I know from my early days as a systems engineer, that power and grounding can sometimes cause vexing, intermittent issues. It’s like chasing ghosts, which is an apropos analogy with Halloween occurring yesterday here in the Western world.
I caught up recently with Dan Jacobsmeyer, a specialist for Emerson’s FlexConnect solutions. These solutions connect pre-engineered cables to the existing automation system termination panels and marshall the field signals to DeltaV I/O.
Dan shared a recent story where a process manufacturer was having some grounding issues with 1-5V analog I/O cards. Ground loop currents were causing a voltage offset, increasing the error on what the I/O card was reporting to the control system, versus its actual measured value. A solution discussed was to convert the I/O over to 4-20mA.
Dan pointed out that replacing the 1-5V cards with 4-20mA cards is a way to solve the immediate voltage offset problem. However, not fixing the ground issue may result in future problems that may be very difficult to diagnose.
Dan recommended the following course of action based upon DeltaV site preparation documentation.
A proper earth ground is critical. It safely conducts stray electrical current to earth for personal safety and good electrical noise control. Building steel must be part of a good earth ground system to obtain an equal-voltage-potential ground between the steel and the automation system ground networks.
Automation system faults are often the result of poor or faulty grounds. IEEE Standard 1100-1992, Recommended Practice for Power and Grounding Sensitive Electronic Equipment describes industry-accepted methods.
Isolation transformers should be used for power transfer from plant A/C service to the individual automation systems. A single point ground eliminates ground loops and provides excellent isolation between the automation systems and other plant systems.
Dan sums up his recommendations to use the isolation transformer between the A/C power source and automation system and have each system with a dedicated connection to a true-earth plant ground grid. Make sure no other systems share this connection point.
Paying close attention to these recommendations and not masking over them will save a lot of time chasing ghosts and delivery more reliable and accurate system operation.