As the plant landscape evolves, the rules are changing
Today, 80% of all input/output (I/O) devices in automation applications are supplied with 24VDC power. The second most popular voltage is 120VAC, which is used on 15% of all I/Os. Over the past decade, the industry has seen a transition of power supply development from linear regulated designs to primary switch-mode (nonlinear) designs, and the corresponding installation and grounding requirements for the switch-mode power supply are worthy of review.
Today's high-power drives and inverters now share their 3-phase power sources with a new type of switch-mode power supply — the 3-phase power supply (“The Basics of a 3-Phase Power Supply,” below). Designed to accept 3-phase, 480VAC power, this device isolates and steps the voltage down to 24VDC for use with the majority of sensors, actuators, and controllers on the plant floor.
So let's discuss the grounding requirements for switch-mode power supplies and offer wiring recommendations that improve availability and reduce problems associated with grounding faults, harmonic distortion, and other electrical disturbances.
Grounding. In 3-phase, 480V applications, large inverters, drives, and motors can introduce significant distortion and interference on the power lines. Using a single grounding system can, and will, introduce unwanted problems. Depending on the application, you can choose between a single ground point that ties all AC and DC voltages together, or separate grounds for the AC and DC voltages. In creating a common ground, you typically would connect the incoming AC voltage ground with the negative leg of the DC voltage.
Of course, this common ground compromises the isolation between the AC and regulated DC and ultimately negates the requirement for an isolating transformer. Because a power supply that uses transformer isolation achieves single — and sometimes even double — isolation, the common-ground approach comes further into question.
By using the right circuit layout and components that feature touch-proof connections, you can create insulating boundaries between different voltages. These boundaries maintain the integrity of the power supply and all of the devices connected to its DC voltage output. In short, it's possible to design a safe system with true isolation between AC and DC and two separate grounding systems.
All DC power supplies offer plus (+) and minus (-) output connections that are isolated from the AC input. With the DC circuit, a common plus or minus connection is possible using touch-proof, DIN-rail terminal blocks that allow voltage isolation from the DIN-rail itself. By keeping different voltages physically and visually separated, the need to keep a common ground is no longer important. The physical distance between the various voltages negates the possibility of creating a dangerous situation, unless it's intentional.
Because 24VDC would normally power 24VDC relays, contactors, and proximity sensors or analog 4mA-to-20mA loops, you must make sure that all metal casings and shields are tied to ground, unless otherwise specified. However, in no way should you connect the AC ground to either the positive or negative connection of the 24VDC power supply.
Harmonics. Unfortunately, the high-switching frequencies associated with switch-mode power supplies introduce harmonics on incoming AC lines. These harmonics, which are normally multiples of the switching frequencies, create a host of problems within AC power networks. (“A Primer on Nonlinear Loads and Harmonics,” below).
Consider the implication of having harmonics present on the common DC leg of a power supply. The most severe problem is the overload experienced on the neutral and ground wires. In a 3-phase, 4-wire layout, the sum total current (including all harmonics) flows through the neutral wire — and sometimes the ground. In 310.15(B)(4)(c), the NEC states “on a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.” The gauge size of the neutral should be twice the gauge of the current-carrying live wires in order to carry the maximum possible amount of triplen harmonic currents, which can be 1.73 times the individual phase current. Harmonic current flow in the ground wire is another reason to keep the AC input separate and isolated from the DC output.
Switch-mode power supplies normally feature power factor correction to filter out higher-order harmonics and meet international standards such as those required in IEC 61000-3-2.
Higher common-ground currents. Higher voltage AC systems are an even greater concern. The step-down ratio from 480VDC to 24VDC is 20:1. Unlike 120VAC, where a combined current total of 15A to 20A can occur, the possible maximum current can be considerably higher because of other larger loads connected to the 480VAC power network. These larger loads could include 3-phase motors with related control gear, drives, and inverters, plus other devices powered with 480VAC.
Imagine the implications when 24VDC analog and digital control signals are referenced with heavy-duty 480VAC loads. Again, the best installation for 3-phase, 480V power supplies calls for separate grounds — one for AC and one for DC.
As more and more tasks become automated, the use and importance of 3-phase, 480VAC power supplies will steadily increase. As the technology gains wider acceptance over the next decade, system integrators will debate the pros and cons and hash over the grounding rules, correct layouts, and appropriate insulations for different voltage levels.
Editor's note: The text for this article is an adaptation of an article that first appeared in the June 2002 issue of Power Quality magazine. The version included here includes expanded information on harmonics, line-to-neutral switch-mode power supplies, IEEE standards, and the latest technology available in the industry today.
Offner is the industry standards manager for Phoenix Contact in Harrisburg, Pa.
Sidebar: The Basics of a 3-Phase Power Supply
As noted in this schematic of a typical power supply, AC and DC ground connections are isolated from one another.
Most power supplies come with an AC power input connection that incorporates line (L), neutral (N), and earth (E) connections. The E connection, also referred to as the ground (GND) or protective earth connection, normally ties into the frame (in an open-frame design) or the housing (if enclosed). Voltage conversion in the power supply occurs through a step-down isolation transformer with a 5:1 ratio, specifically 120:24. The DC output terminations become isolated from the input because of transformer isolation.
The basic internal layout of a power supply is shown in the Figure above. Note that the grounding of the iron-core transformer normally feeds into the incoming AC voltage. The corresponding step-down voltage, which is DC after full-wave rectification, is isolated from the input terminations.
Sidebar: A Primer on Nonlinear Loads and Harmonics
According to IEEE 1100-1992, “IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment,” a load whose instantaneous current is discontinuous or isn't proportional to the instantaneous AC voltage is called a “nonlinear load.” The result is the presence of harmonic components of currents of higher frequency superimposed on the nominal (60 Hz) sinusoidal current. All components algebraically added together equal the actual measured waveform.
These components of current aren't in phase with the distribution voltage waveform at each harmonic frequency. These harmonic currents also interact with the power source impedance and typically create voltage distortion, excite power system resonances, and stress power system components on the AC distribution system.
IEEE 519-1992, “IEEE Recommended Practices and Requirements for Harmonic Control In Electric Power Systems,” includes detailed discussions on the resultant disturbances and proposed limits for harmonic currents.
In 3-phase circuits, the triplen harmonic neutral currents (3rd, 9th, 15th, etc.) add instead of cancel, since they're multiples of three times the fundamental power frequency and are spaced apart by 120 electrical degrees. Based on the fundamental frequency, triplen harmonic currents of each phase are in phase with each other, and so add in the neutral circuit. Under the worst-case conditions, the neutral current can be 1.73 times the phase current.