Measuring occupational exposure to electric and magnetic fields at 400 kV substations

The new European Union directive states that if the electric and magnetic field action values of 10 kV/m or 500 muT at the working place are exceeded, the employer has to ensure that the limit value, given as an induced current density in the body, is not exceeded. The aim of this study was to measure field strengths and find out how the new directive will affect working at Finnish 50 Hz, 400 kV substations. Spots where the action value was exceeded could be found at 75 % of the selected eight substations. The largest electric field strength, 14.3 kV/m, was measured between two adjacent busbars carrying the voltages having the same phase angle. Over 500 muT magnetic flux density was measured in one substation at the safety fence around a three-phase air-core reactor set. Fields were measured at a height 1 m above the ground. According to simple ellipsoid calculations the exposure limit, 10 mA/m of induced current density in the head or trunk, will not be exceeded. However, many of the maintenance tasks have to be done closer to the conductors and busbars and thus more in-depth study and measurements will be necessary at least in Finland.