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TT Earth Spike Installation questions

Former Community Member
Former Community Member
I have two outbuildings going up and I am getting myself tied up in knots trying to decide exactly how I going to do it. Bit of background, this is a DIY installation under BC that will be inspected and tested by an electrician appointed by BC. I need to pass on first test as subsequent tests are expensive. I have a number of questions where the experience of the forum members will be helpful.

Q1) cables to the electrode - In selecting a most likely spot where I will get an earth rod into the ground (fairly hard chalk, overlaid with clay type soil) I am getting confused by some of the terms in the OSG. Table 4.2 ( from the 17th, awaiting for 18th) states that I can gor for a little as 2.5mm2. However, when buried, it goes to 4mm2 for mechanical damage protected, 16MM for corrosion protected and 25mm unprotected. What constitutes mechanical and corrosion protection? Does the PVC insulation on single core cable provide corrosion protection? if not, can someone point me at a cable type that has a suitable sheath layer that does? With regards to mechanical protection, I presume I am again looking at cable tiles/concrete sections or steel pipe protection? or given the danger to the person digging is less significant, would a steel flexible conduit suffice? I do appreciate that a cable break would leave a dangerous condition in the outbuilding.

Q2 - Earth rods 1 - I fear I am going to struggle to get the earth resistance down to a good figure, aiming for sub 50 ohms. I am not clear what gives me a better chance, one deep rod or two shallower rods (around 3m apart). Depth may be a real problem into the chalk (peppered with flints!). My soil conditions vary, around one installation, the soil is is fairly rich for about 30cm, then clay mixed with stones for another 30cm down to chalk. The other area is not as good, it doesn't have the soil (well 5cm or so). 

Q3) What is the correct way to connect multiple rods, single cable  CU - Rod1 - Rod2 etc,?

Q4 - Earth rod alternatives - I have two redundant galvanised steel pipes, 1/2 inch that are about 45m long, both are buried at a good depth and completely disconnected at each end. They run in parallel for most of their length and are about 1.5 m apart. Could I use these to supplement my rods? could I just use them without rods? if so, do you think my Inspector/tester will have any issues as long as my Ra is satisfactory. 


Thanks in advance for any advice offered.


Mike
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    Yes, the G/Y PVC insulation on will normally provide corrosion protection to the conductor (presuming the water table isn't so high that it's going to be submersed in water for significant periods or anything like that). Just make sure that the conductor isn't then vulnerable at the rod connection where the insulation is removed to make the connection (I've used a few wraps of self amalgamating tape to sort that).


    Mechanical protection depends on the environment - but for a normal domestic environment with reasonably sensible positioning I would have though that something like rigid PVC conduit into a suitable enclosure for the head of the rod would been fine.


    Rod depth is tricky. One problem is that when the ground freezes in winter the frozen layers have a much higher electrical resistance (despite being wet) - so generally you need to get well below the frost line. As a guide water pipes have to be at least 750mm down to avoid freezing. Likewise upper layers of soil can dry in summer, again leading to increased resistances.  So, depending on where you are, if you have a 1.2m rod driven vertically, perhaps only the bottom half is going to provide reliable service. There are various approaches about digging holes and backfilling with conductive additives, or using a 'ditch tape' under the building foundations in difficult conditions - but other here will know more about those approaches than me.


    I don't think there's any prescribed way of connecting rods - individual cables back to some common point might be useful if you ever need to test them individually - but generally whatever suits the particular layout best.


    45m of buried metal - if reasonably deep - sounds like an ideal electrode to me. Provided you can make a decent reliable (and preferably inspectable) connection to them. BS 7671 specifically lists pipes and 'suitable underground metalwork' as suitable for use as an electrode. The only prohibition on using pipes is if they're carrying gas or belong to a water utility - neither of which seem to apply in this case.


       - Andy.
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  • 0
    Yes, the G/Y PVC insulation on will normally provide corrosion protection to the conductor (presuming the water table isn't so high that it's going to be submersed in water for significant periods or anything like that). Just make sure that the conductor isn't then vulnerable at the rod connection where the insulation is removed to make the connection (I've used a few wraps of self amalgamating tape to sort that).


    Mechanical protection depends on the environment - but for a normal domestic environment with reasonably sensible positioning I would have though that something like rigid PVC conduit into a suitable enclosure for the head of the rod would been fine.


    Rod depth is tricky. One problem is that when the ground freezes in winter the frozen layers have a much higher electrical resistance (despite being wet) - so generally you need to get well below the frost line. As a guide water pipes have to be at least 750mm down to avoid freezing. Likewise upper layers of soil can dry in summer, again leading to increased resistances.  So, depending on where you are, if you have a 1.2m rod driven vertically, perhaps only the bottom half is going to provide reliable service. There are various approaches about digging holes and backfilling with conductive additives, or using a 'ditch tape' under the building foundations in difficult conditions - but other here will know more about those approaches than me.


    I don't think there's any prescribed way of connecting rods - individual cables back to some common point might be useful if you ever need to test them individually - but generally whatever suits the particular layout best.


    45m of buried metal - if reasonably deep - sounds like an ideal electrode to me. Provided you can make a decent reliable (and preferably inspectable) connection to them. BS 7671 specifically lists pipes and 'suitable underground metalwork' as suitable for use as an electrode. The only prohibition on using pipes is if they're carrying gas or belong to a water utility - neither of which seem to apply in this case.


       - Andy.
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