Earthing is an integral and important component of any power system.
Substations, generating stations and transmission lines all require adequate earthing to ensure protection from power system earth faults and lightning strikes, and also to provide an equipotential surface to work from. Inherently, earthing systems can also introduce hazards, due the passage and distribution of earth fault current.
The testing of earth grid systems is necessary to ensure compliance with safety regulations. Testing also confirms the design or determines the characteristics of an earth grid where the actual design or layout is not fully understood.
Off-frequency current injection
Mitton Instruments’ specially-designed earth test equipment is based on low current off-frequency current injection. This is one of the methods described in IEEE Std 81.2-1991 – Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems.
Off-frequency injection provides an ideal method to test earthing systems, since it does not require the use of high test currents, large generators or power line outages.
Off-frequency injection testing provides a reliable method for testing earthing systems, particularly those with low impedance (< 2 Ω) or which are extensive and where noise is present on the earthing system. Portable earth testers are often unsuited to testing such earthing systems, due to limited test current and inadequate noise rejection.