Installing a small TT installation ... need to fit an earth rod =85 is there any rule or decision factor that decides on the size to use =85 standard 4=92 rods come in 3/8=94 or 5/8=94
Maximum current feed to installation is 32A
Bang enough in to ensure a low enough resistance. One might be enough depending on the type of ground, sometimes twenty might be required.
The thicker ones are better if you need to use multiple lengths to get enough depth. If however you can get to permanently wet clay within a couple of feet like in these parts, the thinner ones are fine.
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
For the same length and surface finish, there is very little difference between the electrical performance of a 3/8in or a 5/8in earth rod. But there may be other more important differences.
I use them with radio aerials, so unlike mains earth rods they sometimes get pulled out and can be checked for corrosion. The 5/8in rods have much thicker and better-quality copper plating, at least in my limited experience.
All the 3/8in rods from Screwfix have turned into red rust within two years. Both above and below ground, the thin copper plating was completely gone. (I didn't measure their effectiveness as earth rods, but dread to imagine.)
By contrast, after 10 years in the ground, a 5/8in rod made by Furse came out looking almost like new. The only rust was where the point had met a few stones, and most importantly those bare patches had not spread. Furse have a proprietary bonding process, and the layer of copper is also much thicker than on the cheap rods.
The 5/8in rods sold by TLC are almost certainly made by Furse (TL358):
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).
Yup, good choice. I have used the screwfix ones but they are nowhere near as nice.
Or to make a pilot hole with one of those extra-long, thin SDS drill bits.
Yes, a pilot hole can be very helpful to clear the way for the rod. An SDS drill bit is much better than the point of the rod for nudging stones out of the way, and even breaking them up (it's surprising how much impact energy does reach the far end of a metre-long bit) so the pilot hole almost guarantees that you'll be able to sink a 1.2m rod at least as far as 1.0m.
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.
That's true... but how long might you have before the electrical test?
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...
You'll deserve all you get from the bunny-huggers if you keep your pets buried up to their necks in the ground :-)
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.