Poor or unreliable ground connection of IO modules can cause controller failure, inputs to stop updating and communications to be poor.
Strings of IO modules were installed on each DIN rail in a panel. One IO module per row had directly landed ground wires, but each of the other modules on the row relied on contact between a metal clip on the back of the module and the DIN rail. Observed an average of two I/O modules per panel with inadequate earth ground (where this clip was not making an electrically sound connection). In some cases, resting a hand on one of the modules caused a wide variance in the grounding quality.
- After installing a panel (not just in the panel fabrication shop), measure each IO module's ground terminal to ground. Do not just assume that each IO module is properly grounded to the DIN rail via the clip on the back. And do not just assume that the DIN rail is properly grounded - some DIN rail is plated to reduce corrosion and this can increase resistance.
- If there is (or will be) any vibration or moisture, or there are any other concerns that site conditions may negatively effect grounding of the IO modules in the future, consider grounding each IO module's ground terminal via a conductor to ground.
There is a Technical Product Advisory here that will provide further detail on this.