3-phase mains monitoring for series installation, with or without N-conductor monitoring (selectable), measuring range 300 - 480 V AC, 1 changeover contact for 6 A (AC1), contact material AgNi, monitoring of: overvoltage and undervoltage, over/undervoltage range, phase sequence, phase failure, phase asymmetry (4 to 25%) and N-conductor interruption, colored LED status display.