General Q about earth rods
- Thread starter ban-all-sheds
- Start date
If we`re not working on a substation, conducting lightning or working for the DNO then I think the best way is a 4` rod or two or three.
Extendable rods help you get below the water table but can be expensive and it doesn`t follow that twice as deep means half the resistance/impedance.
Depending on soil types etc 4` rod might suffice but two or three would substantially reduce this especially if placed 1D (depth of rod ) or better still 2D apart.
Rule of thumb (remember rules of thumbs are exactly that and nothing more).
If you have a rod it is likely that 80% ish of its resistance is within the radius of the depth of rod and 20% is to the general mass off earth.
So if you have a rod 4` deep (1200mm) at 100 ohms then another rod in identical soil 2D from it is 100 ohms and a third rod 2 D away ditto.
Coonect two to get 80/2 = 40 + 20 for the mass = 60 total three = 80/3 = 27 + 20 for the mass = 47.
Actually measured readings suggest the metered results might actually be better than this approximation but then again how accurate is the test gear?
One or two or three would be the way I`d go rather than tape but then it depends upon what you`ve got in space and money considerations.
By the way, clay type soil is not the best conductor but it helps keep water in so can be more consitent.
Extendable rods help you get below the water table but can be expensive and it doesn`t follow that twice as deep means half the resistance/impedance.
Depending on soil types etc 4` rod might suffice but two or three would substantially reduce this especially if placed 1D (depth of rod ) or better still 2D apart.
Rule of thumb (remember rules of thumbs are exactly that and nothing more).
If you have a rod it is likely that 80% ish of its resistance is within the radius of the depth of rod and 20% is to the general mass off earth.
So if you have a rod 4` deep (1200mm) at 100 ohms then another rod in identical soil 2D from it is 100 ohms and a third rod 2 D away ditto.
Coonect two to get 80/2 = 40 + 20 for the mass = 60 total three = 80/3 = 27 + 20 for the mass = 47.
Actually measured readings suggest the metered results might actually be better than this approximation but then again how accurate is the test gear?
One or two or three would be the way I`d go rather than tape but then it depends upon what you`ve got in space and money considerations.
By the way, clay type soil is not the best conductor but it helps keep water in so can be more consitent.
Umm.....If the generator truly was floating with wings, then the RCD would not operate if someone was standing on the ground and touched a live conductor......
BUT...
If the generator is sat on soil, being a huge metal frame, it will make contact with the earth......and 30, 100...hell.....even 500mA is not a huge amount of current to find it's way back to the gens frame.
BUT...
If the generator is sat on soil, being a huge metal frame, it will make contact with the earth......and 30, 100...hell.....even 500mA is not a huge amount of current to find it's way back to the gens frame.
Yea, but obviously I was talking as if there was, just no rod.
I was talking about the N/E floating in respect to the ground.
I was talking about the N/E floating in respect to the ground.
Oh, I see where our wires have been crossed 
, I thought we were talking about the small generators (iirc Single Phase up to 10KVA) where the alternator windings are left floating and normaly the protection against indirect contact is by electrical separation.
, I thought we were talking about the small generators (iirc Single Phase up to 10KVA) where the alternator windings are left floating and normaly the protection against indirect contact is by electrical separation.
My 80% figure might have been wrong and should be nearer 90% but remember it`s still only a rule of thumb and there are many variants.
so 100 ohm with one rod might be nearer 55 with 2, 40 with 33 with 4 down to 19 with 10.
So I`d still say 2, 3 or possibly 4 rods if you want a really low reading providing they are placed far enough apart. it might be less hassle/expense than the tape if you`ve enough room to do it.
so 100 ohm with one rod might be nearer 55 with 2, 40 with 33 with 4 down to 19 with 10.
So I`d still say 2, 3 or possibly 4 rods if you want a really low reading providing they are placed far enough apart. it might be less hassle/expense than the tape if you`ve enough room to do it.
All I want to do is to provide a local earth for a temporary generator.....
ban-all-sheds";p="773915 said:
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