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Multiple Earth Electrodes TT System

Steve UKBC
Thoughts or suggestions welcome....


Is there any issue or downside to having multiple earth electrodes installed on a single TT system (all areas have there CPC connected together) when the TT system spans a large area (we are taking about a 400m x 400m area)?


For example


If you had a couple of farm barns/sheds (metal framed) close together with multiple earth electrodes and some few hundred meters away you have a camp site with multiple earth electrodes (one at each power outlet etc) and then connect the CPC from the barns to the camp site to make one large TT system?



  • 0
    In general multiple electrodes would be considered a good thing - as generally a lower overall Zs and likely more reliable too.


    The only flipside I can think of is that having one large TT system rather than several small ones is that the rise PE potential during a L-PE fault will affect a larger area. Normally that's not too much of a concern but could be significant, say if an RCD was faulty and so failed to disconnect at all.


       - Andy.
  • 0
    AJJewsbury:

    In general multiple electrodes would be considered a good thing - as generally a lower overall Zs and likely more reliable too.


    The only flipside I can think of is that having one large TT system rather than several small ones is that the rise PE potential during a L-PE fault will affect a larger area. Normally that's not too much of a concern but could be significant, say if an RCD was faulty and so failed to disconnect at all.


       - Andy.


    Would you suggest using 2 rather than one RCD just after the DNO cutout (identical RCD's different manufactures) could be a way forward to protect against this or would this just be over engineering, considering the size of installation?


    I suggested two identical RCDs in one place due to the possibility of both tripping on a earth fault and it being easier or more convenient to reset both RCDs in one location. Note that the plan is to have a time delayed RCD after the DNO and final circuits using RCBO's and would not want to go down the route of a adjustable RCD to provide selectivity if a 3rd RCD device was to make its way into the circuit.


    hope i made sense 


    Steve


     


  • 0
    Generally more metal in the ground is a good thing, regardless of where, unless it is very near livestock, or the earthing of something else (HV transformer earths can bring in lighting induced surges, and electric fence earths, well the least said about what they can do,  the better really, just treat both sides of the HV pulser with equal respect for a quiet life.)


    If the TT earth is better than the one the DNO have on their transformer then funny things can happen during faults. ( read  this thread if you find it hard to sleep at night)


    If step voltages around a rod are a concern, then the top of the electrode should be insulated and or the ground surface made free draining (gravel perhaps or ungrouted paving).
  • 0
    Would you suggest using 2 rather than one RCD just after the DNO cutout (identical RCD's different manufactures) could be a way forward to protect against this or would this just be over engineering, considering the size of installation?

    There's nothing official that suggests that duplicate RCDs is necessary or even recommended - although it happens 'by default' as it were in a number of situations - often a TT install will have a 100mA-S type (or higher) as an incomer and then 30mA protection for final circuits - should the final circuit RCDs fail then the incomer can act as a backup - usually providing perfectly adequate ADS (although not additional protection) to the final circuits. As most L-PE faults tend to occur in appliances or final circuits rather than in switchgear or distribution circuits, that sort of thing pushes the probabilities quite a long way in the right direction. In some high risk situations - e.g. caravans - the separate requirements for 30mA RCD protection at the pitch and within the caravan again result in duplication.


      - Andy.

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