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Posted By Topic: 2 SWB, 1 Earth Electrode

Sep 20 2016 07:52

In a domestic situation, I have a 3 phase supply from the boundary into the outside meterbox, and only two phases are metered/used. From the meter, I have two phases, each going to separate Switch Boards inside the house. One SWB is located in the Hallway and powers half the house, and the other SWB is located in the Dining room powers the other half of the house. I can only see 1 Earth Electrode and this is connected to the hallway switchboard. The client has no idea where or how the second SWB is earthed and I couldn't see a second earth electrode on the property. It is possible that the second switchboard is connected to the 1 electrode (yet to test) but I couldn't find anywhere in the STD that says this is acceptable.

Sep 20 2016 08:21

Insufficient information; but generally whether the existing set-up is compliant or not will depend on when it was installed.

Sounds as if both swbds are set up as MEN MSBs.
If so, and if they are sharing same electrode, there will be a parallel path since the N of each swbd is connected to the electrode by the MEC. That means the MECs will be carrying normal load current; which is not allowed these days , but used to be common

Sep 20 2016 08:46

Thanks Aleck. Yes Both are MEN swb. The Hallway swb has been upgraded. The Dinning will be shortly. I will install a new MEC/Electrode and organise an inspection. Cheers.

Sep 30 2016 10:31

What about the following situation when connecting two MSBs to one earth electrode;

There is a MSB for the building with an earth electrode outside.

Then in the building there is also going to be an independent supply via solar panels/battery's and an inverter going to its own MSB and supplying a few circuits inside the building.

Is it ok for this independent supplys main earth to also be connected directly to the same earth electrode outside that is being used by the buildings MSB ?

The other way of doing it would be to run the independent supplies earth to the buildings MSB however that will result in higher resistance of the main earth...


Sep 30 2016 12:41

5.3.1 requires that each installation that provides protection by automatic disconnection of supply to have earthing arrangements that include certain specified parts.
One of these parts is an electrode.

So that means if there are two separate installations, each requires its own electrode etc.

UNLESS one of them isn't using automatic disconnection to provide (fault) protection.
Which is not somewhere you'd want to go without some VERY hard thinking


Sep 30 2016 14:33


Not sure if im interrupting this right
5.3.1 does not state that it an installation needs to have its OWN electrode. It just says it needs to have an electrode. So it can use any electrode as long as its below 0.5ohm?

Sep 30 2016 14:35

** does not state that an electrical installation needs to have its own electrode.
Content of that sentence was incorrect


Sep 30 2016 16:48

5.3.1 lists the requirements for earthing arrangement for "an electrical installation".

One of those requirements.-
"Shall include the following parts:"
(e) Earth electrode

Sep 30 2016 17:11

There is no requirement for the earth electrode [5.3.1(e)] to meet any specified resistance value, but it does need to meet the requirements of 5.3.6.

The main earthing conductor [5.3.1(b)] does have a specified resistance and must be installed to 5.5.1.

As such, you can't run the MEC from one switchboard to another, but I can't see anything to say that you can't run the two MECs from two Main Switchboards to the same earth electrode.

Sep 30 2016 18:39

Andrew ;

As such, you can't run the MEC from one switchboard to another, but I can't see anything to say that you can't run the two MECs from two Main Switchboards to the same earth electrode.

Thanks Andrew exactly my thoughts - it doesn't say anywhere you cannot have multiple installations(two MSBs) to the same earth electrode. So its fine to do so according to the wiring rules.

Sep 30 2016 19:59

You may be right. Or not.

But there is NO possible safety or performance advantage, so saving the cost of an electrode is the only difference.
so for the tiny cost of an electrode; why the f%$# would you bother?

Oct 06 2016 14:37

minimal room for another electrode without removing concrete... so how close can you space two electrodes for two different installations?

Im just going off what the book says/doesnt say.

You could say the book may be right or not


Oct 06 2016 17:16

The only item in 3000 that gives a spacing for this situation is 3.11.5 which refers to underground wiring systems.
An earth electrode is an underground part of a wiring system and therefore must be spaced 100mm minimum from any other earth electrode which forms an underground part of another wiring system.

Also seperate earth electrodes for seperate installations would seem to make sense from an ownership / responsibility point of view.

When the electrode eventually rots away who is responsible for replacement and meeting the cost.
When the electrode fails you will have two installations potentially without an earth electrode.

In the event of a neutral fault on either installation the results will be disastrous for one or both installations sensitive electronics especially if the two installations are on different phases from the same transformer. Even worse if it is a rural two phase transformer.