Electronics Guide

Medium Voltage Distribution

Many data centers receive power at medium voltage (typically 12-35 kV) and transform it on-site. The medium voltage equipment, while operating at relatively low frequencies, generates switching transients during breaker operations and creates magnetic fields around transformers and switchgear. Proper placement of medium voltage equipment away from sensitive computing areas reduces magnetic field exposure, while surge protection at the utility interface prevents external transients from entering the facility.

Transformer design significantly affects downstream EMC. Delta-wye transformer configurations provide isolation between the utility and facility ground systems while reducing the propagation of harmonic currents. K-rated transformers designed for non-linear loads maintain efficiency under the harmonic-rich current waveforms typical of computing equipment, reducing heating and associated reliability concerns.

Low Voltage Distribution

Low voltage distribution (typically 480V, 400V, or 208V) delivers power throughout the data center floor. The design of this distribution system profoundly affects EMC performance. Key considerations include:

Bus duct and cable routing: Power distribution paths should minimize loop areas and maintain separation from data cabling. Bus ducts and cable trays carrying power should be physically separated from data cable pathways, with crossing points at right angles when separation is not possible. Maintaining consistent phase arrangements throughout the facility reduces magnetic field emissions from parallel conductors carrying unbalanced currents.

Harmonic filtering: Computing equipment power supplies draw non-sinusoidal currents rich in odd harmonics, particularly the third harmonic and its multiples. These harmonic currents, flowing through the distribution system impedance, distort the voltage waveform seen by other equipment. Active or passive harmonic filters installed at strategic points reduce voltage distortion below levels that could affect sensitive equipment. Typical targets maintain total harmonic distortion below 5% at the point of common coupling.

Neutral conductor sizing: In three-phase systems supplying single-phase computing loads, the neutral conductor carries the sum of the third harmonic currents from all three phases. Unlike fundamental currents that cancel in a balanced system, third harmonics add in the neutral, potentially exceeding phase conductor currents. Oversized neutral conductors accommodate these currents without excessive heating while reducing common-mode noise voltages.

Power Quality Considerations

Power quality directly affects EMC because voltage disturbances can couple into equipment as interference, while equipment-generated noise degrades the quality of power available to other devices. Data centers must manage bidirectional power quality concerns.

Voltage sags and swells from utility events or internal load changes can propagate through the distribution system unless mitigated by UPS systems or voltage regulators. The transition between utility power and backup sources creates transients that must be controlled to prevent equipment disruption. Surge protective devices at multiple levels provide coordinated protection against external transients while preventing internal events from propagating.

Power factor correction equipment improves efficiency but can interact with harmonic filtering and create resonance conditions. The design of power factor correction systems in data centers requires careful analysis to avoid creating new EMC problems while solving efficiency concerns.

Variable Frequency Drive Considerations

Variable frequency drives (VFDs) control the speed of fans, pumps, and compressors throughout the cooling system. These drives employ pulse-width modulation (PWM) switching at frequencies typically between 2 and 16 kHz, generating harmonics extending into the megahertz range. The steep voltage edges driving motor cables create broadband emissions that can interfere with data communications and sensitive control systems.

EMC-compliant VFD installation requires:

• Input filtering: Line reactors or EMC filters at the drive input reduce the conducted emissions propagating back into the power distribution system. The filter selection depends on the required emission limits and the conducted noise spectrum of the specific drive.
• Output cable treatment: The cable between the VFD and motor acts as an antenna for the PWM switching waveform. Shielded cables with the shield properly grounded at both ends contain most radiated emissions. Cable length limitations prevent reflections from creating excessive voltage stress on motor insulation.
• Common-mode filtering: VFDs generate substantial common-mode currents that flow through parasitic capacitances to ground. Common-mode chokes on motor cables reduce these currents and the associated radiated emissions.

Proper VFD grounding is essential. The drive chassis should connect to the facility ground with low-impedance conductors, and motor cable shields must terminate to grounded enclosures at both ends. Ground current paths should not pass near sensitive equipment or data cables.

Chiller and Compressor Systems

Large chiller systems involve multiple VFD-controlled compressors, pumps, and fans, along with control systems coordinating their operation. The aggregate EMC impact of a chiller plant can be substantial, affecting not only the data center but potentially neighboring facilities as well.

Chiller plants typically require dedicated EMC treatment including:

• Isolation transformers at the chiller plant feed to prevent conducted noise from propagating to other facility loads
• Physical separation between chiller plant control cables and data center cabling
• Proper shielding and grounding of control system interconnections
• Attention to the EMC performance of building management system connections

Air Handler Units and CRAC/CRAH Systems

Computer room air conditioning (CRAC) and computer room air handling (CRAH) units positioned throughout the data center floor contain VFD-controlled fans, electronically commutated motors, and sophisticated control systems. Their distribution throughout the facility, often in close proximity to computing equipment, makes their EMC performance particularly important.

Modern CRAC/CRAH units increasingly employ electronically commutated (EC) fans with integral power electronics. These units generate EMC emissions similar to VFD systems and require similar mitigation measures. The control communications between units and the building management system should use appropriately shielded cabling or fiber optics to prevent interference from affecting cooling coordination.

Rack Construction and Grounding

Standard data center racks provide varying degrees of electromagnetic shielding depending on their construction. Open-frame racks offer no shielding, while enclosed racks with solid side panels and doors can provide 20 dB or more of attenuation at higher frequencies. The choice depends on the sensitivity of equipment, the noise generated by neighboring equipment, and regulatory requirements.

Regardless of construction, proper rack grounding is essential. Each rack should connect to the facility ground through dedicated conductors, typically a minimum of 6 AWG copper. The rack ground must not rely solely on power cable connections, as these may be insufficient for high-frequency ground currents. Multiple ground connections around the rack perimeter reduce ground potential differences that could affect signal integrity.

Equipment mounting rails and chassis grounding require attention to ensure continuous electrical contact. Painted or anodized surfaces can insulate equipment from the rack ground, creating potential EMC problems. Conductive mounting hardware and verified ground connections between equipment and rack ensure a solid ground reference for all installed devices.

Power Distribution Units

Rack power distribution units (PDUs) deliver power to installed equipment while providing monitoring, protection, and sometimes switching capabilities. The PDU's location within the rack and its internal design affect EMC performance.

Switched PDUs containing relays or semiconductor switches generate transients during switching operations. Remote-controlled outlets should switch cleanly without creating interference that affects other equipment in the rack. Higher-quality PDUs include filtering and transient suppression to minimize these effects.

The PDU's connection to the facility power distribution system represents a potential path for noise propagation. Filtering at the rack PDU input can prevent noise from equipment in one rack from affecting equipment in other racks fed from the same power distribution branch.

In-Rack Cable Management

Cable management within the rack affects EMC through coupling between adjacent cables and the interaction between power and data cables. Proper cable management separates power cables from data cables, maintains bend radii that prevent cable damage affecting shielding, and organizes cables to facilitate airflow without creating electromagnetic coupling problems.

Power cables routed in dedicated channels on one side of the rack and data cables on the opposite side minimize coupling between these cable types. Where crossing is unavoidable, perpendicular routing minimizes the coupled length. Cable management hardware should maintain cable organization during equipment installation and removal without creating sharp bends that could damage shielded cables.

Equipment Orientation Considerations

Hot aisle/cold aisle arrangements position equipment with front-facing cold aisles and rear-facing hot aisles. This orientation places equipment power supplies and fans (typically at the rear) facing the hot aisle, while networking ports and user interfaces (typically at the front) face the cold aisle.

From an EMC perspective, this arrangement means that the highest-emission components (power supplies with switching converters) face each other across the hot aisle, while more sensitive network interfaces are separated in the cold aisle. This natural separation can reduce interference between equipment in adjacent racks, provided cable routing maintains the separation.

Aisle Containment EMC Effects

Aisle containment systems that enclose hot or cold aisles to improve cooling efficiency create enclosed volumes that can affect electromagnetic fields. Metal containment structures, particularly those forming continuous enclosures with ceilings and end doors, can provide some shielding benefit while potentially creating resonant cavities.

The EMC implications of aisle containment depend on construction materials and geometry. Plastic containment panels offer no electromagnetic benefit, while metal panels provide some shielding. However, the large openings required for rack access limit any shielding effectiveness. More significant is the need to ensure that containment structures do not interfere with grounding systems or create additional paths for ground currents.

Inter-Rack Cabling

Cable routing between racks must accommodate the hot aisle/cold aisle arrangement while maintaining EMC integrity. Overhead cable trays often separate into sections for different cable types, with power cables routed above hot aisles and data cables above cold aisles or in raised floor pathways.

The distance between racks in adjacent rows affects cable routing and the resulting EMC coupling. Typical aisle widths of 1.2 to 1.5 meters (4 to 5 feet) provide reasonable separation, but longer runs between distant racks require attention to cable shielding and routing to prevent interference accumulation.

Cable Pathway Design

Data center cable pathways should provide separation between cable types while maintaining accessibility for installation and maintenance. The traditional approach uses raised floors for cabling, though overhead pathways are increasingly common in modern designs.

Pathway design for EMC includes:

• Physical separation: Power and data cables should maintain a minimum separation, typically 30 cm (12 inches) when running parallel. Cross-connections should occur at right angles.
• Dedicated pathways: Separate cable trays or channels for power, low-voltage control, and data cables prevent mixing that could create interference.
• Shielding considerations: Cable trays with solid bottoms and covers provide some shielding for contained cables. Metal cable trays should be properly grounded at multiple points.
• Accessibility: Pathways must allow cable additions and changes without disturbing existing cable organization or separation.

Cable Selection for EMC

Cable selection affects EMC performance through shielding effectiveness, characteristic impedance, and immunity to external fields. Data center cabling selections should consider:

Power cables: Properly sized cables reduce voltage drops and the associated power quality effects. Where EMC is a concern, shielded power cables can contain fields from high-current conductors. Cable armor provides additional protection in demanding environments.

Data cables: The choice between copper and fiber optic cables has profound EMC implications. Fiber optic cables provide complete galvanic isolation and immunity to electromagnetic fields, making them preferred for sensitive links or routes passing through high-interference areas. Copper cables, when required, should use appropriate shielding (STP, S/FTP, or similar) with proper termination.

Control cables: Building management and monitoring systems use various cable types. Shielded cables with proper grounding practices prevent these systems from picking up interference from power and computing equipment.

Grounding of Cable Shields

Cable shield grounding practices significantly affect EMC performance. The appropriate grounding method depends on the cable type, frequency range of concern, and the grounding system topology.

For data cables operating at high frequencies, shields should typically be grounded at both ends to provide effective shielding across the frequency range. The ground connection should be low impedance, typically through 360-degree connections at shielded connectors rather than pigtail connections.

Lower-frequency control cables may benefit from single-point grounding to avoid ground loops, particularly in facilities with significant ground potential differences. The grounding decision requires understanding the signal frequencies, interference frequencies, and ground system characteristics.

Ground System Topology

Data center grounding typically follows a mesh or hybrid topology rather than the pure star or tree structures used in smaller installations. The mesh ground provides multiple paths for current flow, reducing ground impedance and minimizing ground potential differences between equipment.

The ground mesh connects equipment racks, cable trays, raised floor pedestals (in raised floor installations), and structural steel in a continuous network. Connections should be made at regular intervals, typically every 1.8 to 3 meters (6 to 10 feet), using bolted or welded connections capable of carrying fault currents while maintaining low impedance at high frequencies.

A central ground point or ground bus provides the connection between the mesh ground and the building/utility ground system. This connection must be robust enough to handle fault currents while providing an equipotential reference for the facility.

Raised Floor Grounding

Raised floor installations require attention to the grounding of floor pedestals and stringers. The raised floor structure, when properly bonded, forms an extension of the ground mesh directly beneath equipment racks.

Floor tile surfaces should maintain conductivity to prevent static charge accumulation while limiting personnel current exposure. High-pressure laminate tiles with conductive surfaces, vinyl tiles with conductive backing, or carpet tiles with static control properties are typical choices. The tile-to-ground resistance should fall within the range specified by equipment manufacturers, typically 1 megohm to 1000 megohms.

Pedestal bonding straps should provide low-impedance connections between pedestals and the ground system. Regular testing verifies that all pedestals maintain proper grounding despite the effects of mechanical stress, corrosion, and building movement over time.

Equipment Grounding

Individual equipment grounding connects each device to the facility ground system through multiple paths. The safety ground conductor in the power cable provides one connection, while chassis connections to the rack provide another. Both paths are necessary; the safety ground carries fault currents while the chassis ground provides a low-impedance path for high-frequency currents.

Equipment ground connections should use dedicated hardware that maintains conductivity over time. Painted surfaces must be prepared to ensure metal-to-metal contact, and connections should be verified during installation and periodically during operation.

Ground Potential Differences

Despite best efforts at ground system design, some ground potential difference is inevitable in large data centers. High-frequency currents from computing equipment create voltage drops across ground impedances, resulting in potential differences between equipment at different locations.

Managing ground potential differences involves:

• Minimizing ground path lengths through proper mesh design
• Using low-impedance ground connections that perform well at high frequencies
• Employing fiber optic connections for long-distance data links to eliminate the ground potential difference as a signal integrity concern
• Designing equipment to tolerate expected ground potential differences without malfunction

Floor-Standing PDUs

Floor-standing PDUs, sometimes called remote power panels (RPPs), distribute power from the main distribution to multiple racks. These units may include transformers, breakers, and monitoring equipment. Their EMC performance depends on internal construction and installation practices.

Transformer-based PDUs provide isolation between the main distribution and rack circuits, reducing the propagation of conducted noise. However, the transformer itself creates magnetic fields that could affect nearby sensitive equipment. Proper placement, typically at the end of rack rows rather than adjacent to equipment racks, reduces this concern.

Electronic monitoring equipment within PDUs can be both a source and victim of EMC issues. Microprocessor-based monitoring systems should be properly shielded and filtered to prevent corruption by the power environment they are monitoring.

Intelligent PDUs

Modern intelligent PDUs include network connectivity for remote monitoring and control, outlet-level power measurement, and environmental sensing. The communications infrastructure within these PDUs adds EMC considerations beyond simple power distribution.

Network connections to intelligent PDUs should use appropriate cable types and grounding practices. The management network often carries sensitive data about facility operations, making EMC-related data corruption a security as well as reliability concern. Fiber optic connections for PDU management eliminate this coupling path entirely.

The switching components in intelligent PDUs that enable outlet-level control can generate transients when operating. High-quality designs include snubbing and filtering to minimize these transients, but the facility designer should understand the PDU's switching behavior when planning sensitive equipment placement.

Double-Conversion UPS EMC

Double-conversion (online) UPS systems continuously convert input AC to DC for battery charging, then invert the DC back to AC for the load. This topology provides excellent power conditioning but creates EMC considerations from the switching power electronics.

The input rectifier stage generates harmonic currents that flow back into the utility supply. Modern active front-end rectifiers with power factor correction reduce harmonic generation but introduce high-frequency switching noise. Input filters are typically required to meet conducted emissions limits.

The output inverter creates a synthesized AC waveform through PWM switching, typically at 3-20 kHz. While filtering smooths the output waveform, some high-frequency content remains. The output cables between UPS and distribution can radiate this noise if not properly routed and shielded.

Rotary UPS Considerations

Rotary UPS systems using flywheel energy storage and motor-generator sets present different EMC characteristics. The rotating machinery creates magnetic fields and mechanical vibration that require proper isolation. However, the motor-generator provides inherent input-output isolation that reduces conducted noise transfer.

Rotary systems often include power electronics for speed control and energy transfer to/from the flywheel, generating EMC emissions similar to other variable-speed drive applications. The scale of rotary UPS installations, typically serving entire data centers, means their EMC impact affects the entire facility.

UPS Installation for EMC

UPS installation practices significantly affect EMC performance. Key considerations include:

• Location: UPS rooms should be separated from computing equipment to allow distance attenuation of radiated emissions. Metal enclosures for the UPS room provide additional shielding.
• Cable routing: Input and output cables should be routed separately and away from data cabling. Proper shielding of these high-current cables contains magnetic fields.
• Grounding: The UPS ground must connect to the facility ground system with low-impedance conductors. Ground conductor sizing should accommodate both fault currents and high-frequency ground currents from the power electronics.
• Filtering: Additional input and output filtering beyond what the UPS provides may be necessary to meet facility EMC requirements, particularly when the UPS feeds sensitive equipment directly.

Generator Output EMC

Generator output voltage waveforms differ from utility power in ways that can affect equipment operation. The synthesis of the voltage waveform by the generator's excitation system may produce harmonic content different from the utility supply. Non-linear loads draw harmonic currents that create voltage distortion more severe than with the relatively stiff utility source.

Generator voltage and frequency regulation during load changes can cause excursions that stress equipment power supplies. The generator's response to load steps, characterized by its transient response specification, affects equipment that may interpret rapid voltage changes as interference.

Electromagnetic emissions from generators include both radiated fields from the generator and alternator assembly and conducted noise on the output conductors. Large generator installations should be treated similarly to other high-power rotating equipment, with appropriate separation from sensitive areas and attention to cable routing.

Transfer Switch Considerations

Automatic transfer switches (ATS) that switch loads between utility and generator power create transients during the transfer operation. The brief interruption in continuous-transfer schemes or the longer break in open-transition transfers causes equipment disturbances that UPS systems are designed to bridge.

Transfer switch operation generates EMC events including:

• Contact arcing during switch opening and closing
• Voltage transients from inductive load switching
• Ground reference shifts between utility and generator ground systems

Modern static transfer switches using semiconductor devices eliminate mechanical contact arcing but create their own high-frequency switching noise. The choice of transfer switch technology involves tradeoffs between transfer speed, EMC generation, and other reliability factors.

Generator Synchronization

Multiple generator installations require synchronization equipment to parallel generator outputs safely. The synchronization process involves matching voltage, frequency, and phase between sources, typically using electronic regulators and synchronizing relays.

Synchronization equipment includes power electronic controls that generate EMC emissions. The control signals between generators, synchronizers, and facility management systems should use shielded cables or fiber optics to prevent interference from affecting the synchronization process. Failed synchronization can result in generator damage or facility-wide power disruption.