However, significant voltages can be created when power system currents flow into the earth. The earth potential rise (EPR) can be transferred on to external networks of conductors such as fencing, pipelines, and telecommunications circuits. Testing ensures that new and existing earth grids meet their performance requirements. Computer modelling is a valuable tool for design but it is difficult to include all parameters which may affect the resultant earth grid characteristics. There are a variety of methods available for testing earthing systems. These are generally based on either portable earth testers or higher current injection methods. Portable testers, which inject < 50 mA into the earth grid, are often inadequate due to noise and induced voltages in the test leads. Current injection at the fundamental frequency is not recommended unless high currents are used {eg 100 A) since any 50 Hz noise can introduce significant measurement errors.
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