Earth Rod size


O

Osprey

Installing a small TT installation ... need to fit an earth rod … is
there any rule or decision factor that decides on the size to use …
standard 4’ rods come in 3/8” or 5/8”

Maximum current feed to installation is 32A
 
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C

Colin Wilson

Installing a small TT installation ... need to fit an earth rod =3F is
there any rule or decision factor that decides on the size to use =3F
standard 4=3F rods come in 3/8=3F or 5/8=3F
Bang enough in to ensure a low enough resistance. One might be enough
depending on the type of ground, sometimes twenty might be required.
 
C

cynic

Installing a small TT installation ... need to fit an earth rod … is
there any rule or decision factor that decides on the size to use …
standard 4’ rods come in 3/8” or 5/8”

Maximum current feed to installation is 32A
There is not "normally" a lot of difference in the thinner and thicker
rods performance but the thicker rods are physically more able to
drive straight if the ground contains stones etc. The ground
conditions do have a more considerable effect but your location is
critical to this, i.e. are you on dry well drained chalk or on
permanently damp clay.
You should ensure that the electrical characteristics meet the
requirements which will require at the very least an earth fault loop
impedance test carried out at the origin of your installation. Look
for less than 200 ohms to avoid instability due to drying/freezing. I
usually find the conditions in my location give between 20 and 40 0hms
(Loam over a bed of clay) but your situation may be completely
different.
It is also a good idea to use an earth rod chamber to make the final
connection to the rod as this will give protection and reduce the
surface voltage gradient in the event of a fault. It must provide
access to inspect the termination at intervals.
The conductor you use must be suitably selected/protected to avoid the
effects of corrosion and mechanical damage. Does your local library
have Regs Guidance notes in the reference section?

HTH
 
J

John Stumbles

The bronze earth rod coupler for the 5/8in rods is also worth the money,
to avoid damaging the threads in case you need to go down deeper. It's
easy to make an adapter to drive the rods with an SDS drill (hammer only).
Or to make a pilot hole with one of those extra-long, thin SDS drill bits.
 
J

John Stumbles

However, the pilot hole mustn't be too large, because a loosely-fitting
rod would have poor electrical contact with the ground. A 10mm bit will
make too large a hole for a 3/8in rod, so that is yet another reason to
step up to 5/8in. A 12mm bit is usually about the right diameter, giving
a nice firm fit for the rod.
Unless you're drilling into granite the soil is likely to collapse back
onto the rod within a short period so an oversize hole should not
permanently impair the effectivess of the rod.
 
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C

Colin Wilson

BTW, I did try sinking a rod into almost solid rock, and even when the
ground was soaking wet it measured 630 ohms! Two years later, it had
come down to 580 ohms. Complete waste of time, but we keep it as a pet.
IIRC on substation "hot sites" sinking rods into holes filled with
rock salt helps...
 
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J

John Stumbles

BTW, I did try sinking a rod into almost solid rock, and even when the
ground was soaking wet it measured 630 ohms! Two years later, it had
come down to 580 ohms. Complete waste of time, but we keep it as a pet.
You'll deserve all you get from the bunny-huggers if you keep your pets
buried up to their necks in the ground :)
 

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