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Posted By Topic: Buried earth electrode

codaxx
Apr 15 2018 08:53

Another question for you guys.

How do you guys go about verifing buried main earths? Im in Taupo and most houses, due to the lime, are buried earths so not many earth electrodes/stakes.

I just completed a switchboard replacement with all new gear(existing install), go to do the earthing test and cant find earth.

I can see it is a 25mm2 cable to a lug in meter board and then a 6mm back to the main board, but cant verify that it is completely 7m of earth in the ground.

Best option im thinking, install new earth electrode/stake next to the meter board and run a 6mm to the lug?

Thoughts?
   

AlecK
Apr 15 2018 09:35

So you can see one end of it (and presumably it has a label on the MEC to confirm that this is the MEC / electrode connection).
So you know there is an electrode, and you can measure the resistance of the MEC.
What more do you want?

If it were a vertical electrode instead of horizontal, you'd be in exactly the same situation: access to the MEC/electrode connection for testing purposes.

Your not certifying the electrode, so it being compliant to modern requirements (or not) isn't your worry.
   

codaxx
Apr 15 2018 09:42

Sweet! Makes sense.
Cheers for your help AlecK.


   

pluto
Apr 15 2018 10:07

There is no easy way to verify the lenght buried but one could do a preformance test using an EFLI tester (active to buried earth conductor only and with the buried conductor being isolated from the electrical installation).

The next problem is there is NO resistsnce value specified by regulation or as/nzs 3000 and the actual resistance value required is very dependant on the design of the electrical distrbution system and the protective device(s) used to provide the automatic disconnection of the supply under earth fault conditions.

One could use a "rule of thumb" and say 10 ohms is the value which was the old (but now revoked) value for the resistance from the mass of earth to the distribution system neutral conductor) as a very rough guide only.

The electrical installation fundamental safety outcomes is Section 1.5 of AS/NZS 3000 details the preformance required.

In the event of an active to earth fault of negligible resistance occurs, the shock current from earthed metal to the mass of earth is kept below IEC 60479-1 Fig 20 values.

The current and duration may be limited by RCDs by the automatic disconnection of the supply as devices detect leakage currents and also have a short opoerating time.

But if MCB and fuses are the protective device on the faulty subcircuit, they will require considerably more current and time and this then becomes the minimum resistance required if the neutral connnection to the supply system is broken.

This all leads to a complex calculation to get the correct resistance value.


As a quick check use an EFLI metter Active to the buried earth cable (isolated from the installation) and check the actual resistance to see that the EFLI value of the earth electrode is less than 50 ohms.

The difficult earthing conditions of the Taupo area due to geothermal activity would mean even if a new earth electrode as detaile in table 5.2 of AS/NZS 3000 was installed, the actual resistance using an EFLI test may not give a low resistance value.

It was my understanding that some parts of Taupo and Rotorua due to geothermal activity, that the distrubution system also provided an earth connection as well as the active and neutral connections.

It is also my understanding that the vertical earth electrode should also be a stainless steel rod to prevent corrosion due to geothermal activity.
   

AlecK
Apr 15 2018 13:05

except that until the new edition of "3000" is cited by ESRs, s/s electrodes are not on the list of acceptable electrodes
   

pluto
Apr 15 2018 14:53

Table 5.2 of the current as/nzs 3000 allows for Stainless clad earth electrodes which is not as good as a solid stainless steel rod, so I don't see that a solid stainless rod is not permitted.

I have for the last 20 years specified solid stainless in the Taupo and Rotorua geothermal areas without problems.
   

codaxx
Apr 15 2018 17:24

I will give the EFLI test a go on the main earth next week.

Although i did test the resistance of the lug back to earth bar which passes.

I did do EFLI test on different power circuits in the home, and due to the low resistance values i assume that the earth electrode is layed correctly.
   

codaxx
Apr 15 2018 17:28

Have not heard about SS electrodes until now, most homes i have seen around here either have buried or older homes, galvanised rods. AlecK, do you know why they have decided to leave them out? Conductive properties not as good?
   

AlecK
Apr 15 2018 17:40

Yes s/s clad is in the current table; solid s/s is not - but it will be in the new edition.
ONLY those listed in the table are acceptable under clause 5.3.6.2.

I have no idea why solid s/s was omitted from current table. Perhaps they just never thought anyone would want to use it.

   

Sarmajor
Apr 15 2018 21:04

Codaxx
I did do EFLI test on different power circuits in the home, and due to the low resistance values i assume that the earth electrode is layed correctly.

Nope you just proved that the MEN link is in place and effective. Most of the earth loop that you test at the power point is actually the phase and neutral of the supply mains. The only earth wire in the EFLI loop tested from a power point is in the wire from the switchboard to the socket outlet.