Dealing with AC Line Harmonics in Three-phase Inputs

These days, more and more loads connected to AC power lines draw currents that are not sinusoidal. The typical example is the AC supply for a computer. When AC voltage is applied to the power supply’s diode bridge (see Figure 1), the voltage is rectified by the bridge, and the capacitor charges to near the peak of the rectified AC voltage. The result is a current waveform containing multiple harmonics (see Figure 2).

HarFigure 1monics introduce a number of undesired consequences. They do not transmit power, which means they produce wasted power in the form of heat without increasing the DC power supplied.

Harmonics increase the RMS (root-mean-square) current by as much as 50 percent and produce excessive heat in wires, contacts, fuses, and circuit breakers, which necessitates the added expense of using larger fuses and circuit breakers, and even heavier power lines. If the total harmonic current is large enough to distort the supply waveform, proper operation of the equipment can be compromised. Figure 2Harmonics affect the power factor, which is the ratio of useful current to the total current. (If, for instance, the RMS current is 50 percent larger than the useful current, the power factor is 0.67.)

Obviously, current harmonics on the input to any unit are problematic. For low-power single-phase inputs, the ideal solution is electronic-power factor correction devices. These devices, which are available as power modules up to about 1 kW, force the input current though a pulse-width modulation (PWM) scheme to be sinusoidal and in phase with the input voltage. The design of these devices can be incorporated into any power supply; and in fact, there are many circuits available for doing so.

The approach above does not work for three-phase inputs, although three single-phase converters could be used in place of a three-phase input when the input is a WYE and the load is very symmetrical. The real problem comes into focus when the power level is much higher and the input is a DELTA. Figure 3Without a neutral, three single-phase converters cannot be used. However, when rectifying three-phase power, the harmonics are less than what would occur in single-phase power rectification.

When a transformer is used on the three-phase input, the unit can be configured to have a multi-phase output. For instance, if the secondary of the transformer has a WYE and a DELTA, it would have the equivalent of six phases. A secondary with three sets of WYEs and DELTAs properly phase-shifted will have the equivalent of 18 phases. An 18-phase rectified output is referred to as having 36-pulse rectification since it would use 36 diodes (see Figure 3). With so many pulses, the choke becomes reasonably small and is easily realizable.

Figure 4Not many devices have 18-phase, 36-pulse rectification capabilities. One that does is the BL+ 120 from Behlman Electronics (see Figure 4). The BL+ 120 is a 120 kVA frequency converter with a specially-wound input transformer that produces an 18-phase output that is rectified and filtered for 36-pulse rectification with the resultant DC supplied to a switching frequency inverter. It can produce low-distortion sine waves from 45 to 500 Hz, and up to 2000 Hz where required. Input voltage can range from 120/208 VAC to 277/480 VAC in either WYE or DELTA connection. The unit can be controlled manually or through an optional computer interface via RS-232, IEEE-488, USB, or Ethernet.

Problems caused by AC line harmonics are receiving more attention as a critical power quality concern with the growing percentage of electricity now passing through loads drawing non-sinusoidal currents. Clearly, today’s engineers must be aware of the negative impact of AC line harmonics in their systems, and of the solutions available to address problem.

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The Theory Behind the Behlman BL Motor Test Option

In our last blog post (please see Is Your Factory Power Source Corrupting Your Product Testing?), we discussed the importance of having a consistently reliable source of AC power when testing new electrical motors. In it, we mentioned that having a special motor-test option (with fold-back/soft-start capabilities such as in the BL 1350 and BL HP lines) can save you a lot of time and money when doing production testing on consumer products. In this post, we’ll explain how and why.

It is generally difficult to start motors because the peak current required to start a motor can be in the range of 3-6 times the current needed to run the motor at full load. After the peak current has just started the motor turning, there is a period of several seconds where the current falls to the run value. The problem is that if you size the power supply so that it has just sufficient capability to run the motor and load after start-up, you often find that it cannot start the motor without severe output voltage distortion or possibly even going into an over-current shut-down. If you size the power supply so that it can start the motor without significant output distortion, then you have purchased a power supply that is 3-5 times larger than what is needed for most of its operational time, which is very expensive. The ideal solution is to design a power supply that can deliver the extra, short-term current during startup but is basically sized for the long-term run current needed.

Electric motorIn any power amplifier there are two major factors that limit the output current. The first is the peak current handling capabilities of the output devices, in this case an output insulated-gate bipolar transistor (IGBT). The second is the requirement to dissipate the internal heating caused by losses in the amplifier. There are usually two types of current-limit features built into a power supply to protect it. One senses peak current through the output device and rapidly limits the peak output current to prevent damage to the IGBT. The other senses the RMS (root mean square) output current and, somewhat more slowly, limits the RMS output current to keep the unit from over-heating. In a typical Behlman BL unit, the RMS current limit is set at about 105-115% of the rated current.

A motor start sequence in a typical BL unit may go something like this: The motor is turned on. If the peak current is too high, the BL shuts off because it sees what it thinks is a short circuit. In this instance, there is no possibility that the BL can start the motor. If the BL doesn’t shut down because of the high peak current, then the motor wouldn’t start because the small initial peak current couldn’t overcome the bearing friction and get the motor turning, or the BL sees a short and just runs at zero output voltage and heats up the motor until the motor over-temperature sensor disconnects the power. The motor still wouldn’t start. The last possibility is that the motor starts turning but doesn’t get up to speed. The BL runs at full current and at some reduced voltage, but things just never get fully started.

The peak current capabilities of an AC power supply are usually limited by the size of the output device, in this case the output IGBT. Behlman doubles the rating of the output IGBT in BL units with the MT (motor test) option; and, since the circuitry that limits the peak current in the output is basically part of the IGBT, doubling the rating of the IGBT doubles the peak current available to provide the initial startup load. That large peak current rating is essential to getting a motor started.

The peak current gets the motor turning, but the power supply must be able to supply enough RMS current to get the motor up to speed. In MT option units, we also delay the action of the RMS limit a few tenths of a second to allow more time at a higher current before current limit restricts the output voltage. Several conditions allow this, first the larger IGBT allows more current. Second, internal heating takes some time to be a problem. Third, motors present a low power factor to the output, meaning that the internal heating of the amplifier is low compared to a resistive load of the same current.

For example, a 1-HP motor is rated at 746W but would need about 1000VA to run it due to motor losses and inefficiencies. A typical BL 1350 can provide 10Arms at 120V and approximately 25-30A peak. This may not be enough to start the motor. Yet, a BL 1350 MT can provide approximately 50-55A peak and a higher RMS current for a sufficiently period to have a much better chance of starting that motor.

We hope this brief explanation will encourage you to contact us to learn more about our BL Motor Test technology as an option in selecting a reliable power supply for your product testing needs.

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Celebrate Manufacturing Day

The second annual Manufacturing Day arrives on October 4, and we’ll be quietly celebrating the achievements of American manufacturers by manufacturing some of the finest power electronics in the world. It’s not that we don’t care (we do!); it’s just that we’re busy.

Just this month, we received orders for the U.S. Army and the U.S. Navy for power supply equipment that we must deliver in a timely manner.

Just because we aren’t openly participating in the activities surrounding Manufacturing Day this year, we don’t want to dampen the enthusiasm around such a worthy cause. As a manufacturer, we take pride in the work we do and appreciate the hard work that other U.S. manufacturers do.

Begun just last year, Manufacturing Day was launched to address common misconceptions the public has about manufacturing. “By working together during and after Manufacturing Day, manufacturers will begin to address the skilled labor shortage they face, connect with future generations, take charge of the public image of manufacturing, and ensure the ongoing prosperity of the whole industry,” the organizers wrote. Sponsors include: the Alliance for American Manufacturing, the Fabricators and Manufacturers Association, the Manufacturing Institute, the National Association of Manufacturers, the National Institute of Standards & Technology, and the Precision Metalforming Association.

The goal of Manufacturing Day is to educate the public as to:

  • What modern manufacturing facilities are really like these days.
  • What companies located in your community make and who they sell to.
  • What kinds of jobs are available in manufacturing.
  • What skills and education are needed to qualify for today’s manufacturing jobs.

Last year, over 240 organizations hosted events in 37 states that attracted more than 7,000 visitors. This year, organizers expect to double those numbers. We wish them well in their efforts.

And maybe next year, we’ll have the opportunity to join in the celebration fully.

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Kennedy’s Speech Was a Preview of the Future of Space Exploration

On September 20, 1963, President John F. Kennedy gave a speech before the General Assembly of the United Nations in which he called for the United States and the Soviet Union to cooperate on a program to send the first humans to the moon. Fifty years later, that idea sounds somewhat curious to us; but, in reality, it was a prognostication of what was actually to come.

Originally, Kennedy had been a firm advocate of the U.S. going it alone to achieve history’s greatest feat of exploration. Just two years earlier, he had given a very different speech that had roused the public during the height of the “space race” with the Soviets. “We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard,” he argued.

Yet, in his 1963 speech, Kennedy proposed that the two world superpowers cooperate in mounting “a joint expedition” to the moon. “Why should the United States and the Soviet Union, in preparing for such expeditions, become involved in immense duplications of research, construction, and expenditure?” It was a controversial stance to take in the middle of the Cold War; but it was realistic (even the Soviets thought so).

As we all know, history intervened two months later, and the notion of major nations cooperating to share their engineering and scientific expertise in space would have to be shelved for years. Eventually, though, Kennedy’s more clear-eyed stance on space exploration became the model for international efforts in the “high frontier.” First, the Americans and Soviets cooperated on a project called Skylab (one of the world’s first space stations) in 1973, which was quickly followed by the joint Apollo-Soyuz mission of 1975. Then they opened participation to all willing nation’s in 1993 with the announcement of the International Space Station (ISS).

Today, we still have individual nation’s making major strides in mastering the technology needed for manned spaceflight, but we also have many programs in which a variety of countries (and even private enterprises) work together to get things done up there, such as equipping the ISS. And this is likely to be the favored model for most future spaceflight breakthroughs going forward.

We were thinking about all this, because the folks here at Behlman have had some small measure of participation in the U.S. space program. Several years ago, we supplied some of the power supplies used to test the electronics associated with the launch rocket.

All these decades later, space exploration still has the power to fascinate us, as well as to remind us that we all share a very small planet.

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Powering the (More) Electric Aircraft of the (Near) Future

The futuristic concept of the Electric Aircraft is becoming a little less far-out these days. At the recent Paris Air Show in June, a variety of suppliers exhibited electric-power technologies that “are nearly ready for primetime.” The real heavy lifting, though, has already come from the two major commercial aircraft manufacturers, Airbus and Boeing.

In Paris, for example, Honeywell and Safran demonstrated their Electric Green Taxiing System (EGTS) prototype, which uses an APU generator to power motors on an aircraft’s main wheels, enabling the jet to push back and taxi without starting its main engines. “The key attractions at the Air Paris Show were the electric prototypes from big industry players,” said Frost & Sullivan Aerospace and Defense Analyst Alix Leboulanger. “If developments go according to plan, then all-electric commercial aviation could take off by 2035 to 2040.”

Really, the bigger story, though, has already happened, in the switchover to variable-frequency power electronics by Airbus and Boeing. Traditionally, commercial aircraft have used bleed-air from their engines to run a motor that supplied fixed-frequency power (3-phase 115 VAC at 400 Hz) to run their electrical systems. Maintaining a constant 400 Hz requires equipment (such as a constant speed drive) that converts the varying engine speeds to the constant speed, adding weight to the plane. Plus, fixed frequency limited the capabilities of the onboard electrical components to handle advanced applications. In recent years, the two avionics industry heavyweights have begun to adopt the more-sophisticated variable-frequency power architecture as part of a wider effort to build what has come to be known as the More Electric Aircraft (MEA) concept.

Airbus moved first with the A380, which uses four 150- to 380-kVA variable-frequency (380 Hz to 800 Hz) electrical generators. Its power system is fully computerized and many connections have been replaced by solid-state devices for better performance. Airbus has also imposed more stringent electromagnetic interference requirements. And the aircraft is touted as being the first commercial widebody to deploy power-by-wire flight control actuators.

Boeing has followed with the 787 Dreamliner, which completes a transition for the company from bleed-air power to a more-electric architecture.  Engine start, APU start, wing ice protection, cabin pressurization and hydraulic pumps are all powered by variable-frequency (360-800 Hz) generators. Boeing claims that this offers a 300 percent improvement in its electrical systems.

In power electronics, wide variable-frequency often goes by the name “wild” frequency (or AC Wild). We wrote about wild frequency in an article for Avionics Magazine back in 2007 entitled Frequency Change. In it, Ron Storm, president of Behlman Electronics, noted that the adoption of wild frequency will offer “significantly reduced complexity compared to the constant speed hydro-mechanical devices run by previous fixed-frequency power systems.”

“Logically,” Mr. Storm wrote, “if wild frequency is to become widely accepted as the prevalent AC power in avionics, ground support and test equipment capable of providing wild frequency power ranges will be essential.”

To help in this transition, Behlman created a Wild Frequency Information Center to provide background material on the topic for those interested in learning more. We will also try to keep you updated going forward with additional blog posts on the new era of power electronics in aviation. Welcome to the near future.

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Obsolescence: The Problem that Never Goes Away

In our last blog post (please see Form-Fit-Function Solutions: A Modest Proposal for Defense Department Cost Savings), we discussed a method in which replacement parts can be customized to supply the same functionality (or better) of an old part that may no longer be in production or may have become obsolete. While preparing that blog, we came across an article in New Electronics that discussed further problems with obsolescent technology and steps that can be taken to reduce the risk that it poses.

In the article Dealing with the Problems of Obsolescence, Graham Pitcher (an editor at New Electronics) writes that his magazine’s readers listed obsolete electronics as one of the biggest problems facing their businesses. ObsolescenceThe head of the Component Obsolescence Group (COG) told Pitcher that projects that need to “keep something in the sky or under the sea for 30 years” are increasingly experiencing problems with obsolete components. While designers today feel the pressure to shorten the upgrade cycle of their offerings, project managers are feeling the equal-but-opposite pressure to extend the life cycle of their ventures.

“While military projects were an early driver of obsolescence management, COG is now dealing with issues in the oil and gas, rail and nuclear industries, amongst others,” COG Chairman Stuart Kelly told the magazine. “These industries face the same issues, as do any with long-life projects.”

Kelly noted that the best approach to the problem is to meet it head on. COG’s website states: “Most companies deal with obsolescence in a reactive way, where they suddenly find that a component is not available. COG promotes proactive management where tools and procedures are put in place which monitor the life cycle of components and aim to have as much information as possible available on their bill of materials.”

Here at Behlman, we provide solutions to the problems of dealing with obsolete power supplies for a variety of applications. If you find yourself facing a problem involving obsolete power electronics, contact us for a potential solution.

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Form-Fit-Function Solutions: A Modest Proposal for Defense Department Cost Savings

As the federal budget sequester rolls along with no end in sight, pressure mounts on the U.S. Department of Defense (DoD) to trim its spending. Recently, the Senate Armed Services Committee warned that unless the Congress and the White House could agree to a long-term plan to reduce government spending that the Pentagon would be looking at $450 billion worth of spending cuts over the next nine years.

That got us to thinking about ways in which the DoD has actually been using some common-sense approaches to the difficult task of balancing cost savings with military preparedness in recent times. One of these methods is called COTS (commercial off-the-Cost savingsshelf) requisitioning (please see our previous blog entry Best Practices for Choosing a COTS Power Supply Vendor). Another method is referred to as FFF (form-fit-function) replacement. Here’s some background on how we view this second approach to practical cost savings in defense requisitioning.

Some of the military’s weapon, communications and support systems can date back half a century, and may need replacement parts that no longer exist. The original equipment manufacturers (OEMs) may even have gone out of business. We often find that electronic parts used by the original power supply manufacturer have become obsolete, printed wiring boards and castings are no longer available, or original schematics no longer exist.

When the military faces system obsolescence or costly, temporary repairs of existing power supplies, Behlman may be asked to create form-fit-function replacements. The result is a newly manufactured power supply designed to match the “form” (shape, materials and interfaces), “fit” (size and all connectors) and “function” (delivering the same output power from the appropriate input power) of an original power supply. These meet the specifications of the original but also present opportunities to address defects or shortcomings in the original design, as well as improvements in performance and reliability through the application of modern technologies and manufacturing techniques.

For example, an aerospace equipment manufacturer needed a form-fit-function replacement for a 30-year-old power supply used in Air Force E-3 AWACS systems. Over 300 units were extant in aircraft, and the power supply failure rate had become unacceptable. By utilizing existing modules and circuits to replace those used in the original AWACS power supply, Behlman kept development time to a minimum and delivered two qualification units in record time. We also integrated several upgrades that improved performance, maintenance and manufacturability. This economical solution enabled the AWACS to perform its mission long after it would otherwise have become obsolete.

While we can only speak for power supplies, there can be little doubt that electronic and mechanical form-fit-function solutions of all kinds can greatly extend the useful life of military systems. And, due to advances in technology, design and manufacture since the original equipment was produced, it is reasonable to expect a replacement part to significantly outperform the original.  To achieve this higher level of performance, vendors must have the ability to:

  • Identify and outline all alternatives and options, as well as explain the pros and cons of each
  • Look beyond the provided specifications and make recommendations to improve performance, manufacturability and cost
  • Draw from and implement a wide array of proven COTS technologies
  • Ensure that the replacement meets all appropriate standards introduced since the original was manufactured (in power supplies, for example, MIL-STDs for shock and vibration or EMI/EMC compliance standards).

While limited program funding has made sustaining legacy weapon, communication and support systems more challenging than ever, taking the time to research and select the right replacement vendor is well worthwhile, as it can definitely keep our nation’s armament on the front lines long after the OEMs have vanished – and all on budget.

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Using Inverters to Back Up SCADA Systems in Power Substations

In today’s increasingly automated world, system controls must be highly reliable; and the more critical the resource is, the more foolproof the control system must be. For power utility substations, the need for continuous facility control under any circumstance demands that a failsafe electric supply be always available.

Utilities automate substations for obvious reasons, such as lower operating costs and enhanced reliability. This is why utilities have been upgrading their substations for years with ever-more sophisticated industrial control systems. Digital protective relays, remote terminal units, programmable controllers and high-speed computer systems utilizing SCADA (supervisory control and data acquisition) have all been part of this upgrade cycle.

In an automated substation, the reliability of these devices depends upon the reliability of the power being supplied to them. Substations use banks of batteries as their reliable source of power because the substation must operate even when AC power is lost. The battery voltages are 48, 125 or 250 VDC. The batteries are kept charged whenever AC power is available. In order to power the equipment for the automation process, the battery voltage must be converted to AC. The device that does this conversion is called an inverter. A battery and inverter system is similar to an uninterruptible power supply (UPS); but an UPS is not preferred because the substation supplies the batteries and charger and, therefore, the inverter is the device of choice. The power requirement for today’s substation electronic devices is approximately 860 watts. Allowing for contingencies and growth, an inverter should be able to supply between 1000 to 1200 watts. (Note: This is not VA but watts. Most units are rated in VA, with watts equal to about 0.65 VA. For all Behlman’s inverters, VA equals watts.)

There are three types of inverters. The first type is the on-line inverter that has an AC input (normally 120VAC at 60 Hz) and a DC battery input. The output always runs from an internal inverter that produces the AC output. If there is a loss of AC input, the batteries take over seamlessly. The second type is the standby (or off-line) inverter that has the same inputs as the on-line unit except that the AC input goes directly to the output via a bypass. Upon loss of the AC input, the output is switched to the internal inverter output supplied by the batteries. There is a short switchover time when there is no output. Usually the electronic devices have internal hold up so that this loss of input has no effect. This cannot be guaranteed and must be verified for each type of electronic device.

The third type is the DC battery input only. This unit supplies the AC power from an internal inverter that is always powered by the substation battery bank. The Behlman ACDC-1200 inverter is the on-line type. The Behlman INV-1200 is a battery-only type that can be configured as a standby unit. Both inverters supply a pure sine wave output. Behlman’s INV-1210 is identical to the INV-1200 except that the INV-1210 supplies a modified sine wave with peak and root mean square (RMS) equal to a sine wave.

Behlman’s low-cost inverters offer: rugged construction for use in a substation environment; proven reliability for long mean time between failure (MTBF); and small size for use in equipment racks.

Contact us for more information on how we can help you in this critical area.

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The VPX Story

If you’re reading this page, you’re probably well-acquainted with the power-electronics market. For the sake of common cause, though, let’s take a few minutes to review one of the most significant developments in recent years in the field of embedded systems: VPX.

The VPX (a.k.a., ANSI/VITA 46.0-2007) standard specifies the architectural basics for VMEbus-based systems with support for switched fabrics over a new high-speed connector. Sponsored by the VITA industry group, VPX specifically addresses the needs of next-generation embedded systems, particularly those with industrial, medical and military applications.

Engineers from the member companies in VITA spent years perfecting an answer to the problem of I/O limitations with parallel-bus VME connections. They agreed to start from scratch on a new architecture and implemented a serial-bus, or switched fabric, solution to the connector bottleneck, the MultiGig RT2. The VITA Standards Organization (VSO) eventually agreed on: retaining the 6U and 3U Eurocard form factors; implementing the new 7-row high-speed connector (rated up to 6.25 Gbps); offering a choice of high-speed serial fabrics (e.g., PCI Express, RapidIO, Infiniband or 10 Gigabit Ethernet); including FMC (VITA 57), PMC and XMC (VITA 42) mezzanines; and developing hybrid backplanes to accommodate VME64, VXS and VPX boards. The new specification also offered improvements in power, cooling, and board real estate.Full VPX100CD

The VPX base specification and sub-specifications also enable power electronics modules to provide: 115 watts at 5 volts, or up to 384 W at 12 V or 768 W at 48 V. To upgrade cooling requirements, VITA advanced the Ruggedized Enhanced Design Implementation (REDI) standard to provide for improved cooling methods such as forced air, conduction- and liquid-cooling arrangements. Moreover, VPX accommodates legacy VME technology with hybrid designs that map the VMEbus onto the VPX J2 connector.

The OpenVPX specification enhances the VPX standard by improving interoperability between various VPX system providers, leveraging the various dot upgrades to the overall standard. (For more on OpenVPX, please see its FAQ at VITA.)

As a member of VITA (serving on its power supply committees), we at Behlman have been longtime backers of VPX development efforts. Our VPXtra™ line supports the design, manufacture, operation and cooling of high-density 6U OpenVPX-compliant COTS DC-DC power supplies and a COTS AC-DC front-end module. They are OpenVPX Vita 62 compliant with a wide input range and typical efficiency of 90 percent. The VPXtra™1000CD series delivers 1000 W of +12V DC power with a 3.3V AUX. The VPXtra™1000CM series delivers 700 W of DC power via five outputs. In both, the +12V outputs can be paralleled for higher power and redundancy, and both can accept a 28-VDC input, IAW MIL-STD-704.

For more information on our VPXtra offerings, please feel free to contact us at or 1-800-874-6727.  We’ll be happy to discuss the latest in VPX technology with you.

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Keeping the Railroads Running

Upon hearing that this is National Transportation Month, we thought we should praise our friends in the railroad industry, the men and women that keep people and product moving. After all, we’ve been working with them for more than 20 years.

One of our specialties is providing power supplies for railroad signaling applications. This is a mission-critical area in which absolute reliability is the standard. There is no room for error when it comes to railroad safety. And our clients have come to trust in our equipment so much that we have become the brand name in this specialty. Before going any further, though, let’s take a moment to discuss the basics of railway signaling for our friends in other fields.

Railway signaling goes back to the mid-nineteenth century when signalmen used flags and a variety of semaphore signs. By the latter part of the century, the myriad railroad operations began to unify around a set of guidelines called the Standard Code of Operating Rules (SCOR), produced by the Association of American Railroads. To this day, the SCOR guidelines are the grand-daddy of all North American railroad signaling rulebooks. Now, most Class I railroads in the U.S. use one of two sets of rules: the Northeast Operating Rules Advisory Committee (NORAC) rulebook and the General Code of Operating Rules (GCOR).

Amtrak, Conrail and several commuter and short-line railroads in the northeastern U.S. use the NORAC rulebook. The GCOR is used by every Class I railroad west of the Mississippi River, most of the Class II railroads and numerous short-line tracks. A few operators, including CSX, Norfolk Southern (NS), Illinois Central, Metro North and Florida East Coast, have adopted their own rulebooks. In the case of NS and CSX, the NORAC rulebook was integrated into their existing rules structure with the Conrail merger. Metro-North uses a rulebook based on NORAC. The Long Island Rail Road still uses a rulebook that is based on SCOR. So railway signals can vary somewhat from one system to another. Here’s a page displaying the various NORAC signals as an example.

OK, now that we’re all on the same page, let’s take a moment to thank our customers in the U.S. transportation sector for their hard work in keeping the railways safe for passengers and efficiently moving the goods we need across the country. Their efforts are truly vital to us all.

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