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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
  • 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.
  • 0
    Wolfie100:

    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


    Your lengths of redundant disconnected galvanised steel water pipe will provide good earthing and can serve as earth electrodes, and will no doubt be better than any 4 foot long earth rod. 542.2.2.(vii). You can join them together if you wish.


    I presume that you are providing a TT earthing system at the outbuildings with 30mA R.C.D.s for all final circuits, as any earth rod or other type of electrode will not in itself  normally comply if you are just using M.C.B.s or fuses.


    Table 54.1 refers to BURIED earthing conductors. So if you provide earthing pits and the earthing conductor is not buried it will not apply. We normally use a 16.0mm2 green and yellow single cable for earthing conductors connected to earth electrodes in domestic properties. 


    It sounds as though you need to employ an experienced and qualified electrician for this job.


    Z.






     


  • Former Community Member
    0 Former Community Member
    Thank you. I am going to give the galvanised pipe a go. I will have use of an earth resistance tester this weekend so will check it out. Apologies for more questions, what would you use to connect to the pipe? I assume a standard bonding strap in unsuitable. Most connectors I have seen so far are for 5/8 or 3/4 rods. I could drill the pipe where exposed and get a nut and bolt through with a plate to form a clamp. Do I need to be concerned with the reactivity between copper/zinc?
  • Former Community Member
    0 Former Community Member
    Thank you, I am certainly going to make use of the pipe, may include a supplementary rod as well. I can assure you the installation will include RCDs all suitable for a TT set up and all will be subject to inspection and test by a Building Control appointed electrician.
  • 0
    Wolfie100:

    Thank you. I am going to give the galvanised pipe a go. I will have use of an earth resistance tester this weekend so will check it out. Apologies for more questions, what would you use to connect to the pipe? I assume a standard bonding strap in unsuitable. Most connectors I have seen so far are for 5/8 or 3/4 rods. I could drill the pipe where exposed and get a nut and bolt through with a plate to form a clamp. Do I need to be concerned with the reactivity between copper/zinc?


    I would use a B.S. 951 bonding clamp for the connection to the earth electrode old water pipes, in an earth pit with cover and warning label.  The standard size is up to 35mm dia. according to the box details on mine.  Larger straps are available. The outdoor "wet" type are necessary to avoid corrosion. (Blue label or better). I would not drill holes into the old water pipes as that may attract corrosion.


    Z.


     


  • 0
    I'd not bury a BS 951 - they may be the 'standard' but they are not that great a design in the outdoors. If the pipe or a screw on fitting will take a cross bolt of about M6 or more, then do that, with a star or spring washer to give it some decent 'graunch' and then once it has been checked out protect with grease or paint ( not both ! ) Try and locate (bend the pipe tail or add a right angle fitting ?)  to give you an accessible join.  Ideally the joint needs to be not totally exposed to the elements, it will rot off eventually of course, but we'd like that later than sooner.

    There are proper little earth terminal covers that bury with just the cover showing (example) if you need to lose a connection into a concrete path or something and need to be able to  inspect without a trip hazard.

  • Former Community Member
    0 Former Community Member
    Thanks, I understand reticence to use that type of connection is this type of use. Indeed, it was why I asked in the first place. However, I also appreciate Z's comments on drilling and corrosion. I am accessing the pipe at approx. 1/3 of its length  I have some U bolts that would give a good tight mechanical connection and avoid drilling/sawing the pipe. What do you think? Will it pass muster with the inspecting electrician? The connection will not be buried. I will leave it uncovered by earth and accessible, albeit at nearly arms length, via a 300mm x 300mm access chamber with a lid. An earth pit/terminal cover may be too small to get a hand on and I already have some access chambers.
  • 0
    If you mean the things that look like small exhaust clamps, then yes, a couple of thse should be fine - either fan the strands out and do not tighten it to the extent it actually cuts the wire,or crimp the wire end solidly into a lug and bolt that on. I cannot see it though, so ideally ask the person  who will be inspecting.
  • Former Community Member
    0 Former Community Member
    Hello again. This weekend I have got myself a Peaktech 2700 to test my TT earthing proposals. Whilst it appears to be a nice little tester, the instructions were poor and I used some information from Megger to conduct my tests.

    I have a point close to the garage where I can access the redundant pipe and form a good bonding connector using a U bolt clamp. However, one of the limitations of the tester is cable length, I could not be sure I was getting the mid probe outside of the influence of the pipe due it the pipe runs underground, Hence, I extended the pipe over the top of the ground using insulated 10mm2 copper to give me in effect, a different access point from where I could conduct the test. Please see picture below.

    I conducted various tests moving the mid probe and also going out at various angles. I was somewhat limited due to hard surfaces as to direction of test. My results were great, 28 ohms average and all results with 3 ohms until I moved probe much closer the the outer probe where I started to see an increase. Would value your opinion on the way I have conducted the test.

    However, I may have a problem and would appreciate your view. The redundant pipe is cut off as it exits the floor plate in the house porch. It is in contact with the house floor plate. Do I need to break that connection by cutting pipe off prior to entry to the house?
    dddc681ed071bf4be9e2889327eb562e-original-earth-test.png
  • 0
    You are doing quite well if your results are within 10% of each other, results like 50+/-20 are quite possible from a batch of tests on the same electrode with the tester spikes moved around a bit, especially with an electrode layout that has no rotational symmetry. and probably water tables interrupted by building foundations and so on.

    In the end the only one that counts is the AC resistance back to the neutral/earth of the supply, as that is what fires the RCDs, and usually you can play a  trick with most of these testers to link things either internally or externally   get into a 2 wire mode were you can do that test too, Maybe the translated instructions are not clear on how.


    If it is easy to do, I'd be tempted to cut the pipe branch that goes to the house, just so you can say the systems are truly separated, though in reality the risk is small - a fault in the house has to make the person leaving the building 'live' relative to ground, and then they touch the pipe that is outside potential - it is not that likely. And actually if a person is at live potential relative to ground as they leave the house then  you have other more serious problems as soon as feet touch the path outside, so it is not the kind of fault condition likely to go un-noticed.

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