Weird situation on plant. One bus out of three of 9 contactor on SLC 5/05 not working as it suppose to. It is connected to Relay output module of SLC 500. 24 V dc is available at the coil (no volt coil) terminals of all the contactor. If just flicker the on off switch then start working. While the plant is running, suddenly stop whole bus of 7 contactor. But 2nd and 3rd bus stays at normal working. Electrically tests every thing is ok.