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Electric Power Engineering

Ground Detection vs. Insulation Monitoring for Ungrounded LV Systems by GFHunter
Posted: 14 Oct 03

In the US & Canada, the practice of ground or 'ground fault' detection on ungrounded (usually delta) AC distribution systems, as well as ungrounded DC systems, still utilizes the outdated 'light bulb' method.  Although this technology has been around for many years, there are more modern alternatives which provide more effective sensing of grounds, and make the equipment safer for operators.  Insulation Monitoring Devices (IMDs) allow O&M managers, engineers, and technicians to view the actual insulation level between their electrical conductors and the ground grid.  Therefore, instead of 'detecting' a ground on their systems(s), the plant can now 'monitor' the condition of it's insulation in order to determine the level of breakdown when it does occur.  Also, there are some monitors designed to 'look through' AC-to-DC conversion devices which are often designed into motor controls (VFDs/VSDs), fuel cells, battery chargers, and welding equipment.  The old 'light-bulb' method, and most of the 'voltage imbalance' technologies used to currently sense grounds, do not provide the necessary protection required on most modern controls.

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