A fail-safe device is disclosed for ensuring compatibility and reliable operation of a Voltage Indicator System (VIS) for a medium- or high-voltage apparatus in presence of a monitoring system with: a first and second fail-safe device terminal; the first terminal being connectable to an output terminal of a coupler, the coupler being provided in a medium- or high voltage portion of the apparatus, and the second terminal being connectable to an input terminal of the VIS, which is provided in the low-voltage portion; a third and fourth fail-safe device terminal, wherein the third and fourth terminals being electrically connectable to first and second input/output terminals of the monitoring system; and an electrical circuit connecting the first and second fail-safe device terminal and being adapted to compensate for electrical failure modes of the monitoring system such, that the VIS is operable in case the electrical failure modes occur.