Appreciate any help on solving a long standing intermittent problem. We have an instrument control panel that serves a treatment plant. We monitor gas pressures on seven analogue control loops. All the loops and PLC digital I/O's are supplied from a 24 VDC supply, and the 0 Volts is floating (not connected to earth). To explain one of the loops:
24 VDC Positive to an Intrinsically Safe Barrier (MTL 787SP), out to field instrument, back to Intrinsically Safe Barrier, then to analogue input module on PLC, out of PLC, back to 0 Volts. There are seven of these loops. Intermittently, the PLC sees +20mA coming from the field instrument, and they all do this at exactly the same time. On checking at the field instruments, they are giving out nothing like 20 mA. As a quick check, I bypassed the IS barriers, and everything works fine. At first I thought that the zenner diodes may be leaking. So, changed all barriers and the DC power supply. Rechecked, PSU supplying correct voltage and not overloaded. On connecting the inputs to the IS barriers, with the outgoing loops disconnected, the plc is picking up milliamps and it is erratic. My next thought is that something in the plant is leaking to earth, and returning back through earth to the IS barrier earth. This is what the PLC is picking up. The IS barriers are commoned together with an earthing strip. This earthing strip is connected to the cubicle earth. If I remove this earthing connection from the barrier strip, everything returns to normal. Or if I earth the 0 Volts so that it is not floating, everything returns to normal. The PLC sees 0 mA with the outgoing field loops disconnected. I will probably never find the earth problem causing this, as it is a very large plant, and the PLC controls everything. Would installing a permanent earth to the 0 Volts on the PSU be the answer to my problem, or is there anything else I haven't considered. TIA