Earth Wire To Ground Rod: How Long Can It Be?

[anatta]

I want to replace the ground rod that the builder has located under the house in dry sandy soil (and may well be short and of poorly conductive metal). I'm thinking of running the earth wire (10 sq. mm?) from the CU out of the house through the air to a nearby concrete pole, then down to a decent 2-metre copper sheathed rod in soil that never fully dries out. The total length of the earth wire would then be about 20 metres: Would that be OK?

Please advise.

[bazmlb]

I dont think you need a 10mm feed, especially if you use a RCD in the board as it only takes milliamps to trip them. But 10mm should be fine, the area really should be as moist as possible, the earth stake should be a bare copper rod of around 2 meters or more depending on how dry the soil gets, I am not an electrician, so get some professional advice it could save your life.

[electau]

10sqmm Copper conductor (green/yellow) is quite satisfactory.AS3000 requires only 4sqmm minimum. 10sqmm appears to be the minimum under MEA/PEA requirements.

Max resistance of the main earth conductor from the main earth bar to the point of conection at the electrode, 0.5ohms.

Electrode should be copper clad steel minimum depth in soil 1.8M. Connection should be protected by metallic paint.

Electrode should be exposed to the weather and not encased in concrete.

[Crossy]

I would install your rod next to the house as close to the CU as practical. As Electau notes 10mm seems to be the MEA/PEA requirement but 6mm would be fine.

It's a fallacy that the rod needs to be damp, at 2m down the ground is always damp anyway (even sandy soil) if you're worried put it near the condensate drain of an aircon.

It's quite possible that your existing ground is more than adequate but to verify it you need specialised test gear (and someone who knows how to use it).

[T_Dog]

Annata, Also consider that if you run your ground line in the air for some distance, you make it more vulnerable to getting damaged and cut. Better to keep it short and contained. Have you considered using a better quality rod and longer length in its present location? As Crossy says, the ground is most likely to be be fine where a longer rod ends up.

[anatta]

It's quite possible that your existing ground is more than adequate but to verify it you need specialised test gear (and someone who knows how to use it).

Thank you for this.

I know that "rough and ready" isn't a good mindset for approaching electrical safety, but I seem to remember reading somewhere about a method of testing the quality of a domestic electrical earth by connecting a light bulb across live and earth (to blow it)? Am I mistaken? (If not, would I be foolhardy to try this?)

[anatta]

Annata, Also consider that if you run your ground line in the air for some distance, you make it more vulnerable to getting damaged and cut. Better to keep it short and contained. Have you considered using a better quality rod and longer length in its present location? As Crossy says, the ground is most likely to be be fine where a longer rod ends up.

Thank you. I'll definitely explore all alternatives before deciding to go 'aerial'.

The existing rod is in the (recently land-filled) earth under the ground floor. The crawling space is really too low for hammering in or pulling out a rod of 1.5 or 2 metres' length. I'm fairly sure that what the electrician put in was a copper-painted (!) rod one metre long.

[electau]

AS3000 does not require the resistance of the electrode to the surrounding soil to be tested.

The resistance from the main earth bar to the electrode must be less than 0.5ohms.

The minimum size is 12.7mm copper clad steel for the electrode.

However where the TT earthing system is in use the regulatory authority may require testing. An earth resistance tester is the instrument used.

The actual values will depend on type of soil and moisture content, it is advisable to locate the electrode so as it is exposed to the weather, eg against the outer wall of a building.

The earth conductor should be protected where subject to mechanical damage.

[Forkinhades]

AS3000 does not require the resistance of the electrode to the surrounding soil to be tested.

The resistance from the main earth bar to the electrode must be less than 0.5ohms.

The minimum size is 12.7mm copper clad steel for the electrode.

However where the TT earthing system is in use the regulatory authority may require testing. An earth resistance tester is the instrument used.

The actual values will depend on type of soil and moisture content, it is advisable to locate the electrode so as it is exposed to the weather, eg against the outer wall of a building.

The earth conductor should be protected where subject to mechanical damage.

Must disagree,

A TT system must be used in conjunction with an RCD, where the resistance of the rod can be as high as 200 ohms ( in fact 1667 ohms would still be adequate to trip an 30 mA RCD, but anything over 200 ohms is considered unstable BS 7671 2008)

Using a rod without an RCD is in effect useless.

[electau]

AS3000 does not require the resistance of the electrode to the surrounding soil to be tested.

The resistance from the main earth bar to the electrode must be less than 0.5ohms.

The minimum size is 12.7mm copper clad steel for the electrode.

However where the TT earthing system is in use the regulatory authority may require testing. An earth resistance tester is the instrument used.

The actual values will depend on type of soil and moisture content, it is advisable to locate the electrode so as it is exposed to the weather, eg against the outer wall of a building.

The earth conductor should be protected where subject to mechanical damage.

Must disagree,

A TT system must be used in conjunction with an RCD, where the resistance of the rod can be as high as 200 ohms ( in fact 1667 ohms would still be adequate to trip an 30 mA RCD, but anything over 200 ohms is considered unstable BS 7671 2008)

Using a rod without an RCD is in effect useless.

I was referring to testing of the main earthing conductor and electrode only. Not to the installation of RCDs which are protective devices. Yes, RCDs must be used with the TT system to comply with legislative requirements.

[electau]

The main earth consists of two parts:

1. The main earth conductor and the electrode. Max resistance 0.5 ohms. 12.7mm copper clad steel electrode 1.8M in the ground

2. The contact resistance between the electrode and the general mass of earth.

With the MEN system (AS3000) only item 1 requires testing.The TT system is not used under AS3000. The MEN system is mandatory.

However in other countries where the TT system is employed testing of the contact resistance may be required by legislation to meet the minimum requirements. (BS7671/2008).

In the MEN system any current flowing in the main earth in the event of an earth fault may be ignored, as the fault path is through the MEN link and main neutral conductor to source of supply.

In the TT system the fault current flows through the main earth and general mass of earth to the point of supply (neutral). This is generally not sufficient to trip an MCB so RCDs must be used.