در سيستمهاي صنعتي ،مقاومت پيشنهادي براي سيستم زمين بين 2 تا 5 اهم است. بر اساس توصيه ABB Switchgear Manual مقدار 2 اهم بدون محاسبه قابل پذيرش است. در صورتي كه بخواهيم مقادير بزرگتري را اختيار نمائيم مي بايد محاسباتي به شرح زير انجام شود. In TN or TT systems, the total earthing resistance of all functional earths should be as low as possible to limit the voltage rise against earth of all other conductors, particularly the protection or PEN conductor in the TN network if an earth fault occurs on a phase. A value of 2 ?is considered sufficient in TN systems. If the value of 2 ?cannot be reached in soils of low conductivity, the following condition must be met: 
In the TT system, the implementation of overcurrent protection devices is problematic because of the required very low continuous earth resistance. In the IT system an earth resistance of <15 ?is generally sufficient when all metallic enclosures of equipment are connected to a common earthing system. If a supplementary equipotential bonding is required in an electrical installation, its effectiveness must be verified by the following condition: 
براي اطلاعات بيشتر مي توانيد به Grounding riddle NO.19 در بخش انگليسي همين سايت مراجعه فرمائيد.
