Avoid Instrumentation Problems by Properly Installing Low-Voltage Wiring

It happens over end over. A complex control system involving many instruments is installed and the connections seem to be electrically correct, yet there are problems. Some devices do not function at all, while others perform erratically or inaccurately. The problems seem to be particularly acute when sensitive solid-state analog or digital instruments are involved. What could be wrong?

What's wrong is the designer and installer did not give adequate attention to the details of instrumentation wiring during design and installation. Some of these important wiring details include proper treatment of each type of instrument, correct wiring for instrument signals, proper instrumentation grounding, and sound specifications for instrumentation wire and terminations.

Before we get into the detailed discussion, let's clarify one point: The wiring practices described here are for the low-voltage instrumentation typically found in industrial process control or laboratory situations, not switchgear instrumentation, which involves instrument transformers (i.e. potential transformers and current transformers). Because of the potentials involved, switchgear instrumentation wiring is done to a completely different set of standards.

Wire and cable

It's common to use twisted pair wire when wiring process control instrumentation. When two wires are twisted together, many of the effects of electromagnetic interference are canceled out, so twisted pair wiring is more resistant to electrical noise than untwisted wiring. A tighter twist (i.e. a higher number of twists per inch) results in greater immunity, so this is a specification worth paying attention to.

To add another level of protection from electromagnetic noise, a grounded shield is added over the twisted pair wires. When this is enclosed in a protective jacket, the package is called two-conductor shielded twisted pair cable, and this cable is recommended for most instrumentation field wiring.

There are typically two types of shields, the braided type and the foil type. Because it gives 100% coverage, the foil type is preferred. Be sure that shielded cable also has a drain wire, which is a bare conductor wound inside the cable jacket and in continuous contact with the shield. The drain wire makes terminating the shield easy.

The actual conductors in the cable should be stranded plated copper, and a conductor size of at least 18 AWG is recommended. Although most instrument signals can be carried by wiring smaller than this, keeping the conductors 18 AWG or larger increases reliability and makes terminations easier. The drain wire should also be stranded plated copper and should be at most one size smaller than the conductors, but again at least 18 AWG for reliability.

Insulation should be high-quality thermoplastic and rated for the voltage to be used (most instrumentation circuits operate below 30 VDC). The standard colors for two-conductor instrumentation cable are red (positive) and black (negative), but other colors are available.

The cable jacket should be rated for the intended use of the cable, and instrument cable is available for all of the common uses (e.g. conduit, tray, outdoors, direct burial, etc.). Also, be sure the jacket is resistant to any chemicals or oils that it may encounter. If the cable is to be run in conduit, make sure the jacket is of the smooth, slippery variety. The soft, rubbery jackets make pulling difficult and can lead to cable damage.

Fillers are nonconducting fibrous strands that are wound into a cable to fill any empty space. They are not usually used in instrument cable, but if fillers are present, make sure they are nonhygroscopic. This means that they will not absorb moisture and draw it into the cable, an obvious advantage.

Cable terminations

Terminations are an important part of instrument wiring. Proper terminations result in reliable connections and help to eliminate problems like ground loops and electromagnetic interference. Instrument terminations are usually made to screw terminals or compression terminals, and either one can be made reliably. Two-conductor shielded twisted pair cable is terminated in two different ways, with the drain wire connected to a terminal or with the drain wire cut off and insulated.

Fig. 1 shows a typical two-conductor shielded cable prepared for termination to screw terminals with the drain wire cut off. This is usually done at the field end of an instrument cable where no shield grounding is desired. Note that insulating tape or heat-shrink tubing is used to protect the cable from contamination and to prevent accidental grounding of the shield or drain wire. An accidental ground at this point would almost certainly create an undesirable ground loop.

[Figure 1 ILLUSTRATION OMITTED]

Fig. 2 shows a typical two-conductor shielded cable prepared for termination to screw terminals where the drain wire is to be connected. Note that the drain wire, which is an uninsulated conductor, is sleeved with insulating tubing to prevent accidental grounding. The crimp-on lug is valuable in this instance to retain the tubing. Insulating tape or heat-shrink tubing is again used to protect the cable from contamination and to prevent accidental grounding, since any accidental connection between the drain wire and a chassis, frame, or enclosure would almost certainly create a ground loop.

[Figure 2 ILLUSTRATION OMITTED]

Instrumentation wiring practices

In addition to good terminations, some general wiring practices need to be followed when designing or installing instrument systems. First, splices in instrumentation cables should be avoided whenever possible. Although splices at intermediate terminal strips often make installation or troubleshooting easier, they cause more trouble than they are worth. Terminations of this sort are prime areas for corrosion, loose connections, accidental shield grounding, and the introduction of electromagnetic noise. A single, unbroken cable from a field device to the controller or control system is always the most reliable solution.

Be careful when installing instrument wiring in proximity to higher voltage wiring. Any wires carrying an AC signal of 120V or more are a possible source of electromagnetic interference, and instrument signal cables should be installed a safe distance from them. If instrument cables must cross over AC power and control cables, the two should be separated by an adequate distance, and the crossing should be made at right angles to minimize induction.

Instrumentation cabling should always be installed in conduits dedicated to instrument signals only. When a tray is used, it should at least be divided. Separate trays for instrumentation are a better solution, and often do not increase cost much. Two 12-in. trays can be installed on common hangers for nearly the same cost as a single 24-in. divided tray, and the advantage of having a tray dedicated to instrumentation cables is usually worth any additional cost.

For maximum protection, install all instrumentation cabling in steel conduit because this type of conduit, when properly grounded, provides an excellent electromagnetic shield as well as an inductive damping action due to the iron content. Also, it's almost impossible to induce noise into an instrument cable installed in steel conduit.

Grounding in instrumentation systems

Most instrumentation systems have two grounds: The electrical or power ground and the instrument ground. It's important you realize that these two grounding systems have entirely different purposes. The primary purpose of the power ground is safety. All metallic or conducting equipment should be connected to this ground, and the rule here is "the more the merrier." Code requires, for example, that a ground grid, ground rod, building steel, and water pipe all be tied into the power grounding system if they are present.

The primary purpose of an instrument ground, on the other hand, is to protect instrumentation from electromagnetic interference. To do this successfully, any part of the protective shielding system must be connected to ground at one point and one point only. Whenever the shielding system becomes grounded at two or more points (to the instrument ground and to a grounded enclosure, for example), a ground loop is formed. Current can flow in a ground loop due to inevitable potential differences between separated grounds. Ground loops cause noise to be transmitted along the very shielding system that's supposed to be protecting the sensitive instrument signals, defeating its purpose.

To avoid ground loops and electromagnetic contamination of the ground system, all instrument ground wiring, including cable shields and drain wires, should be treated like sensitive current-carrying conductors. All instrument ground wires should be insulated, not bare, and the same wiring practices should be observed with ground wires as with other sensitive signals. Care must also be taken when designing instrument wiring to ensure that each shield is connected to only a single ground point. You should establish this point at a central location, like a control panel or PLC cabinet, and to avoid all connection to grounds in the field. An instrument ground is sometimes referred to as an isolated ground (an oxymoron) for this reason, but the term single-point ground is more accurate.

Instrument signals

The most common instrument signal used in industry is the 4-20mA current signal. A value of 4mA typically represents the zero level of a variable, while 20mA represents the maximum value.

Current signals are preferred to voltage signals because they are inherently more immune to noise, and the 4mA zero offset helps further with signal integrity. While maintaining a perfect OmA signal would be nearly impossible in the presence of noise, it is possible to "bury" the noise in the 4mA offset signal and represent the minimum value cleanly.

Voltage signals are also used, but they are usually confined to relatively noise-free areas like control panels or laboratory environments. Typical voltage signals are 1-SVDC, 2-10VDC, 0-SVDC, and 0-10 VDC. The first two signals again employ the offset zero, and are therefore more noise immune. They also have the advantage of being easily generated from a4 -20 mA current signal by simply passing the current through a resistor of the proper size.

Instrument signals are also classified as isolated or nonisolated. An isolated signal is not referenced to any ground or common reference. Isolated voltage signals are often called differential signals because the information is represented by the difference in voltage between two points, not the absolute voltage to common.

Nonisolated signals are reference to some signal common, and are often called single-ended because the information is represented by the voltage between a single point and common.

It's important that you know if a device is isolated or nonisolated. Two nonisolated devices in the same current loop, for example, will cause the loop to be referenced to ground or common in two different places and will almost certainly result in improper operation of the loop.

Instrument types

Transmitters are common input field devices. A transmitter typically consists of an electronic package that interfaces to a sensor, which, in turn, measures some physical quantity in a process. The electronics interpret the signal from the sensor, do any conversion necessary, and then transmit a signal that is proportional to the quantity being measured. A temperature transmitter, for example, may measure a temperature with a thermocouple, perform compensation and linearization, and transmit a 4-20mA signal representative of the temperature to a controller.

Transmitters have four common electrical configurations, as shown in Fig. 3. A field powered transmitter takes power for the electronics package from a source in the field and transmits a signal (usually isolated) on two dedicated wires. A four-wire transmitter is similar, but it takes its Dower (typically 24VDC) from a central source, and the power is transmitted to the device along the same cable that carries the transmitted signal back to the control system. Three-wire transmitters use two wires for transmitter power and transmit the signal back on a third wire. The signal is nonisolated (referenced to common) so only a single signal wire is required. Two-wire transmitters "steal" power from the instrument signal itself without affecting its accuracy. Two-wire current transmitters are very popular because of their simplicity, and the standard zero off set guarantees that electronics will always have at least 4mA to operate on.

[Figure 3 ILLUSTRATION OMITTED]

Output field devices are almost always two-wire, and respond to either voltage or current signals. These devices are usually isolated, but it's wise to make sure because nonisolated devices can cause the problems mentioned earlier. Typical output devices are control valves, which position themselves proportional to the incoming signal, and variable speed drives, which control the speed of a motor to the value indicated by the incoming signal.

Typical instrumentation circuits

Fig. 4 shows some typical instrumentation circuits, how the electrical connections are made, and how the shield drain (DRN) wires should be connected to avoid ground loops. The diagram shows how a straight splice through a terminal strip should be done (even though this usually is not recommended), and how voltage signals are properly distributed. It also shows the proper interconnection of multiple devices in a current loop, and how a current signal is converted to a voltage signal with a resistor. Instrument-grade resistors with an accuracy of a half percent or better are available in the commonly used values (250, 500, and 1000 ohms). The terminations in the figure should be physically done as shown in Figs. 1 and 2.

[Figure 4 ILLUSTRATION OMITTED]