محمد سعید جعفرپور
بطور کلی از نظر صحت محاسبات منجر به 2 اهم ذکر شده، یک حداقل فاصله بین دو الکترود مطرح است که از طول میله الکترود نباید کمتر باشد. اما آنچه حداکثر فاصله الکترودها را محدود می کند گرادیان افزایش پتانسیل ناشی از تخلیه الکتریکی در زمین است که به پارامترهای متعددی از قبیل مقاومت خاک، پراکندگی الکترودها و میزان جریان اتصال کوتاه بستگی دارد و بهترین راه اطلاع از آن انجام محاسبات کامپیوتری با نرم افزارهای اختصاصی است. در این ارتباط می توان به BS 7430 و IEEE 80 مراجعه نمود.
در محاسبه مقاومت زمین کلیه الکترودهای مدفون به هر منظوری در محاسبات موثر هستند.
9.6 Potential gradient around earth electrodes
9.6.1 General
Under fault conditions an earth electrode is raised to a potential with respect to
the general mass of Earth that may be calculated from the prospective fault
current and the earth resistance of the electrode; this results in the existence of
potential differences in the ground around the electrode that might be
damaging to telephone and pilot cables, whose cores are substantially at earth
potential. Such a risk should be considered mainly in connection with large
electrode systems, as at power stations and substations. The potential gradient
over the surface of the ground is should also be considered because personnel
or livestock can be in contact with two points sufficiently far apart that the
potential difference constitutes a danger to life; cattle are most at risk.
Figure 16 may be used to predict the ground surface potential gradient around
an isolated 3 m rod electrode. The high potential gradient within the first few
centimetres of the rod is clearly seen and, depending on the electrode voltage,
a barrier to prevent contact by personnel or livestock may be advisable out to a
radius of 1 m or 2 m.
NOTE Further recommendations on reducing this gradient by placing the top of the
electrode below ground level are made later in this Clause.The potential gradient on a perpendicular to a line of three electrodes may be
taken from Figure 16; this shows that, for a given electrode voltage, although
the initial gradient within about 2 m of the rods is slightly reduced, the general
effect at greater distances is to increase both the ground potential and the
gradient. However, for a given fault current the multiple electrode arrangement
may be used for a lower resistance compared to a single electrode and results in
considerably smaller ground potentials and gradients than those of a single
electrode.
The distribution of ground surface potential in the vicinity of a horizontal
electrode (wire or strip) may be assumed to have gradients much smaller than
those of a vertical electrode, the highest gradient occurring at a distance from
the electrode equal to its depth. Likewise, the potential gradients between
horizontal electrodes may be assumed to be smaller. Depth of laying, within the
range normally used, may be assumed to have little effect on ground surface
potential.
It should be borne in mind that the ground potential contours associated with
installations of more than one electrode consist of isolated areas around each
electrode where the potential and its gradient are high and possibly dangerous;
between these areas there are troughs of minimum potential which can
nevertheless have a value which presents a danger to a person or animal
touching conducting items in contact with either the electrode or the mass of
earth at a more remote location, e.g. a metallic fence.
Such variations in ground potential within and close to an installation may be
considered unacceptable and measures to reduce them might be necessary.
These measures may be more conveniently implemented by the use of
additional electrodes in the form of buried horizontal strip or wire,
possibly 0.25 m to 0.6 m below the surface (see Clause 5 for further comment on
the laying of such electrodes).
The large fraction of the electrode potential developed over the few
centimetres of soil next to the electrode indicates how important it is to
consider the resistivity of the material in this region. In high resistivity soils, for
cases where the use of additional rods is not practicable and it is desired to
reduce the electrode resistance by soil treatment or by replacement with
concrete, only the soil in this region should be considered. Since soil conductivity
is affected by soil density (except for very wet soils), good compaction, or lack of
disturbance, of soil in contact with the electrode is important.
Earth electrodes should not be installed close to a metal fence, unless they are
used for earthing that fence; this is to avoid the possibility of the fence
becoming live and thus dangerous at points remote from the substation, or
alternatively giving rise to danger within the resistance area of the electrode by
introducing a good connection with the general mass of the earth.
In rural areas, danger to animals in the vicinity of earthed poles may be
minimized by the use of insulated earthing connections and by having no earth
electrode less than 0.6 m below the surface of the ground.