Underground » Maintenance
There have been an increasing number of electric shock incidents reported during switching of electrical equipment in systems where no faults were present. These electric shocks occurred during closing and opening of circuit breakers and contactors. A mechanism has been identified for transient touch voltages leading to electric shocks that is consistent with the reported incidents. The estimated effect of these transients, based on modelling of the mechanism for each incident, ranged from perceivable to extreme pain. Fortunately, none reached the level for which fibrillation may occur according to AS/NZS 60479.2. However, the possibility of secondary effects, such as injury from falling, cannot be understated.
This report describes how transient touch voltages are generated and the methods developed to reduce the effect so they are not perceivable, ensuring safe operation under all conditions. The result is guidance on the safe use of switchgear to reduce or eliminate transient touch voltages. The proposed solution is a high frequency roll off common mode filter installed on equipment closest to the incoming supply. This has the effect of shunting transient currents during switching back to the phase conductors rather than passing along the earth conductor where a touch voltage would be developed.
The results from a hardware simulator and measurements on a shuttle car are presented demonstrating the effectiveness of a device to minimise transient touch voltages. Details of the device are provided to allow manufacture of the device so that they can be installed in mining equipment.