Get the Power and Grounding Right First

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.

Posted Thursday, November 1st, 2007 under Modernization, Power & Grounding.

2 comments

  1. I’d like to say that your essay should be obvious, but apparently it isn’t. Schools don’t teach this stuff, and much of it is passed along by folklore and habit.
    By the way, many 1-5 volt systems really 4-20 mA circuits in disguise. The difference? One has a precision 250 ohm resistor on the outside, and one has a precision 250 ohm resistor on the inside (Ok, it isn’t always a 250 ohm resistor for the 4-20 mA gear, but you get my drift).
    And as for the grounds, they only work well when everything uses them properly. We practice careful isolation where I work. We expect a dry contact from the MCC, and we provide dry contacts with sufficient current switching capacity for controls.
    Our biggest vulnerabilities are those 4-20 mA circuits. Sadly, many technicians and a surprising number of engineers seem to think that if one surge protector is good, that more must be better. What they don’t realize is that if more than one protector fires on the same line against two different grounds, you’ll have a massive current flow. The result will be roasted electronics with some carbon on the side.
    Going further, some of our 4-20 mA circuits travel over 1000 feet from the instrument to the RTU. There are very few happy things one can do in situations like this. If someone is interested in selling an optical isolator for a 4-20 mA circuit, I suspect they’d sell a few…

  2. Jake, Thanks for your comment! I agree this that this is pretty basic stuff and it’s not something usually taught in the universities. I know as a EE by degree, I learned a lot about many things, but not enough about power and grounding.
    I worked on offshore platforms back in the mid-80s. True earth is great on these masses of steel, but we still had issues–mainly if I recall right, multiple grounding points. In my inexperience, I dealt with transients in the signals by applying filters on the PLC I/O channels. Smart electricians finally clued me in to the right way to solve the issues we had.
    Hopefully, this post plus this follow on conversation will help the next inexperienced engineer when they are googling for help.
    Take it easy, Jim

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