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tank/hv earth electrode and nearby concrete pad with mounted lighting column

psychicwarrior
Hello - good day to all !


Any experience/experiences that might provide a few tips/watch out fors etc, most gratefully received.


Contemplating the run of buried SWA cabled over 100m for outdoor lighting (and electric gates) passing by a pole mounted transformer, where it's been confirmed by the distributor that there is a Tank/HV earth electrode setup  at the pole. Whilst explaining this was not a rigid rule (more a guide and just as well as it shuts off the only feasible route), it was suggested best [in this case] to stay out of a 9m radius if burying cables - but the situation would be helped if this had to be compromised, by running the cabling in ducting/conduit when in the ground.


What are opinions on the siting of concrete pads with threaded rod to secure [conductive] column lighting in this context; this would seem a more serious concern within the 9m advice, is it a no-no, is there anything that can be done to mitigate (aside to using all non-conducting materials), or is it a non-issue (I feel not) ? 


I have no experience of such a long distance, but any tips and advice on pulling SWA along duct for over 100m would be uiseful too :-)   \\m/


Thank you all

Habs

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    mapj1:

    Or  for light duty situations  the all-insulated lamp post  You could do worse than call them to see what they offer for your situation Tel +44 (0)1623 511 111  I have no connection with the company other than our maintenance folk ordered a replacement post for a funny situation from them once.


    Quote:

    ”GRP does not conduct electricity, it is an insulator. As our columns are constructed entirely from GRP, they are a class 2 product. Being Class 2, our columns require no electrical earthing connection, therefore affording savings on installation. There is no risk of electrocution from touching the column, making it an ideal solution for low / medium voltage areas.”


    so so how do you install a cable without armour buried in a duct?


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    For what it's worth I too wouldn't be comfortable with a TT solution for equipment installed within the hot zone and supplied from an external LV source for the reasons that Alan and Andy set out. Also worth bearing in mind that we're not just talking about operators immediately in the vicinity of the hot zone: By installing conductors you may well create situations where the HV substation local earth potential is exported through the LV installation to others where precautions you might consider reasonable around a lampposst (e.g. shoes) aren't being taken. In other words you might be extending the substation hot zone into the house at the far end of the LV cable...


    For substations where the auxiliary LV supply is derived from an external LV source (e.g. 33kV sub fed from local network for resilience), it would be normal to feed the equipment via an isolation transformer, with a locally arranged TN earth.


    To put it into context Openreach require the installation of isolation transfomers at each end of each cable where a new substation imposes a hot zone that crosses their existing telecoms network, because of the risk to their remote equipment (and personnel).


    I would suggest, if possible:

    a) Confirm with the DNO whether the substastion really is hot, and if so where the hot zone contour actually is and what their rules are

    b) Re-route to avoid the hot zone

    c) Install cables within suitably insulating ducts and derive any equipment within the hot zone from an isolation transformer (rated for the EPR of course)


    Lastly do be careful if you're digging a trench across the hot zone as you might well expose - or worse damage - the electrode, especially if it's laid horizontal, which could well present a hazard to site operators.

     
    mapj1:

    Or  for light duty situations  the all-insulated lamp post  You could do worse than call them to see what they offer for your situation Tel +44 (0)1623 511 111  I have no connection with the company other than our maintenance folk ordered a replacement post for a funny situation from them once.


    Hmm, interesting, one for the potentially useful products library, thanks!


  • 0
    @ mapj  ..."Actually a concrete column or a one with a fibreglass clad base might be quite sensible, both exist. .."


    Just back on that info, if you have a link (fibreglass grp clad base) for me to check out, please post it whe you get a chance.

    I've done a search and am not seeing what I'm expecting, so at least with a link I can see what you mean specifically :)


    Thank you.

    Habs
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    I'll have a good look tonight for a link on-line , but basically it is really a "by request" option applied to the standard metal poles at the factory, I think originally  to make them last longer when installed at seasides and in contaminated ground with high acidity or whatever. The process is that either just the roots, or the whole thing, is covered in in a thin overlayer of glass fibre and resin, which as well as being weather proof is also a  very much an insulator. It is then over painted.  I imagine it is one of those 'call us for special options' type of things, as I cannot quickly see it in the catalogues.

    Also on the 'options' front many of the steel post designs can either have a root for burial like a spike, or can be fitted with a welded flat flange about a foot square with holes for large bolts, so you do not have to have metal deep in the earth if you do not mind forming  a large concrete cube instead. Equally the forces are substantial so that requires some mech eng input to do safely.

    The more common (=cheap) and more or  less standard  corrosion proofing  is that the end of the thing is dipped into some trouser-wrecking sticky gunk that looks like tar, but that is not what you want at all.

    I need to back track on what I said about concrete posts - apparently no one wants them any more, so new ones are not being made- I imagine the sheer weight and perhaps what they do in a car crash has lead to their falling out of favour.
  • 0
    Sparkingchip:..

    <snip>

    so so how do you install a cable without armour buried in a duct?




    If you are are a DNO,  earthed concentric wiring, but if you are to BS7671 then you can still use an armoured cable - the armour can remain earthed at the origin and linked to be continuous all the way along, and will do its duty and  blow the supply fuse when a digger finds it, but until that moment, the armour does not have any connection to the terra-firma around it.

    PS I'm not a street light expert, I'm transplanting knowledge of things done near radio transmitters, where earthing and and what is at what voltage also matter. Things like aircraft warning  lights on transmitter towers are something else, and involve isolation transformers in strange places.


  • 0
    So far I have only found a link to a standard offering of  insulated roots - on aluminium posts  perhaps not surprising as aluminium posts are more vulnerable to electrolytic effects.
  • 0
    @mapj   thank you.  Dont spend time on it. I am going to have to discuss further with property owner in any case.


    Thank you all for the contribution - as always, most helpful.


    For completeness and in case anyone is interested, the distributor has effectively stated clearly to avoid their 'hot zone' of any metal work rising from the ground etc  and anything near to the boundary to be all insulated - with any buried cables kept as far away as poss and if they cross, to have them in non-conductive ducting.    They were unable to provide details due to not having enough soil resistivity records, but did once again stated in this TX case  the 9m radius is their recommendation.   I did get back some figures of,  for the 20 ohm electrode, expect worst case to be a rise of earth potential of ~4900V.


    It would be folly without any other engineering expertise and calculation etc...to ignore the advice, so Im happy to adhere to it !


    Thank you again all.

    Habs

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