Concept: Performing a control system retrofit without completely understanding the electrical characteristics of the old and replacement system I/O cards is a fast-track path to disaster. Very subtle differences can create start up nightmares.
Details: This tip is really an extension of Tip #22 (“Details matter”). When an engineering team is faced with the task of replacing an existing distributed control system (DCS) with a new one, the first step should be an exhaustive study of the I/O card designs in the existing system and in the proposed replacement. Any differences should be investigated in detail. Here is a partial list of the items that should be checked:
- Are the points grounded or isolated from ground? (Grounded analog input cards can create ground loops and if the point is tied to ground, wiring analog signals in series with other systems may not be possible.)
- Do the analog cards source voltage or not? (Some systems allow selection of two wire or four wire connections on a point by point basis. Does the replacement system have similar options?)
- Are the points on a given card all the same type? (One older system allowed the user to set each channel on a card to be two-wire analog inputs, four-wire analog inputs, analog outputs, pulse inputs, and even 24V digital inputs and outputs.) Does that new system have this ability?
- What is the impedance of the analog input card? (Older systems may have had low impedance – 50 ohms or less.) New systems tend to have 250 ohms to allow HART communications. If field indicators are present, the additional impedance can pose voltage drop problems.
- How are the shields landed and grounded. (Different cards handle the shields differently.)
- Does the analog cards measure the current leaving the system or returning to it? (If the old system measured the current going to the field, then transmitters will continue to function even if their return wiring is shorted to ground. Such a ground would keep the signal from reaching the DCS if it measures the return current.)
- Obviously, the voltage ratings must match. Using 120VAC cards for 24VDC signals will not be a good choice.
- What is the current rating of the outputs? Are the points fused? (Be sure to check the individual channel rating, as well as the rating of groups of channels on a card. Some cards have a high per-channel rating but have a comparatively low rating for the collective current of all the points.)
- Does the digital input card source or sink voltage? Some cards provide voltage that loops through the field contact and returns to each channel. Others provide voltage on a channel-by-channel basis and look for the field to ground it out. Still others provide no voltage and expect it to be provided externally. Some cards can take voltage from isolated sources; others require all channels (or channel groups) to use a common voltage. The new system must be compatible.
- Does the digital output card source or sink voltage? A relay card can generally switch externally powered 24V and 120V signals on a channel by channel basis. Other digital output cards provide voltage on each channel that may be fed from external sources or fed from the card. Still other cards ground out each channel providing a current path for externally fed power. As with the digital input cards, the cards come in many different flavors that may or may not be compatible.
- Beware of leakage currents. Some digital input cards will turn on when an input receives even a tiny fraction of current (<1mA). If the field device leaks current (as many proximity switches do) or if the signal is fed from another system digital output card that leaks 1 or 2 mA of current, the digital input will turn on and never turn off.
Intrinsically Safe Systems
If the automation retrofit project involves existing intrinsically safe barriers, then the opportunity for error is huge. A book could be written on the subject, but as a start, here is a quick list to consider:
- Voltage drop – Intrinsically safe system voltage drops are high in general, but if the new system has a higher impedance then the existing barriers will probably not work.
- Grounding – Some barriers are grounded internally. If the system measures returned analog signals, they may not reach the system if the barrier is grounded.
- Supply voltage – Some barriers use an elevated supply voltage to provide more voltage to the field. Is this compatible with the new system?
If the project plan calls for a reuse of the existing barriers then the team would be wise to obtain a sample of each barrier and test it with the new control system I/O cards. Many barriers may not function when wired to the new system.
Watch-Outs: Many vendors provide interface hardware or special connectors that supposedly allow the user to “instantly” connect the existing I/O into the new system. Evaluate these offerings carefully. In many cases, the opportunities for hardware incompatibility mentioned above still exist and need to be evaluated. (Differences in grounding, voltage drops, impedances, and so on can still wreak havoc.)
Insight: Start with detailed schematics of the old system and the new. Investigate current limitations, leakage currents, impedances, and particularly grounding.
Rule of Thumb: The electrical analysis of the systems should NOT be assigned to a junior engineer. If just one detail is missed, whole banks of I/O may fail to function on start-up.