Oh dear not really a DIY job but may as well tell you.
If the earth wire is protected and not under ground min size is 6mm² but 16mm² or even 25mm² if the earth wire is not protected. If it is ever converted to PME than it will need to be 10mm² so I would suggest you use at least 10mm².
As to the testing there are two methods there is a special meter for testing earth rods but it needs around 30 meters of ground to test it against but has the advantage of not being dangerous to use.
The other way is with an earth loop impedance tester this can be dangerous as you need to used the 230 volt supply but does not need the room to put in test stakes.
There is a table 41.5 to work out results required but in general 200 ohms is required assuming you are using an Earth Leakage Trip on all circuits.
Since the meters cost around £70 to hire I think it is unlikely you will do the job yourself so at this stage I am not going any further.
the wiring regs state if you have 16mm size tails supplying your home then the earth must be equal or if 25 mm tails supply the home then 16mm earth cable must still be used so the minimum size earth you can use is 16mm unless you know how to use the hepatic equation to find out if you can use 10mm earth but i suggest you take my advice and use 16mm earth to comply with the wiring regs.
There are so many rules about what can and can not be done with earthing and they have also changed so many times if you wants to quote “Regulations say” one really does also need to quote with regulation.
514 is the general area but first though some definitions:- Illustration 2.1 shows the wire between the earth rod and main earthing terminal is called “earthing conductor” the “main protective bonding conductor” goes between the extraneous-conductive-part and the main earthing terminal.
Next we move to Table 41.5 which with note 2 gives maximum resistance of earth rod at 200Ω.
Table 51 tells us the cable should be “Green-and-yellow” and 514.13.1 tells us that it must be labelled.
542.3.1 tells us how PME is different to TT and 544.1.1 is only for PME. And also table 54.1 is only for cables buried in the ground.
542.4.2 refers to disconnection points of the earth rod in order to test.
544.1.1 It the one I am really interested in where it says how when PME is not used the cross-sectional area required is not less than 6mm² and table 54.8 is only for PME.
Now with so many cross references it is of course very easy to read the wrong regulation and one must also consider if buried Table 54.1 comes in which could require 25mm² copper if not protected against corrosion but although there may be a regulation under certain special circumstances that requires a larger conductor the limit where the conductor is protected as I read it is 6mm².
Since the earth rod is likely to be between 21Ω (Min outside consumers installation) and 200Ω and only take enough current to operate an earth leakage trip I can understand why the cable is so small. When working in Point of Ayr gas terminal we required 8Ω for each earth rod and I had to run out the cables to the test spikes and check them all 6 months work and we needed between 4 and 8 x 1.2 meter rods to get this reading. Although in a sandy salty soil and this will differ area to area I would not think one would ever get enough current to flow through a single earth rod to cause even a warming of a 6mm² cable. Especially in the 40 milliseconds it takes to trip a 30ma trip and even the S type 100ma trip would hardly allow enough Joules of energy to warn a 6mm² cable.
As I said in my first post not really a DIY job having used a earth rod testing tool getting the spacing of the spikes correct to arrive at a static reading takes some skill, and if an earth loop impedance meter is used instead, it can, if not done very carefully, put 230 volt onto items which should be earthed, and so very dangerous if not done correctly. Pluss the expense of hiring the meter at around £70 or buying at £250 so I did not go too deep into how to do it.
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