode resistance.

Regards

Locket

 

[somersetsparks]

You don't have to do a dead earth electrode test on a normal installation. Just a normal EFLI test on the stake will do. Then put the method of measurement down as 'EFLI'

Testing otherwise i.e. for star point of a genny, would be done with a bridge meggar.

 

[IQ Electrical]

Cheers Spin,
Im still trying to understand what your saying,

When you do a earth electrode test , say the dead test, are you just trying to measure the rsistance of the actual metal rod, not the earth its sunk into. If this is the case when you do the live test with the E.F.L.I tester isnt that measuring the phase,earth and transformer resistance as well as the metal earth electrode resistance.

Regards

Locket

You have to be mindful that the resistance to earth of the electrode is likely to be vastly higher than the sum of the other parts of the measured loop, therefore, the measured figure can still be termed as Ra

 

[Deleted member 26818]

What you would be testing is the resistance between the soil and the electrode (soil/electrode interface).

 

[fire]

i wouldnt worry much, if its for a TT system i would stick the rod in the ground and connect a minimum of 16mm earth on it with the approroate clamps, take that to the MET and then do a Ze test from there if its less than 21ohms that will comply with the regs but if you do a earth fault loop impedence test it should be less than 200ohms

 

[somersetsparks]

i wouldnt worry much, if its for a TT system i would stick the rod in the ground and connect a minimum of 16mm earth on it with the approroate clamps, take that to the MET and then do a Ze test from there if its less than 21ohms that will comply with the regs but if you do a earth fault loop impedence test it should be less than 200ohms

Well by regs, 4mm earth to the rod is the minimum without mechanical protection, 2.5 with.

 

[IQ Electrical]

i wouldnt worry much, if its for a TT system i would stick the rod in the ground and connect a minimum of 16mm earth on it with the approroate clamps, take that to the MET and then do a Ze test from there if its less than 21ohms that will comply with the regs but if you do a earth fault loop impedence test it should be less than 200ohms

What? 21 Ohms? 16mm Earth?

Someone is rather confused.

 

[somersetsparks]

21ohms is a figure which you would want to meet for a PME stake, or as spin says the earthed star point of a genny or transformer.

1666.6 is the theoretical maximum for a TT system although you would reccomend action if reading was above 200ohms.

Personally, I always TRY to get readings down to 10ohms or below, although I readily admit this not always possible!

 

[snowplough]

Cheers Fire,
I think what im trying to understand is when you do the dead electrode test if your just testing the resistance of the electrode cant you just pull it out and put yyour leads on either end of it and do a continuity test to find the resistance rather then this 3 electrode test

regards

locket

 

[IQ Electrical]

Cheers Fire,
I think what im trying to understand is when you do the dead electrode test if your just testing the resistance of the electrode cant you just pull it out and put yyour leads on either end of it and do a continuity test to find the resistance rather then this 3 electrode test

regards

locket

Are you serious?

You're not testing the end to end resistance of the electrode itself, you're testing the electrode and soil resistivity to ensure that you have a means of earthing for the installation.

 

[somersetsparks]

Fire, Why would you ever want to do this?
There is no point in testing a 3ft copper rod man....
Thus disturbing it and making its already fairly poor earthing ability worse!
You just need to do an EFLI test.

 

[snowplough]

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

 

[Deleted member 26818]

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

No good at all, unless you know the composition and resistivity of the ground.

 

[Engineer54]

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]

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]

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]

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]

Have a look at this it may well help