AS/NZS2081 and AS/NZS4871 present competing requirements for the application of earth continuity protection in earth fault limited mining systems. AS/NZS2081 requires that earth continuity protection be capable of operating within 100ms to quickly protect an outlet in case of live uncoupling. Meanwhile, both AS/NZS2081 and AS/NZS4871 require that the earth continuity protection should also be capable of monitoring incremental changes in the resistance of the earth return path to ensure that leakage currents do not result in dangerous touch potentials. It is sometimes not possible to achieve a practical earth continuity protection system that satisfies both of these requirements, if static time and trip settings are used.
We’ve developed a new method of earth continuity protection that utilises a ‘trip curve’ to discriminate between earth disconnection faults and incremental increases in the earth return impedance that occurs due to equipment degradation. Using this method it is possible to provide an “instantaneous” trip for energised uncoupling of cables that significantly reduces the chance of an arcing fault or exposure to live terminals. For gradual increases in the impedance of the earth return path, a time delay is introduced to allow discrimination between noise induced onto the cable’s pilot-earth loop, and an actual increase in loop impedance.
Take a fresh at earth continuity, safety and AS/NZS2081 in this downloadable PDF or watch the below presentation.