Earth Rod Testing

[XtraRaptor]

Hi,


Is it possible to test an earth rod using an earth loop impedance tester?

Can you get an accurate result?

How should you carry out the test?


Thanks,


Nick.

 

[Adam_151]

If this is for a TT system (as opposed to tie down a generator in the middle of a field, then yes you can)

The same way that you normally test a Ze, but remember to switch main isolator off and take the earthing conductor out of the MET to test it

 

[XtraRaptor]

It is actually the "ground" point for the cutting table on a cnc profiling machine, the plasma unit that does the cutting needs a ground rod close to the table, with a reading of 0.5 Ohm or less, to dissapate the HF pulse it produces at start up.




Nick.

 

[bernardgreen]

The DC resistance is important but the high frequency AC impedance is more important.

So the cable from table to earth rod needs to be laid in as straight as possible. The reason is that any bend in the cable will add inductance and increase the impedance at high freqency.

 

[XtraRaptor]

The DC resistance is important but the high frequency AC impedance is more important.

So the cable from table to earth rod needs to be laid in as straight as possible. The reason is that any bend in the cable will add inductance and increase the impedance at high freqency.

Thanks for that...


But don't think it actually answers any of the questions I asked.



Nick.

 

[Spark123]

You won't get a streight answer - this is a specialised subject as opposed to a DIY one!
You'll probably need specialist equipment to measure the impedance of the rod and connecting cables at the correct frequency, what do the manufacturers say?

 

[XtraRaptor]

That the work table requires a ground rod within 7M of the table, with a resistance of no more than 0.5 Ohm.


I know it is a DIY forum, but a lot of people who post here are professionals, I was assuming that some of them would have some experience in this area (testing earth rods) and could offer some guidance.

I know I can use a dedicated earth rod tester, but as the machine is in the middle of an industrial estate there is not too much scope for driving in test spikes.

I could also use one of these, http://www.kewtechcorp.com/products/earth_resistance/KEW4200.htm but they are a bit expensive.


Nick.

 

[ericmark]

Yes you can measure the resistance with a EPI tester. But at 0.5 ohms you may have problems.

The earth loop impedance is as it says a loop. So it is also measuring the rest of the loop as well as the rod.

So to get an accurate figure you need to know the resistance of rest of loop. Where you need around 60 ohms as with an earth rod used in a house TT system the few ohms the rest of loop take is so small it can be disregarded.

But for 0.5 ohm it is a different story. I would not expect you could get 0.5 ohm on a single earth spike. When I was putting in earth rods it needed 3 to 8 x 1.2 meter rods to get 8 ohm reading so for a 0.5 reading you will likely need to sink 4 sets of rods and put a mat between them.

And to read 0.5 ohms you will need the special earth rod tester and put out the two probes.

You could try to measure if you have a really big supply to the plasma unit and it will tell you if you are somewhere near the mark with a ELI tester but it will read higher than true reading. So a reading of 1 ohm will in real terms be more like the 0.5 you are looking for. But if you are having to dig up concrete to install the mat is it really worth the chance I would hire proper tester.

 

[jeepmadmike]

0.5 ohm for a earth rod! you will be very lucky, its not a percise science soil conditions will have a massive effect on it.

To obtain that type of resistance i would expect an earth mat will be required buried in bentonite.

there are lots of methods but you will need a proper instrument to test them all.

I have one!