house earth vs garage earth spike

Discussion in 'Electricians' Talk' started by steve go, May 26, 2016.

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  1. steve go

    steve go New Member

    Hi all,

    Am just about to kick off training to become an electrician - having dabbled a bit, I realised how much I don't know and so want to get it sorted properly. In the mean time though, my old man had a question for me which I couldn't find the answer to easily so hoping you guys might help.

    We have a house that has a garage, 20m away with a run of what looks like 4mm SWA from one of the house consumer units, which is RCD protected and on a 32amp MCB. It is 2 core, with the armour earthed at the house end, but not connected at the garage end.

    At the garage end, it runs to a new consumer unit, which is earthed via a spike in the ground behind the building. That consumer unit serves the garage (ring, lights, electric door) but also another CU in a pump room for a swimming pool, which has a socket spur for the pump, lights for the room, and a transformer to drive a 12v pool light (and some v complicated looking timing, and remote starting stuff for the socket for the pump!).

    We have had two electricians come and quote for replacing the garage CU - and both said almost completely opposite things.

    One said that with a 20m run, and for such limited use, there was no need for the separate earth spike - and in fact, the spike is right next to the building, under the overhang of the roof, and in a spot where no rain gets to - it is very dry earth. He said that might be an issue as spikes are best in damp ground.

    The other said that we did need the earth spike, and it was fine where it was.

    They both agreed at least that the two should be kept separate, in case of any pd between the two earths.

    So which is right, and can someone point me towards the right part in the regs?

    All help v much appreciated - I strongly suspect that the training I am going to do isn't going to scratch the surface and I think I will need to get a decent apprentice job to learn as I go!

    thanks

    Steve
     
  2. philthespark

    philthespark Active Member

    This is an arguement that has gone on for ages,there are all kinds of myths about not exporting earths from the main building to an outbuilding,in actual fact it isn't exporting an earth it is extending one,in any situation you would want to see what reading you are getting from your rod.for a tt system(rod) the acceptible figure is quite high,and obviously you want your earth resistance as low as possible,so if it was just a garage,I'd take the earth from the house.However where pools are concerned this can be a different situation,I've heard of people complaining of getting "tingles" when being around the pool,obviously this isn't a good idea,but the reasons for it are a bit complicated.If using a rod then a common mistake a lot of people make is to just bang one 3/8ths one in,then the reading is going to be high and quite possibly unstable,if you connect 2 or more rods and go deeper then you'll get a more stable earth value.I won't start quoting numbers to you as if you haven't got a meter they'll mean nothing anyway.
    I don't normally have any issues with earthing but then again the type of "sheds" that I work on have massive steel frames concreted into the ground,getting a good loop value is never an issue.
     
  3. Coloumb

    Coloumb Screwfix Select

    I believe the swimming pool tingling is caused by the volt drop back to the trannie causing a slight difference in potential between earth and the installation. I think it's only PME earth types that exhibit this quirk.
     
  4. philthespark

    philthespark Active Member

    Agree mate,I knew it was down to a PD but didn't want to risk confusing the OP, we don't know how much he knows.
     
  5. steve go

    steve go New Member

    Thanks both - that is really helpful. I think he is TN-S for the main supply and house, but will double check later as think it has to be labelled where it comes into the house doesn't it? so am assuming that I have a separate earth supply into the house and is not tied to neutral. For some reason, when the installation was done, the guy doing it must have assumed that the extension of the house earth to the garage wasn't sufficient and so installed the earth rod at the garage. I would rather get the best earth possible, and if TT is usually high resistance (relative to other options) and the fact it is a rubbish, single rod in dry ground then would think common sense would be to use the earth from the house? What was a little odd was that neither of the guys that came to quote bothered to test the earths - which i would think would pretty quickly tell the answer; and not having shelled out for any kit yet, I haven't got an earth tester myself so can't do it!

    Would love to understand the tingling point - but maybe sounds a bit advanced for me given where I am on the learning curve.

    Although an interesting point to throw into the mix is that the transformer is a 30 year old metal thing, which is badly rusted and I would expect to be replaced. It has an input point for earth, which makes sense for a metal fitting into the mains I guess, and then also has an earth output that runs to the light, even though the light is only 12V.

    From digging around in the regs, I think that a 12v fitting probably doesn't need an earth individually, but given it is a pool that my kids might end up in, would rather be safe than sorry (particularly given the exisiting fitting spec obviously needed one!). The complication is that I just took a look at the specs for the manufacturers current identical replacement for that transformer, and it is a plastic, double insulated job with no earth in or out. It is advised to be fitted with their new light fittings, which are also DE and don't need earthing apparently.

    But - my dad, and I, have no intention of changing the fitting - so would assume the installation when done will just take the earth direct from the CU to the light fitting, and the live/neutral to the light from the transformer....

    I didn't get into that conversation with the sparks that came round as was more interested in the earthing point!

    As if it needs to be any more complicated, it was originally a three phase supply that he got another sparks to put back to 2 phase when he had new CUs put in a few years ago - we aren't using him again for this as he did a pretty rubbish job! add to that the fact that across the property, there are no less than 7 separate consumer units, and a generator supply too, I don't think this would be the type of location I would start on! [does give some interesting theoretical challenges though!)

    Thanks again for the help and advice, very much appreciated.
     
  6. JP.

    JP. Screwfix Select

    12v/230v/pool/shed/spikes etc etc = qualified sparks to do job/s..circumvention of said is not an option Mr Go.
     
  7. steve go

    steve go New Member

    it will be - I just suffer from wanting to understand too, allied to the fact that I will be training to become one myself so hoping to learn on the job. That said, I am sure the last thing the electrician I get in to do the work will want is me hovering over his shoulder! Might offer myself as a free apprentice though!
     
  8. fire

    fire Well-Known Member

    Yup this is a old and disputable issue but in today's day it is pretty simple when you look at it and follow the regs.

    As is in the reg book less than or equal to 50Volts is the value to be used for earth fault calculations. Anything over 50v is deemed dangerous. In the days when RCD's were not used getting that resistance of the earth rod down was very important. Of course when an earth rod is placed in damp or wet soil the resistance will be lower so it is correct on one hand to place the earth rod in a place where constant moisture will be. No one can dispute this in respect to resistance.
    We can go into great depth here on how to install earthing rods some methods even prepair the soil with a copper sulphate solution sink away to increase soil conductivity.

    Today with RCD's the fault current required to open the device is in most cases 30Ma so taking the 50v from the regs and using ohms law we can divide 50/0.03 = 1666.6 Ohms(1.67Kohms) so if the resistance(impedance) of the earth is bellow this then the RCD will open in the event of a fault at 50v or less.

    Connecting the two types of earth together can create issues, you have to use a suppression device which only connects the earth rod once a certain voltage has been reach, usually 5,000-10,000volts by what is called a gas discharge device, that is the only way you will benefit from the two earthing systems connected to the same installation. This is literally entering into the surge and lightning protection and is well beyond the scope of your installation.

    The real world(theory aside) will be down to what the values read on your tester and what supply configuration you have into the dwelling. If it is a TNCS and your resistance(impedance) is acceptable using the earth from the SWA cable run then this is what you should use. I have seen it many times where a separate 10mm2 earth cable is run back to the dwellings earth block in parallel with the armour of the SWA cable.
    It has also been said on this forum that the use of a 3 phase SWA(3 core) cable is to be used so that the 3rd core can be sheathed with green and yellow and used as a CPC earth core along with the armour.

    It should also be noted that earthing systems create a whole new set of challenges for networking and transmission equipment, electro magnetic interference and so on. Something that is rumoured to be being looked into in depth and will be dealt with for the forthcoming 18th edition regs. Splitting the earthing up can resolve many issues in regard to this but has not been dealt with in the BS7671 very much over the past few years, that is changing due to the new EMI directives. Ofcom rarely if ever have been involved in the BS7671 but are very much digging into it recently so i expect to see allot of changes regarding this in the near future.

    So between the two electricians you have had look at your job, neither is wrong but i would favour the electrician who pointed out that the rod should be in damp moisture prone soil as this is a very valid point, it without doubt does reduce resistance(impedance).
     
  9. nffc

    nffc Active Member

    I stand to be corrected but I do not think it would be permissible to 'extend' the EBZ in the ops example due to the electric door.

    It would need to be TTd.
     
  10. Rulland

    Rulland Screwfix Select

    Am I really that blind?, door?.
     
  11. nffc

    nffc Active Member

     
  12. peter palmer

    peter palmer Screwfix Select

    What door? and why would a door affect electrics.
     
  13. peter palmer

    peter palmer Screwfix Select

    Oh that door, well I've heard of not doing it if metal pipes or structural steel are sunk into the ground etc but never come across not being able to export due to a door before.
     
  14. nffc

    nffc Active Member

    You were talking about extending the EBZ. Not exporting.
     
  15. leesparkykent

    leesparkykent Well-Known Member

    Having the swimming pool in the outbuilding means I suspect you would have extraneous conductive parts that require bonding? If this is the case then the SWA armour would not be adequate to be used as a bonding conductor and a work around would of been to make the installation at the end of the distribution circuit TT other wise a separate bonding conductor would of have to been installed or an adequately sized 3core SWA utilising the 3rd core for earthing/bonding.
     
  16. peter palmer

    peter palmer Screwfix Select

    Well if you export the earth you create a bigger equipotential zone and if there is anything extraneous in there it woud need bonding so same difference isn't it.
     
  17. nffc

    nffc Active Member

    No.
     
  18. Coloumb

    Coloumb Screwfix Select

    I see no reason not to use the SWA for earthing providing it's SWA CSA meets the requirements of table 54H. I'm always open to learning though so if you think otherwise...
     
  19. leesparkykent

    leesparkykent Well-Known Member

    yes for earthing but in most circumstances the armour will not be adequate for bonding. If a 10mm copper bond was needed then the armour would have to have a csa of least 85mm to be used as a bonding conductor
     
  20. Coloumb

    Coloumb Screwfix Select

    Why's that then mate? Surely it depends on table 54H. For a PME the incoming size of the neutral at the origin and for a TNS by an adiabic or reference to 54.8 (sorry not 54H (old skool)).
     

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