Abstract: An apparatus for monitoring grounded states of multiple DC loads of a relay system, comprising a power supply circuit for inputting a commercial AC power and supplying DC voltages necessary to the DC loads and (+) and (-) DC circuit lines to which the DC loads connected, a low frequency signal generating circuit for transmitting a low frequency AC signal over the (+) and (-) DC circuit lines, the low frequency AC signal having the range of having no effect on different components, main (-) and (+) grounded state detecting circuits for detecting (-) and (+) grounded states of the whole of the (+) and (-) DC circuit lines and the DC loads, respectively, an overvoltage detecting circuit for detecting an overvoltage at an input stage of the apparatus to protect the apparatus from the overvoltage, a grounding current detecting circuit for detecting individually the low frequency AC signals from the DC loads when the DC loads are grounded, and a load fault signal generating circuit for generating load fault signals