Electrical Bus

2. Climate
Obviously, arid and good rainfall climates are at opposite extremes for conditions of soil
resistivity.

3. Seasonal Conditions
The effects of heat, moisture, drought and frost can introduce wide variations in “normal” soil
resistivity. Soil resistivity usually decreases with depth, and an increase of only a few percent of moisture content in a normally dry soil will markedly decrease soil resistivity. Conversely, soil temperatures below freezing greatly increase soil resistivity, requiring earth rods to be driven to even greater depths. See Table 2 for variations of soil resistivity with moisture content, and Table 3 for variations of soil resistivity with temperature.

4. Other Factors
Other soil properties conducive to low resistivity are chemical composition, soil ionisation, homogeneous grain size and even grain distribution - all of which have much to do with retention of soil moisture, as well as providing good conditions for a closely packed soil in good contact with the earth rod. In view of all the above factors, there is a large variation of soil resistivity between different soil types and moisture contents.

Every earth is an individual and the only way to know that an earthing installation meets code
requirements is to carry out proper resistance measurements on site. There are a variety of test instruments available , however, they can be generally categorised as threeterminal of four-terminal test instruments.

TYPES OF EARTH RODS
At one time or another, all manners of conductor materials and shapes have been installed in the ground to provide an electrical earth. These materials range from cast iron plates, tubes, galvanised steel stakes, copper strip, metallic rod, wire and water pipe. Taking into account conductivity, high resistance to atmospheric corrosion and soil attack, ease and economy of installation and overall reliability, the steel rod clad with either copper or stainless steel has proven its superiority over all others. The clad steel rod is simple to install, its connection to the earthing system is easily made, and the installation is readily accessible for inspection and test. Additionally, by the use of deep driving techniques, extendible earth rods gave been developed to reach underlying strata of low permanent resistivity unaffected by seasonal drying.

STEEL CORE EARTH RODS HAVE THE BEST ATTRIBUTES
Electrically, a good earth rod should have a low intrinsic resistance and be of sufficient section to carry high currents without damage when called upon. Mechanically, its physical properties should exhibit strength, have a rigid core for easy driving and be of durable, corrosion resistant material.

EARTH ROD LENGTH MORE IMPORTANT THAN ROD DIAMETER
Apart from considerations of mechanical strength, there is little advantage to be gained from increasing the earth rod diameter with the object in mind of increasing surface area in contact with the soil. The usual practice is to select a diameter of earth rod, which will have enough strength to enable it to be driven into the particular soil conditions without bending or splitting. Large diameter rods may be more difficult to drive than smaller diameter rods. The depth to which an earth rod is driven has much more influence on its electrical resistance characteristics than has its diameter. This is because it is not the actual area of contact with the soil that counts, so much as the total resistance area of the sheath or shell surrounding the earth rod.