Predictive maintenance techniques have proven to be effective strategies to reduce unexpected machinery failure. Vibration monitoring is by far the most widely used predictive maintenance technology due to the significant amount of machinery condition information provided.

Most plants that implement a vibration monitoring program begin with a portable data collector and a pre-determined route of data collection points. Vibration data is gathered and trended. Maintenance action then is determined based on machinery condition trends. Very often the new vibration information is reviewed and compared to trended data and no anomalies or exceptions are noted. Vibration data was just taken on healthy machines.

Plant size and the number of points of machines to be monitored can make implementing a vibration monitoring program a formidable task. Determining the machinery routes and the frequency of data collection also can be a difficult undertaking. These issues, as well as machinery with different rates of failure-that is, the time to machinery failure once excessive vibration is detected, direct many plant managers toward investigating continuous monitoring solutions with permanently installed instrumentation.

These investigations often reveal that existing continuous condition monitoring programs provide a tremendous amount of information for predicting machinery failure and diagnosing and analyzing root cause. This gives the plant engineering staff the most reliable and complete information available to help assess the health and condition of the plant's machinery.

If the only detail maintenance managers are interested in is determining if a machine is good or bad, these continuous condition monitoring programs may provide too much information and may be cost prohibitive. Also, many systems are not compatible with existing plant monitoring instrumentation and require implementation of proprietary equipment and significant duplication.

Another approach may be for on-line condition monitoring systems to be implemented using existing factory process control equipment. This concept would monitor overall vibration levels using general-purpose accelerometers routed to vibration transmitters that convert the vibration signal into a 4-20 mA output compatible with plant process equipment. The programmable logic controller (PLC) then could send alarms to the vibration analyst when these levels become excessive. These alarms would alert the predictive maintenance team of the need for closer investigation with portable analysis and diagnostic equipment.

This approach may be much more cost effective. Since process monitoring systems are widely used in many factories, vibration channels can be added at a fraction of the cost of dedicated on-line condition monitoring systems. Other costs, such as installation and training, also are reduced since the monitoring network system already is installed and personnel are in place to manage the system.

Once the decision is made to implement a vibration monitoring program using existing process control instrumentation, the next task is to determine the equipment to be monitored and to define the machinery faults that need to be detected.

Answers to the first point are relative to the particular equipment: the cost of repair, rate of failure, and its importance to the production process. Answers to the second part require a basic understanding of the typical modes of failure of machinery and their respective vibration signatures.

Typical machinery faults
The first step in implementing any condition monitoring program is to know the equipment. Research the machinery to be monitored in order to be familiar with its operation and understand its potential failure modes.

There are many failure modes for machinery. The more complex a piece of equipment, the more complex the failure mode can be. Four basic failure modes most commonly found in standard equipment are imbalance, misalignment, bearing faults, and gear mesh failure. Each machinery fault has its own unique vibration signature that helps to identify the particular fault. Each fault has specific fault frequencies that help determine the mode of failure while the amplitude of the vibration helps to determine the severity of the problem.

Imbalance and misalignment most often occur at low frequencies. Mechanical looseness and process loading also can produce faults at low frequencies. These machinery failures demonstrate high vibration at one, two, and three times running speed. These low frequencies are typically in the 2-1000 Hz range for equipment operating around 1800 rpm.

Since the mechanical defect is a result of a physically massive rotor or shaft, the amplitudes are relatively high. A good range for trending vibration is from 0-1 in./sec RMS.

Bearing faults occur at nonsynchronous multiples of machinery turning speed. Specific bearing fault frequencies are unique to the bearings and depend on the physical parameters of the bearings. Specific measurements such as the pitch and diameter of the bearing, the number of balls, and the turning speed are all needed to calculate the fault frequencies of bearing failures such as inner race and outer race defects as well as ball bearing defects. Bearing defect frequencies are available from most bearing manufacturers, but as a rule of thumb one can estimate the frequencies to be near 50 percent of the product of the number of balls in the bearing times the machinery turning speed.

Vibration amplitudes for these faults are very low as the mass of the moving parts is relatively small compared to the rotor or shaft mass. Bearing fault frequencies range from 200-5000 Hz with relatively low amplitudes. Trending acceleration data instead of velocity data is desired since velocity accentuates the lower frequency vibration and attenuates the higher frequency vibration while acceleration data gives stronger signals at higher frequencies and is better able to measure the lower amplitudes of bearing faults. A typical acceleration range for bearing fault detection may be 0-10 gs peak.

Gear mesh faults occur at even higher frequencies than bearing faults. Gear mesh frequencies are the product of the number of teeth times the shaft's turning speed. Depending on the particular machine, these gear mesh frequencies can range from 100 Hz to over 10,000 Hz. As mentioned previously, acceleration data is preferred over velocity data as the acceleration measurement emphasizes the higher frequency vibration and de-emphasizes and is less sensitive to the lower frequency mechanical defects and process loading conditions. A typical acceleration range for gear mesh fault detection may be 0-50 gs peak.

Selecting the proper transmitter
As discussed previously, it is imperative to know the machinery in order to effectively implement a condition monitoring program. Current machinery operating conditions, expected modes of failures, and potential machinery faults are all factors to consider when monitoring equipment.

Selecting the proper frequency band to trend over relative to the particular fault of interest is critical in order to actually detect the given machinery fault and eventually predict machinery failure. Determining the amplitude ranges within the given frequency band is also important so that alarms will provide an early warning when machinery condition has degraded.

Another critical concern for vibration monitoring equipment and alarms is that a time delay be available for each measurement point. A time delay would be used to avoid false alarms that could be set off as a result of transient vibration caused by local traffic, process changes, and even ancillary equipment. Also the time delay should be sufficient to avoid setting off alarms during machinery start up and coast down. During start up and coast down the equipment could move through mechanical resonances and high amplitude vibrations could be present. Transient time delays should be on the order of 5-10 seconds while time delays for machine start up and coast down should be greater (approaching 1 minute). It may be desirable to de-activate the vibration transmitters and their alarms during start up and coast down to avoid inadvertently setting off alarms.

A final selection criterion necessary for vibration monitoring instrumentation is that the raw vibration signal is made available for further diagnostics. The trended overall value within a given fault frequency band will be a good indication of the machine's condition relative to that general fault condition but it will not reveal specifically the details of the pending fault. For example, if an alarm is tripped in the lower frequency band where misalignment or imbalance may occur, in order to effectively make repairs prior to total machinery failure, the maintenance staff must understand what the exact fault is.

Many specific faults could occur in that broad low frequency band including mechanical looseness, oil whip, and oil whirl and even belt failures as well as the already mentioned misalignment and imbalance. Detailed diagnostics is required to pinpoint the exact failure mode. This is accomplished by inputting the raw vibration signal from the installed sensors into a portable diagnostic instrument for further analysis by a qualified vibration technician.

Machinery condition monitoring is an important facet in modern maintenance. Avoiding unscheduled downtime is critical to maintain corporate competitiveness. Low-cost on-line condition monitoring of rotating machinery using industrial accelerometers, vibration transmitters, and plant process equipment is an excellent method to gather information to help determine the overall health of a plant's machinery. MT

Eric Saller is a field applications engineer for PCB Piezotronics' Southwest Office and a certified vibration specialist with the Vibration Institute. He can be reached at 2646 E. Rockledge Rd., Phoenix, AZ 85048; (480) 759-4939.

Failing to plan and schedule maintenance is a common shortfall in many maintenance departments in a variety of industrial settings. The maintenance planning and scheduling function is often overlooked or poorly defined.

There is an urban legend about a group of people who built an entire house in only 24 hours. When asked how they could accomplish this task, the foreman answered that they had spent three days planning.

We interviewed a number of practitioners and consultants who all stated that any successful maintenance and reliability operation is built on the basic foundation of maintenance planning and scheduling.

According to Tracy Strawn, senior maintenance and reliability consultant at the Marshall Institute, Raleigh, NC, "Maintenance planners are change agents. Even though they typically have no direct authority, they have a great deal of influence over others through their conversations, their actions, and their attitudes."

What a planner and scheduler does
To understand these functions, look at two simple definitions:

• Planning-how a job will be done and what resources will be required
  
• Scheduling-when a job will be performed

A maintenance planner and scheduler finds the best ways to minimize wasted travel time between jobs, makes sure all the required materials and procedures are available, and then schedules the tasks.

The importance of planning
Labor is one of the largest resource areas and expenses in the maintenance department. If this resource is not being utilized effectively and efficiently, a great deal of money is being wasted.

Ricky Smith, president of the Technical Training Division of Life Cycle Engineering, a Charlotte, NC-based training and consulting company, states that most North American maintenance departments operate at between 10 percent and 40 percent efficiency. He also reports that some maintenance craftspeople spend up to 75 percent of their time searching for repair parts and traveling to jobs.

Why is the importance of maintenance planning and scheduling often overlooked? When a plant is operating in a reactive mode, it is very difficult to see the value of placing a potential "firefighter" behind a desk with a computer. Neil Juhnke, corporate maintenance manager at American Crystal Sugar, Moorhead, MN, states, "It's the tyranny of the urgent. Today's issues overshadow longer term priorities."

Implementing a maintenance planning and scheduling function is also a major paradigm shift. Gross inefficiencies are identified and procedures will be changed as a result.

What is the standard ratio for maintenance planners to craftspeople?
There is no hard and fast rule; however, we found that an average figure of 20 maintenance craftspeople to one planner is standard. This figure should be adjusted depending upon issues such as highly regulated environments where procedures require extensive documentation, the age of the plant, or the maintenance program itself.

What other tasks should an effective maintenance planner and scheduler be responsible for?
None. The maintenance planner and scheduler should be dedicated to two functions-planning and scheduling of maintenance tasks.

Strawn adds, "Good planning requires that the planner go to the shop floor to examine and plan future jobs. Some supervisors load their planners up with additional responsibilities that make the planner desk-bound or unable to go to the shop floor to plan. An example would be a supervisor or manager who makes the planner a relief foreman in the absence of the regular foreman or volunteers the planner to attend different departmental meetings because he is conveniently available."

The maintenance planner who will produce the best results is allowed to focus on planning and scheduling. The planner will look ahead to maximize the utilization of people, which will produce the biggest gains.

Planning and scheduling tools
An effective computerized maintenance management system (CMMS) is the best tool to manage multiple repairs and work orders, craftspeople, and parts inventory and, most importantly, to track and report effectiveness and results. In smaller settings without a CMMS, a spreadsheet program such as Excel could be used.

Planning and scheduling is an information-intensive job and computers must be used to manage resources, track multiple tasks, and generate reports.

Measuring the effectiveness of a planner
A key ratio to measure the effectiveness of the maintenance planning function is the ratio of planned to unplanned work. If the planning function is working at peak efficiency, reactive work should fall to a low single digit percentage. Another important measure is planned/scheduled work compliance. An effective program will be above 90 percent.

Although planning and scheduling seems simple, it requires absolute management support on a level equal to the support that safety programs receive. In other words, it must be allowed a budget and dedicated personnel who will receive ongoing skills development training. Active management support is vital to the program's success.

Benefits to an effective maintenance and planning function
"One of the biggest benefits has come from shutdown management," Juhnke says. "We had much tighter budgetary control and, for the first time, we were able to start up all five plants within two hours of our target. We also have been able to reduce our use of contractors because planning has freed up our maintenance craftspeople to perform the work themselves."

By its very nature, an effective maintenance and planning function will move away from reactive toward proactive maintenance work. The benefits to this type of environment are increased safety, worker morale, and job satisfaction.

Many planners and schedulers also serve a CMMS data quality assurance function as they close work orders as reported by maintenance craftspeople and ensure that the data is accurate and in the correct format.

Skills for a maintenance planner
One of the most important skills the maintenance planner will possess is the ability to communicate effectively with others. The planner serves as the center point on a hub between maintenance, operations, storeroom, supervisors, and engineering.

The planner usually will meet with the maintenance supervisor at least once per day to review past and future work and to deal with any required changes.

Once the maintenance planner is in place, a long-term training program should be developed that exposes the planner to various quality improvement skills such as the use of Pareto charts, root cause analysis techniques, and problem solving methods.

How to select training resources
It is important to gain a comprehensive understanding of what is required for a successful maintenance planning and scheduling program. A formal training program can be useful to everyone who will be involved, including management. Look for programs that cover the basics well. If you can implement the basics well, you will have an effective maintenance planning and scheduling program.

Be sure to ask the training company for customer references and follow up with the supplied names. Ask about the actual real world experience of the trainers in maintenance planning and scheduling.

Implement before anything else
There are many popular buzzwords and maintenance management paradigms such as reliability centered maintenance (RCM), total productive maintenance (TPM), predictive maintenance (PdM), and condition-based maintenance (CBM) that seem to hold many of the answers needed to improve machinery reliability and overall asset management.

Juhnke notes, "Most programs and technologies need to be applied on a planned and scheduled basis."

Without a solid foundation of maintenance planning and scheduling, many of these programs will fail or will not live up to their full potential. As Smith says, "If you think planning and scheduling won't work in your organization, you are right. If you think planning and scheduling will work in your organization, you are right. What do you think?" MT

Information supplied by Terrence O'Hanlon, publisher of Reliabilityweb.com

Nearly everything we do in life and in business follows a process, a series of steps. Sometimes these steps are well defined and sometimes they just "happen" and eventually we get an "output" (How consistent is the process to get work done at your site?).

When mapping a process, we need to identify the "inputs" and the "outputs" for each step. Y (output) = function of (inputs X1, X2, X3, &Xn). Simply stated, Six Sigma is aimed at producing key Ys within a specification range (upper and lower control limits) by reducing the variation in the key influential Xs. When Y is centered about the targeted (desired) value and Y's spread of variation is 33 percent of the spread of the upper and lower control limits, we have a Six Sigma process.

The catch is understanding the influence of the many Xs, both planned (controllable) and unplanned (noise). Six Sigma extensively uses statistics, the science of observation. A student of Six Sigma keenly learns the science of observation using many statistical tools to characterize the influence of the Xs on the Ys. It can get sticky when there are interactions occurring among the multiple Xs that influence Y. It gets even stickier when we have an incapable measurement system. (Is the measurement of the Ys and the Xs real? After all, we are observing them through the "lens" of flawed measurement systems.)

This leads us to the stages of Six Sigma variation reduction: measure, analyze, improve, and control.

In my opinion, the measure element carries about 50 percent of the importance in successful variation reduction. So, what should we be measuring to reduce the variation in our assets' performance? The answer is: "it all depends." I'll leave you hanging for a while and then this answer will make sense.

Let's start with the desired output, big Y. What are the principal subprocess steps (activities.); what are the outputs of each of the activities (little Ys); and what are the inputs that influence each activity? In a 10-step process, it's easy to identify 100, 200, or more inputs.

A process map is fundamental to the Six Sigma method. It is the foundation, the starting point, of measure. In manufacturing, a good process map can be facilitated only with a knowledgeable and participative cross-functional team of operators, maintainers, process engineers, supervisors, and environmental and safety engineers.

Why, you might object, do we need to compile a process map when we know the process and run it every day? Because a Six Sigma black belt or green belt can facilitate the mapping process in such a way that we see, perhaps for the first time, the possible influence of these 100 or 200+ inputs, inputs that were never considered nor understood before which may or may not contribute to the variability of big Y, the desired outcome.

I've witnessed many instances where a trainees' Six Sigma project became an overnight business bonanza by merely constructing a process map. Light bulbs went on among the team and "quick hits" could be implemented within a week with big gains. But they didn't stop there.

What is the big Y for asset dependability? Uptime? MTBF? MTTR? Cost? Percent emergency work? It depends on your business drivers. But I would contend that Y (uptime and/or cost) = function of (MTBF, MTTR, percent emergency, etc.). Another layer of mapping would analyze Y (MTBF) = function of (MTBF pump 1, MTBF compressor 2, MTBF control valve 3, MTBF motor 4, etc.).

Robert C. Baldwin, CMRP, Editor

In his current bootstrapping situation, my friend says RCM, predictive maintenance, and other higher-level strategies will have to wait until he gets some planners on board. He knows he will not be successful without being able to plan and schedule his work.

His logic parallels a point I tried to make during my presentation at the recent annual meeting of the Machinery Information Management Open Systems Alliance (MIMOSA). I presented a slide that listed Abraham Maslow's theory of hierarchal human needs:

1. Physiological-the need for food, clothing, and shelter
2. Safety-the need to be free from physical danger and emotional harm
3. Social-the need for affection, to be accepted, and to belong
4. Ego or Esteem-the need for self-respect, to be heard, to be appreciated, and to be wanted
5. Self Actualization-the need to achieve one's potential (doing things)

According to Maslow, one of the early humanistic psychologists, self-actualization is the highest motivator, but only after lower levels of the motivational hierarchy have been met. For example, the musician engaged in self-actualizing activity of making music will eventually become tired or hungry so that physiological need for rest or food becomes a primary motivator.

I suggest that a similar hierarchy of needs governs the behavior of reliability and maintenance organizations.

Physiological needs must be satisfied first. These include people, skills, and money. Unless you have some, you can't make much progress toward self actualization. Unfortunately, changing worker demographics and current economic behavior are making them harder to get. This means the self-actualized manager may have to step down a few levels to make sure these bread-and-butter issues are met. On the other hand, perhaps having enough people, skills, and money to get the job done is the real meaning of self actualization. MT

Today's new control systems are delivering higher productivity to meet ever-increasing expectations of equipment performance. These systems typically contain more electronics, much of it adopted from non-industrial applications, and almost all of it more sensitive to electrical disturbances than the equipment being replaced.

Often, the operating environment for these sensitive systems experiences a wide variety of power and electrical noise problems from aging power generation and distribution facilities, both inside and outside of the plant. The following discussion provides information needed to develop a strategy for protecting sensitive mission critical equipment from the effects of a poor industrial power environment.

Mission critical elements
The first task in protecting mission critical elements is to identify them. While each system is unique, the mission critical components are usually easily recognized.

Typically, programmable logic controllers (PLCs), industrial computers, and electronic motor speed controls serving in the control loop of a manufacturing process are the first components put on the "mission critical" list. But sensors, data communication equipment, actuators, and even production planning systems also must be included to achieve a high level of customer satisfaction and minimize costs due to downtime.

As each component is evaluated for inclusion on the critical component list, remember it is "mission critical" if its downtime causes lost profits.

Protection strategy
Once the list of mission critical components and systems is identified, the next step is to determine the necessary level of protection. When making this decision, it is valuable to look at achieving three distinct levels of protection: defense against instantaneous destruction, protection against long-term degradation, and defense against disruption.

Defense against disruption-those unexplained soft failures, system lock-ups, and resets for which no specific cause can be identified-is perhaps the most important level for most industrial systems. As more devices containing volatile memory find their way onto the production floor, guarding against such disruptive events becomes even more necessary to ensure that these costly interruptions do not occur.

If satisfied customers and controlled costs are of primary importance, there is little question that systems must be protected to the third, and highest, level. To accomplish this, it is critical to place a "bubble of protection" around mission critical systems. To create a bubble of protection, each input and output line, whether power or data, needs to be examined and appropriately protected against likely hazards. Achievement of this level of protection usually requires the use of industrial grade components, along with a combination of devices such as surge protectors, power conditioners, and power conditioned uninterruptible power supplies (UPSs), as well as appropriate grounding techniques.

Power line issues
Power line problems that can cause the destruction, degradation, or disruption of mission critical equipment can originate either inside or outside of the facility. Outside problems include inclement weather that produces lightning-induced transients or power line outages due to high winds or ice. Power problems also may come from routine utility operations such as capacitor switching to effect power factor correction, or from the clearing of line faults.

While outside events are the most obvious and spectacular, it is estimated that in industrial facilities, up to 80 percent of power related problems originate on the plant side of the meter. Inside problems are caused by a wide variety of factors including stopping and starting of motors, welding equipment, electronic motor speed controls, poor grounding, and some of the same problems facing the utility company-fault clearing and capacitor switching. The results of these events show themselves in many ways including voltage interruptions, sags, and the more disruptive voltage transients.

Power interruption
Among the most noticeable power quality problems is a power interruption. While power interruptions are relatively infrequent in most locations, their effect can be dramatic and obvious because everything grinds to a halt.

Solutions to combat power interruptions include alternate power feeds to the facility, local back up generating capability (diesel or gas powered generators), and the addition of UPSs on selected equipment. While alternate power feeds and local power generation may not be practical for every facility, the addition of UPSs, particularly to software-controlled devices, is an important component in a total protection strategy.

When properly selected, the UPS will ensure that the attached devices are kept active during an outage. With proper communications interface software, these devices also can smoothly and automatically shut down all running software applications and the operating system to ensure a clean restart of the process-a factor particularly important in batch processing applications.

Voltage sags
Voltage sags, and to a lesser extent voltage swells, are reported to be the most measured power line problem. A study of one site estimated that up to 62 voltage sags down to a limit of 80 percent of nominal voltage, and an additional 17 sags down to a limit of 50 percent of nominal voltage, occurred yearly at that site. In another study of a large industrial facility, more than 500 sags of various levels were recorded at the input to key control equipment over a 31/2 month period. In the same study, only about 100 such sags were recorded during that period on the input power line to the facility. Both of these studies also reported that the recorded voltage sags affected individual pieces of control equipment quite differently.

As with power interruption, solutions can be applied both locally and plant wide. Plant-wide solutions include layout of power distribution to minimize the number of sags induced on critical equipment from internal causes such as starting motors and fault clearing. Since studies show that up to 80 percent of sags are caused within the plant, such solutions, while expensive, can greatly aid in protecting critical control components from unwanted sags. To combat sags induced from the utility, new devices such as the dynamic voltage restorer (DVR) and other solid state devices developed in conjunction with the Electric Power Research Institute (EPRI) may be installed.

Typically, however, a more practical approach for protecting controllers is the application of a voltage control device in the power path supplying the control system. Because these local devices can compensate for sags generated both inside and outside of the facility, using them is usually more reliable and less expensive than attempting a plant-wide solution.

At least three basic types of devices that provide local sag protection are available. They include devices that store energy in a transformer (constant voltage transformer), devices that use boost windings to raise voltages during sags (tap switching transformer), and devices that supply energy from batteries during sags (uninterruptible power supplies). There are also devices that use some combination of these three technologies to combat sags.

While each of these solutions has its advantages and disadvantages, some are better suited than others to today's electronic control systems. In the past, the most common device applied to control sags was the constant voltage transformer (CVT). This device, which also typically provided the step down voltage function, was an excellent choice when most control devices used linear power supplies, most sags were not too severe, the attached control system "crashed" well, and the CVT was presented with a relatively constant load.

The new power environment
Today, however, control systems have changed. Loads are more typically switch mode power supplies (SMPS), and sags (particularly with deregulation) are likely to become more severe. In addition, control systems are often no longer based on proprietary software that crashes well, but on commercially available operating systems that need to be properly shut down in order to start up smoothly. Power system load requirements also change more often as control schemes are updated frequently with the latest technology in order to gain additional performance from existing tooling and equipment.

While changes have been made in many CVTs to adapt to this new environment, the best solution is one that was designed specifically to power SMPSs and has more energy to ride through sags than is available in a typical CVT. Such a device is a UPS containing a low impedance power conditioning transformer that, if required, also can perform the voltage conversion function. Typically such UPS devices are more efficient, provide longer ride through than a CVT is capable of, and can interface with the control system to provide an orderly shut down in the case of long-term power loss.

Transients
By their very nature, transient voltages on power lines, below the level of those that cause massive destruction, are difficult to measure directly. Among the most difficult transients to measure are the high-speed transients that are the most likely to cause disruption of electronic equipment. To further complicate the situation, transients often occur randomly and special power quality monitoring equipment is usually required to capture the high-speed impulse and oscillatory events that can cause sensitive electronic equipment to be disrupted. While often not discussed or considered, this "least measured" power quality event can be a major contributor to those random errors and lock-ups that occur in a control system.

As with many industrial power quality issues, most of the high-speed transients that cause system disruptions are not supplied through the power utility, but are generated inside the facility. This conclusion can be reached not only by observation, but also through examination of the typical transient's high frequency content and its interaction with the intrinsic impedance of power distribution lines. The one obvious exception is lightning, which is clearly a natural and external, or "outside," event. Typical inside causes of transient events include switching devices such as contactors, motor starters, compressors, variable speed drives, and the switching of capacitor banks for power factor correction.

While these transients are clearly a threat to a mission critical system's overall reliability, not every transient will cause a system disruption. The transient's frequency, edge speed, the mode in which it appears to the equipment, and where it occurs in the effected equipment's clock or processing cycle will all determine its immediate effect.

Clearly, almost all transient events are ignored by electronic equipment. If they were not, it would be almost impossible to keep a computer running. However, in mission critical applications the goal is to push disruptions as close to zero as possible, and the reduction or elimination of these transients is critical in achieving this result. Thus, in mission critical applications, reducing the amplitude and edge speed of all transients becomes paramount in achieving the desired system reliability.

In order to better understand the specific methods that may be used to control the amplitude and edge speed of transient voltages, it is useful to review how transient noise appears to electronic equipment.

Power line noise
Transients are said to be normal mode noise when they appear between the line (hot or phase) and neutral conductors supplying the equipment. While somewhat troublesome, noise appearing in normal mode often can be controlled by a combination of transient voltage surge suppressor (TVSS) devices and filters. Typically, individual pieces of equipment often make some provision for controlling this noise mode within the control equipment itself.

The far more difficult noise mode to control is common mode. In this situation, there is noise between the neutral line and the ground line connected to the equipment. While the neutral and common are bonded at the service entrance or at an intermediate transformer, noise in this mode is quite common, and very disruptive. Common mode noise typically occurs when current is "dumped" into the ground lead by other equipment -(input and output filters to suppress high frequency line noise are a typical cause) or protective devices such as TVSSs.

Control of common mode noise usually requires a transformer-based power conditioning device that provides a "separately derived" source of power in which the neutral and ground wires are locally rebonded.

Almost all such commercial power conditioning devices also include appropriate components to control any normal mode noise that is present. These devices, which are typically available as traditional power conditioners or as power conditioners with battery backup, accomplish the necessary reduction in amplitude and edge speed of transient noise sources to help ensure that equipment in mission critical systems is not unnecessarily affected by transient events.

In addition to installing an appropriate power conditioning device, proper care must be taken in system layout and wiring. In particular, it is critical that the wiring to the power conditioner not be run with the power from the output of the power conditioner. Running these wires in the same conduit or wiring tray will significantly reduce the benefits provided by installing the power conditioner.

It is also important that, whenever possible, all critical devices-including sensors-be powered from the same power conditioner as the controller, and that sensor and peripheral equipment grounds be connected at a common point. Finally, data communication cables should be run in conduit or wiring trays that do not contain power, or, at a minimum, do not contain unconditioned power.

Communication line issues
Today's typical control system uses communication lines for several purposes. Control busses such as DeviceNet or Profibus are becoming more popular; data lines to peripheral devices such as human machine interfaces (HMI) and connections to plant-wide information systems are becoming more common. While not subject to all of the problems of power lines, communication lines are often more likely to cause system disruption due to transients. In addition, grounded (nonisolated) communication schemes such as RS232 provide an opportunity for an additional path of disruption known as ground skew.

As with power lines, a user must be concerned about destruction, degradation, and disruption when addressing communication line protection. In communication lines, minimizing the chance of destruction or degradation is best addressed by the use of a communication line protector (CLP).

Typically, the semiconductor devices associated with communication lines are not designed to withstand the high voltages or currents that can be induced from power lines or other noise sources, and thus need to be protected with a CLP.

System considerations
CLP selection should be done with care to ensure that the clamping voltage is lower than the point at which damage will occur, but higher than the maximum voltage that can be applied to the line for normal communication. In addition, when using systems with the higher transmission speeds now available, care must be taken to ensure that the insertion loss due to the added capacitance and inductance of the CLP will not cause unacceptable signal level reductions.

Use of external CLPs is often suggested to improve system reliability, even if a communication port is internally protected by a TVSS against over voltage. This approach can lead to improved reliability because a typical CLP will have a grounding lead that can be wired to direct transient noise away from the chassis ground of the control device. Redirecting this transient noise current will avoid introducing potentially disruptive common mode noise into the equipment, a situation that can occur if the internal TVSS is triggered.

For this scheme to have value, however, the external CLP will be required to activate at a lower voltage level than the internal protective devices. While proper selection of an external CLP will provide this result, the selection requires investigation into the internal protection levels for each piece of equipment in order to ensure proper coordination.

While CLPs can provide protection against system destruction and degradation, they do little to assist in reducing disruptions from transient voltages that are below the level of component destruction, but above the disruptive level that interferes with routine communication. Protection against such disruption can be addressed in several ways.

First, it is critical that system grounding follows good practice, and meets the equipment manufacturers' guidelines. With grounded communication schemes in particular, a small grounding problem can lead to very inconsistent communication.

A second key factor is cable routing, which should be done in a manner to avoid inducing any noise into communication cables from other sources. In particular, to maximize system reliability, do not run communication cables with power cables, and when crossing power cables, if at all possible, do so at right angles.

Ground skew
Addressing ground skew is the next step in improving communication reliability. Ground skew problems occur when noise currents flow in a ground path between two pieces of equipment connected by more than one ground lead.

In grounded communication systems, the primary connection is the power ground, while the second ground lead is the shield and/or common lead in the communication cable. When ground currents flow in the power ground, they cause a voltage difference (ground voltage skew) between the two locations, thus causing a voltage differential to be reflected in the communication cable. This voltage differential, and the resultant current flow in the communication cable, can cause serious disruption of the communication path, and can even destroy devices not protected by a CLP.

There are two solutions available to eliminate or reduce ground skew related problems. The first, most expensive, and often most difficult to implement is full isolation on the communication port. Such isolation typically requires separate power supplies be added at each end of the line, in addition to adding the appropriate isolation device. While commercially available, such devices are relatively expensive and take time to install. To avoid such costs, an alternative solution is desirable.

One alternative solution to ground skew induced problems is a ground skew protective device in the power path. Such a device is available from multiple sources, each with slightly different, and patented, implementations. The device works on the principle of creating a high impedance in the ground path at high frequencies while maintaining a zero impedance (Oneac technology) or low (other implementations) impedance at power line frequencies.

By increasing the high frequency impedance in the ground line, the resultant voltage produced by high frequency ground currents is substantially reduced, thereby reducing the opportunity for disruption or destruction of the communication line. In order to ensure proper protection, one ground skew device should be placed in the power path of each device containing a grounded communication port. Commercially, ground skew devices typically are sold as an internal option to power conditioners and power conditioned UPS.

Ensuring reliability
In order to provide the highest level of confidence in the reliability of a mission critical industrial system two overall steps are required. First, robust equipment designed for use in an industrial environment must be selected. While this discussion covers techniques to minimize the effect of electrical anomalies on the system, items such as working temperature range and mechanical ruggedness are also important to ensure long-term system reliability. Once the proper equipment is selected, installing it with the proper bubble of protection on power and communication ports becomes of paramount importance to provide a system that is as failure free as possible.

When installing equipment with the goal of achieving a bubble of protection, it is important to protect each power and communication port into the system and provide a grounding scheme that is in accordance with the National Electrical Code and the manufacturers' guidelines. In a well-protected system, each power port should be protected with a low impedance transformer-based power conditioner to control both common and normal mode noise. On some power ports a low impedance transformer-based power conditioner with batteries (UPS) may be the proper choice to provide protection against extended sags and outages when sensitive controllers need to be shut down in an orderly fashion.

In addition, each communication line should have a CLP installed that has the appropriate voltage breakdown level and controlled insertion loss for the type of communication port being protected. When grounded communication lines are involved, either ground skew protection devices or full isolation of the ports should be considered.

Finally, remember that once a system is properly installed and protected, vigilance is required to maintain the level of integrity that was originally designed in. One single "on the fly" addition or change can leave a system with an unprotected path, and subject to the disruptive effects of power and communication line anomalies. MT

Paul Haake is vice president of engineering for Oneac Corp., 27944 N. Bradley Rd., Libertyville, IL 60048; (800) 327-8801. The company supplies products that protect against all types of power and data line disturbances.