Dual power paths and single-cord equipment

Protecting your data center against power failure is crucial to providing maximum availability. Power loss or poor power quality is a major contributor to data center server down time. This task takes more than simply having UPS and a backup power source, such as a generator. Dual Power Paths: A Crucial Element in a High-Availability Data Center article tells that in in high availability environments, a common way to provide redundancy is to supply two independent power paths to each piece of computing equipment. All Dual Feeds Are Not Created Equal white paper explains the classification system for data center power feeds and provides the pros/cons of each configuration.

The use of dual power path architecture in combination with IT equipment with dual power supplies and power cords is an industry best-practice. Data centers designed and built utilizing Tier IV requirements are by definition “concurrently maintainable,” which means any system or component in the data center may be shut down for maintenance or may fail without affecting the delivery of services to the end user. In the case of a dual-powered data center, this typically is achieved by delivering at least two power circuits to each cabinet, one from the A power source and one from the B power source. The equipment accepts the two power feeds via independent, parallel power supplies that are sized such that the equipment will continue to operate with only one power path. To make this work the equipment to be powered need to have Redundant Power Supplies.

Redundant Power Supplies are one advanced feature available on high-end server machines. In essence, this is a power supply that actually includes two (or more) units within it, each of which is capable of powering the entire system by itself. If for some reason there is a failure in one of the units, the other one will seamlessly take over to prevent the loss of power to the PC. You can usually even replace the damaged unit without taking the machine down.

Design Tips for the Dual-Powered Data Center and Four A-B Design Failures to Avoid in the Dual-Powered Data Centerarticles gives some more tips how to design dual power paths. You need to be careful in designing dual-powered data center. Failure to properly design, size and implement dual power infrastructure at the cabinet may lead to breaker trip during restart (starting current of many computing devices and storage systems could easily exceed 200% of the running load for some time). You need to have enough spare capacity, but on the other hand failing to fully load power circuits to their rated capacity may not result in downtime but may drive power subscription costs higher than necessary. Proper power planning and budgeting involves loading every circuit to the proper rated capacity while respecting safety margins.

For dual power paths approach to be effective, you’ve got to meet two requirements:

  • The protected equipment must support dual power feeds and operate with one feed faulted.
  • The loading of breakers within each power path must always be less than 50% of trip rating during normal conditions, so the breaker doesn’t trip if the alternate path has to take on the full load.

Meeting these two requirements can be a challenge. Especially because some computing and networking equipment is only available with a single power cord. It’s good design principle to disallow the use of single power cord computer devices in a high-availability data center environment, but there are case where those can’t be avoided. For example some network products or legacy servers may only have single power supplies.

One good way to over-come single power input equipment problem is to use an Automatic Transfer Switch (ATS), which generates a single feed from two inputs. These single power supply devices can still be used with reliability by utilizing automatic transfer switches. Redundant Power Supply article has the following picture to show the use if automatic switch.

Powering single-cord equipment in dual path data center environments article tells about a new white paper from APC-Schneider Electric addresses the concern of powering single-corded equipment in dual path data center environments. However, equipment with a single power supply introduces a weakness into an otherwise highly available data center. Transfer switches are often used to enhance single-corded equipment availability by providing the benefits of redundant utility paths. You need to understand the use of power transfer switches well because there are several possible configurations how to use them, with their pros and cons.

Powering Single-corded Equipment in a Dual Path Environment white paper goes on to describe three fundamental approaches to powering single-corded equipment in a dual path data center environment. There are a number of options for integrating single-corded devices into a high availability dual path data center. Powering Single-corded Equipment in a Dual Path Environment paper explains the differences between the various options and provides a guide to selecting the appropriate approach. The conclusion is that Power availability to the single-corded equipment below 10 kVA is optimized by bringing utility redundancy directly to the rack. This can be done by using a rack mount static transfer switch or a rack mount ATS, and the optimal solution is a rack mount ATS.

A well designed adaptable rack enclosure power system would be able to support a single or dual path environment or a hybrid of both single and dual equipment. Automatic Transfer Switch (ATS) used in data center is typically rack mountable and occupy 1U or rack unit of space. They feature dual input cords and are able to switch from one power circuit to the other in a few micro seconds when power failure is detected on one of the input leads.

The idea to write about this topic to this blog came to me after reading Powering Single-corded Equipment in a Dual Path Environment white paper.


  1. Tomi Engdahl says:

    OR controller wards off ±300-V transients

    Linear Technology’s LTC4371 ideal diode-OR controller provides low-loss ORing of two negative voltage supplies in 48-V telecom systems. It permits seamless handoff between redundant power supplies, replacing power Schottky diodes and associated heat sinks with N-channel MOSFETs, reducing power loss and voltage drop.

    The LTC4371 is designed to withstand ±300-V transients experienced during lightning induced surges, load switching, or supply short-circuit

    LTC4371 – Dual Negative Voltage Ideal Diode-OR Controller and Monitor

    Controls N-Channel MOSFETs to Replace Power Schottky Diodes
    Low 15mV Forward Voltage Minimizes Dissipation

  2. Tomi Engdahl says:

    APEC 2016 – Plenty of hardware, but where was the software?

    Power management, of course, is the theme of this event and component-level product advancements certainly need to be given their proper recognition. Indeed, the ability of converters to step directly from 48V to 1V or less, for example, is clearly attractive to drive efficiency gains in distributed power systems

    Technology-wise, GaN remains a mainstay of the show, except that where previously it’s always been on the cusp of becoming mainstream, now it finally seems to have arrived with GaN devices providing the basis for real products

    However, digital power products by themselves do not deliver what can truly be considered as Software Defined Power ®, which is what CUI sees as the essential next step for digital power in moving to the system level and adding intelligence outside of the power supply. Other industries are layering software on top of hardware to advance system-level solutions for greater efficiency, lower capital and running costs, and other benefits. Considering that 10% of the world’s electricity is now consumed by data centers, efforts to manage power on a system-wide basis are well overdue. Strategies like “peak load shaving” combined with backup capabilities are needed to even out load fluctuations and eliminate single points of failure.

  3. Tomi Engdahl says:

    Paralleling supplies: good, bad, or ugly?

    Paralleling voltage sources (also called power supplies, but there are also current-source power supplies) is done for a variety of reasons:

    you just found out that your design needs may need more current than originally planned for, and paralleling a supply seems like a good way to have some just-in-case “insurance;”
    it gives you the option of implementing a redundant or N+1 topology;
    it allows you to use the same supply as a single unit or in multiples across various products, thus simplifying the BOM, inventory, and other practical issues;
    it lets you work with one make/model of supply and get to know its inherent characteristics and idiosyncrasies (and they all have some!).

    Unfortunately, paralleling supplies is not as simple as the term suggests. Strange and often unpleasant things can happen if you just connect the outputs “as is.” Generally speaking, you can’t just connect the outputs of two supplies together unless the supplies have been specifically designed for such operation. Problems include output voltage inaccuracy, poor regulation, output droop, all the way to possible damage if the “stronger” supply tries to force current into a weaker one (and no two supplies–even the same model–are identical).

    “Current Sharing with Power Supplies” is an overview of the issues related to using supplies in parallel

  4. Tomi Engdahl says:

    Current Sharing with Power Supplies
    This paper presents the basic characteristics of power supplies and discusses the methods of paralleling units which do not already have special provisions to enable them to be configured in parallel.

  5. Tomi Engdahl says:

    Putting COPS into context

    NFPA 70: National Electrical Code Article 708: Critical Operations Power Systems (COPS) introduced electrical standards for facilities that support critical functions in response to vulnerabilities from natural and human-initiated disasters. However, these standards are less than straightforward and seemingly contradictory with other codes. It’s time to put COPS into context

    Article 708 introduced three new acronyms to the NEC. As one would expect for an electrical code, terms are defined by their electrical, rather than functional, characteristics in 708.2:

    • “COPS” is an electrical system serving part or all of a facility that must operate continuously to support public safety, emergency management, national security, or business continuity. The requirement for a COPS is determined by governmental authorities in accordance with codes or statutes, or by facility engineering documentation.

    • A “designated critical operations area” (DCOA) is an area that, by virtue of the functions performed within it, requires a COPS. Examples include air traffic control centers, police and fire stations, 911 call centers, communication centers, business data-processing centers, and broadcast stations. The NEC, as an installation code, doesn’t determine which facilities require a COPS. Instead, it describes how the system must be installed.

    • “Supervisory control and data acquisition” (SCADA) is a control and monitoring system for the COPS. Long used in industry, this term is new to the NEC with Article 708. It’s defined in the Article, but doesn’t appear among the requirements. Informative Annex G lists requirements for a SCADA system, but these provisions are informational rather than prescriptive. Those requirements, though, are not code; they amount to suggestions for how a SCADA system might be installed. For now, in the NEC, SCADA is an acronym without requirements.

    NEC Article 708 describes a number of documentation requirements including a risk assessment, a hazard-mitigation strategy, and maintenance and testing records.

    Standby power

    NEC Article 708 permits storage batteries, uninterruptible power supplies (UPSs), and fuel cell systems as alternate sources for COPS. These systems are uneconomical and unwieldy for all but a minority of COPS applications. Typically, the project characteristics will strongly push the alternate source selection toward generators.

  6. Tomi Engdahl says:

    IC manages backup-battery switchover

    Home> Tools & Learning> Products> Product Brief
    IC manages backup-battery switchover
    Susan Nordyk -September 10, 2016

    Save Follow
    Linear Technology’s LTC4420 dual-input monolithic power prioritizer for 1.8-V to 18-V systems ensures smooth backup switchover to keep critical circuitry alive during brownout and power-loss conditions. Its 18-V operating capability accommodates a wide range of power sources, including wall adapters, USB ports, supercapacitors, and stacked batteries with lithium-ion, alkaline, or NiMH cells.

  7. Tomi Engdahl says:

    Software Predicts Power Component Failure
    Algorithms to monitor universal power supplies

    Eaton Corp. (Raleigh, N.C.) claims its cloud-based software ensures that universal power supplies (UPSes) will keep your computers up and running 24/7/365.

    Called PredictPulse Insight, the software monitors the UPS’s components with a rule-based algorithm that tracks the batteries’ discharge history and determines when “80-to-90 percent of the UPS’s lifetime has been used up for the battery, capacitors, fans, air filters, and, if equipped, the power module [inverter, rectifier and the insulated gate bipolar transistor],” said Art Mulligan, product line manager at Eaton Corp.

    Eaton is a variety of Internet of Things (IoT) services that can predict at least 80 to 90 percent of the component failures in a variety of devices, such as vehicle transmissions before they happen.

  8. Tomi Engdahl says:

    Critical power: Selective coordination in health care buildings

    Health care facilities, especially hospitals, have more stringent selective coordination requirements than conventional building electrical systems, according to some electrical engineers.

  9. Tomi Engdahl says:

    Although a building’s entire electrical distribution system is important, its overcurrent protection is the most critical to safety. Overcurrent includes short circuits and overloads. During a short circuit, electrical current bypasses the load, taking the path of least resistance. An overload is an overcurrent condition within normal current paths. If an overload is allowed to persist, it can cause equipment or wiring damage and potentially start a fire. Temporary overloads can be harmless; sustained overloads can cause damage. Temporary overloads occur frequently, are often a routine part of system startup or operation, and should be allowed to subside. An overcurrent protective device (OCPD) should not open the circuit during normal operation, allowing motors to start and loads to stabilize.

    Overcurrent scenarios dictate the type of overcurrent protection that should be used.

    Fuses and circuit breakers are available in a variety of sizes and ratings. Their similar—yet different

    Critical Power: Circuit protection

  10. Tomi Engdahl says:

    Preventing arc flash in mission critical facilities

    To address arc flash problems, we turn to codes and standards. NEC 240.87 is an important leap in arc flash safety for the electrical industry, along with NFPA 70E and IEEE 1584: IEEE Guide for Performing Arc Flash Hazard Calculations.

    “Failure is not an option.” This quote from Eugene Kranz, a flight director for NASA during the Apollo 13 space mission, defines the concept of mission critical in five words. Yes, the Apollo 13 mission may be an extreme example of mission critical, where every decision and every action made was essential to the survival of the astronauts aboard that unstable spacecraft.

    When describing the electrical distribution system for a mission critical facility, two of the key components are availability and reliability. The electrical system must be available when called upon to function (24/7) and it must not fail while in operation. Based on this “must not fail” philosophy, most of the protection systems for mission critical facilities traditionally have been designed to keep the system operating. As such, protection devices are set as high as possible to prevent them from tripping and de-energizing the critical load. This philosophy guards against dropping the critical load, but it does not protect the equipment or more importantly, the personnel working on the equipment from potential hazards such as arc flash.

    NFPA 70E and IEEE 1584: IEEE Guide for Performing Arc Flash Hazard Calculations are the two standards used by the industry for guidelines and analysis regarding arc flash and arc flash safety. Arc flash hazards are usually expressed as a unit of incident energy (cal/cm2). Incident energy is a measure of thermal energy at a working distance from an arc fault.

    Mission critical facilities also tend to have generators for backup power with closed-transition capabilities that allow the load to transfer from generator to utility without interruption (or vice versa). During this closed-transition period, the generator and utility operate in parallel and both contribute to the fault current, which results in increased amounts of fault current.

    Because of the larger systems and closed-transition transfers, data centers often have a higher magnitude of available fault current throughout the distribution system, leading to higher arc flash thermal energy.

    The farther away a person can be from a fault, the lower the incident energy exposure. The 24/7 availability and reliability components of a mission critical facility require a comprehensive preventive maintenance program. Often, this preventive maintenance program demands that the equipment be operated or worked on live. For example, using infrared for scanning cable lugs to ensure all connections are tight. Working on equipment live puts the personnel working on the equipment close to the fault; therefore, they are at risk of severe injury.

    Another issue with data centers is that electrical equipment, such as power distribution units and remote power panels, is often located in a data center’s white space, which is occupied by unqualified electrical personnel. In addition to those working on the equipment, the explosive force caused by an arc flash can injure bystanders in close proximity to the arc flash. For this reason, an arc flash boundary is calculated in addition to the working distance.

    Arcing time: Arcing time is the time duration of the arc flash. While arcing time is not as obvious a characteristic of arc flash as the first two parameters, it is arguably the most important—especially in data centers. The duration of the arc flash is determined by the time required for the upstream protection device to trip and clear the fault condition

    Something worth noting is the difference between arc fault current and bolted fault current. Bolted fault currents (two conductors being bolted together with little or no impedance) and short circuit values normally are calculated at maximum. Bolted fault currents are used for sizing the interrupting rating of equipment and setting the protection devices. Arc fault currents are calculated based on the assumption that there is a small gap between the conductors that is bridged by something causing the arc. Because of the impedance caused by the gap and the bridging component, the arc fault current is usually lower than the bolted fault current. Lower current means a longer time period before the protection device clears the fault.

    The failure-is-not-an-option mentality of a typical mission critical facility and/or data center has pushed the configuration and coordination of the protection system toward keeping the system operating.

    High instantaneous devices or devices with built-in time delays are used in an attempt to avoid nuisance tripping. Although this philosophy keeps the electrical system operating as long as possible, it also extends the duration of the arc flash by delaying the time to clear the fault.

    If the arc fault happens with a magnitude to the left of or less than the instantaneous region, the protection device will operate in its time-delay region, extending the duration of the arc flash. Depending on the protection device used, this could result in a clearing time of 2 seconds.

    For many owners of older data centers, arc flash study results are indicating that they have high levels of incident energy and thermal energy within the distribution system. This high level of energy is requiring personnel to wear 40 cal/cm2 PPE protection (safety glasses, hearing protection, leather footwear, hard hat, arc-rated gloves, arc-rated flash suit, arc-rated flash suit hood).

    Due to the critical nature of the facility, the majority of electrical distribution equipment will require maintenance and adjustments while energized. Therefore, something must be done to reduce the incident energy level to within the range of a hazardous risk category so that it can be worked on with the proper protective equipment and clothing.

    Whether it is a new design or existing facility, the preferred method and least costly solution for mitigation is a re-evaluation of the coordination study. Incident energy is the result of short circuit current and clearing time under arcing fault conditions.

    Another method of mitigation is to select or replace older equipment, such as breakers and relays, with units that have quicker clearing times. For example, this could mean using solid-state trip units and replacing thermal-magnetic breakers with devices that have electronic trip units

    Zone-selective interlocking eliminates intentional delay without sacrificing coordination. In a well-coordinated system, longer delays and higher pickups are selected on upstream devices to allow downstream devices to pick up first. This delay extends the arc flash and exposes the system to higher levels of incident energy. Zone-selective interlocking allows the electronic trip devices to communicate with each other so that a fault will be isolated and cleared by the nearest upstream device with no intentional time delay

    Another form of zone protection is current differential protection, most commonly used with medium-voltage systems. This uses current transformers to measure and compare the incoming and outgoing currents. Because this protection system is independent and associated only with a particular zone, it is not required to be time-coordinated with other systems, allowing for tripping without additional delay.

    Optical light sensing

    This system uses a combination of light sensors, which detect the flash of light associated with the arc fault, and relays that detect the high fault current. When both conditions are present, the relay will quickly clear the fault by opening the upstream device without any delay. For protection, both conditions must be present.

    Arc resistant equipment

    Arc resistant equipment is designed to provide protection to those workers surrounding the equipment from internal arcing faults under normal operating conditions. Arc resistant equipment normally provides the required protection by venting the hot gasses and explosive material away from the work area surrounding the equipment.

    The maintenance-mode switch is normally an external switch wired to the circuit breaker that allows the operator maintaining that piece of equipment to modify the trip settings of the device to a lower setting. The lower and faster setting is intended to reduce the incident energy levels downstream of the device.

    Using remote operators, such as hard-wired control switches, programmable logic controller-based human-machine interface screens, and supervisory control and data acquisition systems allows the worker to be completely outside the room and outside the arc flash boundary when breakers and switches are operated.

    Infrared viewing windows are installed in equipment to allow the operator to scan cable connections and other key components of live equipment with infrared thermography devices without having to remove the equipment covers that expose them to hazardous energy

  11. Tomi Engdahl says:

    How an arc flash relay reduces costs
    Wire manufacturer finds a way to eliminate hazard and control labor spending.

    One of those e-houses is a 20-foot-long trailer mounted up a flight of stairs on a mezzanine. It is filled with breakers, PLC panels and had a an arc flash hazard rating of Category 3, which corresponds to an incident energy of 8 to 25 cal/cm2. The Category 3 rating requires anyone working inside it to wear personal protective equipment (PPE) consisting of safety glasses or goggles, hearing protection, hard hat, cotton underwear, fire-resistant (FR) shirt and pants, FR coveralls (in addition to FR shirt and pants), arc flash hood, leather gloves and leather shoes.

    Wearing this level of PPE was a real burden in terms of time and money for the manufacturer and its employees. The PPE was time-consuming to put on and take off, uncomfortable and confining.

    The manufacturer had arc flash labels in place and had the proper PPE on hand, but it looked for a way to reduce the Category 3 hazard rating.

    Each installation, including circuit breakers and parts, cost roughly $60,000. In addition, the reduction in arc flash hazard was insufficient.

    The plant engineering team learned about arc flash relays from its electrical distributor, Mayer Electric, who suggested that an arc flash relay might provide a solution. This relay uses light sensors to detect the light of a developing arc flash and sends a signal in less than one millisecond to open the upstream power breaker. By interrupting the power quickly, it dramatically limits the amount of incident energy, preventing a small arc from growing into a dangerous and destructive event.

    On the front of the enclosure they installed a pair of lights that indicate the status of the arc flash relay (on or tripped).

    For sense input to the relay, they installed four point-light sensors in strategic locations

    Because there were four lines feeding the e-house, it would be necessary for the arc flash relay to trip four breakers simultaneously, which was initially a concern.

    The relay instantaneously tripped the circuit breakers on each and every test. Installing the arc-flash relay saved $30,000 on this installation, about half the cost of the previous attempt, and the company acknowledges that it is a better solution than what they had tried in the past.

    The interior of the e-house now officially has no arc flash hazard. Under last year’s classification system it would have been rated as a hazard risk Category 0, but that category has been eliminated as superfluous. No hazard now simply means no hazard, and workers no longer need to don PPE before they enter the e-house.

    An arc flash occurs when an energized phase conductor with sufficient current available faults to ground or another phase conductor. The result is essentially an electrical explosion, as metal vaporizes to form a cloud of ionized gas that radiates intensely across the electromagnetic spectrum, including visible, ultraviolet and infrared light sufficient to damage eyes

    All this is accompanied by shrapnel—bits of metal, both solid and molten, that are flung at ballistic velocities. The longer the arc continues to burn, the greater the damage. There are several methods for reducing the energy available to an arc flash, including the use of current-limiting fuses. An arc-flash relay uses light sensors to detect the intense light given off as an arc flash begins and within a few milliseconds sends a signal to an upstream breaker to open and shut off the power. This stops the arc flash in its tracks.

  12. Tomi Engdahl says:

    Bloom Energy, Southern Company/PowerSecure form strategic alliance to integrate distributed power generation, smart storage technologies

    ATLANTA and SUNNYVALE, Calif. — On Oct. 25, Bloom Energy, Southern Company (NYSE: SO) and its subsidiary PowerSecure announced a strategic alliance, which will include project investment and joint-technology development to provide behind-the-meter energy solutions. PowerSecure will acquire an estimated 50 megawatts of Bloom Energy Servers under long-term power purchase agreements with high-quality commercial and industrial customers.

    The solution is designed to fully integrate Bloom’s firm 24x7x365 Energy Server platform with PowerSecure’s smart storage solutions. The result will deliver a reliable on-site generation solution tuned to the customer’s precise power requirements

    This fully-supported integrated platform will be designed to meet customer needs on several critical dimensions:

    • Enabling optimized use of power to drive cost predictability and strategic savings.

    • Delivering the quality and exact type of power required by each part of their business, for example, AC power for offices and DC power for data centers.

    • The ability to dial-in the level of reliability required from “no need” all the way up to mission critical reliability for tier IV data centers and critical operations.

    This fully-supported integrated platform will be designed to meet customer needs on several critical dimensions:

    • Enabling optimized use of power to drive cost predictability and strategic savings.

    • Delivering the quality and exact type of power required by each part of their business, for example, AC power for offices and DC power for data centers.

    • The ability to dial-in the level of reliability required from “no need” all the way up to mission critical reliability for tier IV data centers and critical operations.

    Apple’s Campus 2 to use next-gen Bloom Energy fuel cells first deployed in NC data center

  13. Tomi Engdahl says:

    Your questions answered: Critical power: Circuit protection

    Question: Provide clarification on which specific pieces of equipment or systems that need to be fully coordinated per the current NEC standards?

    John Yoon: Selective coordination requirements apply to all portions of an electrical distribution system that serve the following:

    Health care (Article 517.30G)
    Elevators (Article 620.63)
    Fire pumps (Article 695.3)
    Emergency systems (Article 700.28)
    Legally required standby systems (Article 701.27)
    Critical operations power systems (COPS) (Article 708.54).

    Question: How does the ambient temperature affect the rating of circuit breakers? And what’s the impact of that rating on cable sizing per NEC Article 110.14(C)?

    Tom Earp: Higher ambient temperatures will reduce the amount of current that thermal magnetic trip circuit breakers can carry continuously, and low ambient temperatures increase the amount of current they can carry continuously. However, this is not a change in rating. Circuit breakers listed to the UL 489 standard must be able to carry rated load in open air, 40°C ambient. This is their rating, and they do not have alternate ratings at other ambient temperatures. Solid-state trip devices are not affected by temperature variations.

    Yoon: Low-voltage power circuit breakers (LVPCBs) that meet the ANSI C37 standard have two distinguishing characteristics that are pertinent to this question: they are constructed to withstand fault current for a greater amount of time than a standard UL 489 circuit breaker (30 cycles versus 3 cycles), and they have the ability to delay or defeat the instantaneous trip function of the circuit breaker.

    Question: How does an electronic breaker differ from a thermal magnetic breaker or a fuse in terms of TCC tolerances?

    Yoon: While circuit breakers with electronic trip units typically have tighter TCC tolerances than a standard thermal-mag breaker, it depends on what portion of the time-current curve is being evaluated. Most generic thermal-mag circuit breakers have reasonable ±10% tolerances in the overload/long time portion of the TCC. However, in the short time portion of the TCC, the area of uncertainty in the curve gets bigger as the mag portion of the breaker takes over.

  14. Tomi Engdahl says:

    Paralleling generator systems

    When designing generator systems, electrical engineers must ensure that generators and the building electrical systems that they support are appropriate for the specific application. Whether providing standby power for health care facilities or prime power for processing plants, engineers must make decisions regarding generator sizing, load types, whether generators should be paralleled, fuel storage, switching scenarios, and many other criteria.

    Expertise in generator power design for emergency, legally required standby, and business critical loads is an essential skill for an electrical engineer to master. When designing generator systems, electrical engineers must ensure that the generators and the building electrical systems can support the critical loads reliably and effectively.

    This article examines standby systems in which generators serve as backup to the main utility source, such as those commonly installed in airports, data centers, hospitality complexes, water-treatment facilities, and in most life safety institutional applications.

    The need for backup power

    Interruptions of electrical power, even for a short duration, can introduce the potential for situations that could imperil public health and safety. Extreme weather-related disasters often disrupt power to hundreds or thousands of people and businesses, potentially for days. When these situations occur, they call attention to the vulnerability of the nation’s electrical grid and the importance of alternatives. Hospitals, airports, data centers, water and sewage facilities, fueling stations, communication, and transportation systems require alternate-power sources to limit the impact and ultimately save lives during times of crisis. The loss of electrical power due to storms, natural disasters, or high-power-demand issues are increasingly common. The loss of business and the associated economic impact from power outages are significant. Emergency generators are necessary to provide the reliable power required to maintain operations during primary supply system failures.

    Why diesel-powered generators are used

    Diesel-powered generators are considered among the most reliable approaches to providing backup power.

    NFPA 70: National Electrical Code (NEC), Article 517.30, as well as the California Electrical Code require hospitals and critical care facilities to have standby power systems that start automatically and run at full capacity within 10 seconds of power failure.

    When evaluating generator sets for parallel operation, ratings are important because the rating directly affects the efficiency and effectiveness of the selected generator set based on the application

    Using low-voltage generators in medium-voltage systems

    For generators rated 2,000 kW or less, it is common to install 480 V 3-phase generators and step up voltage transformers. The cost of medium-voltage generators is significantly higher-in the order of an additional $80,000 to $150,000 per unit. Additionally, medium-voltage generators generally do not have the UL listing necessary to support emergency power loads.

    Paralleling is the operation in which multiple power sources, usually two or more generators, are synchronized and then connected to a common bus.

    When connecting the generators in parallel or synchronizing with the utility, the following criteria must be met:

    Matched/proper frequency
    Matched/correct phase rotation
    Phase voltages in phase and within specified voltage range.

    Typical parameters that determine synchronization include a voltage difference of less than 5%, a frequency difference of less than 0.2 Hz, and a maximum phase angle of 5 electrical degrees between the sources.

    Closed transition is used when it is desirable to transfer loads with zero interruption of power when conditions permit. It is used when the generator system transfers back to the utility and when load testing the generators with building loads. Closed transition can be either a soft load transfer or a make-before-break transfer.

    The typical soft-load-transfer overlap time is around 2 seconds. The make-before-break transfer will parallel the generators and perform a transfer of load from the generator to the utility.

    Paralleling multiple sources provides increased reliability, flexibility in load management, and maintenance capabilities with little to no disruption.

    Redundancy: The redundancy inherent in the parallel operation of multiple generators provides greater reliability than a single generator unit for critical loads.
    One of the primary purposes of redundancy is to eliminate single points of failure.

    Efficiency: A more efficient system provides more stability and reduces cost and losses. Loads do not remain at a constant level in most installations. Variations in power demand can cause a single larger generator to run at loads of less than 30% of capacity, which could cause wet stacking. The optimum operational point for prime movers is between 75% and 80% of its rated value. At this point, the generator will be at its maximum efficiency.

    Expandability: When sizing generators to match system load requirements, it is often difficult to accurately project increases in load and adequately plan for unanticipated additional requirements.

    Ease of maintenance and serviceability: In an N+1 paralleled generator system, if a generator in the system fails or requires maintenance, individual units can be dismantled and serviced without disrupting the function of the remaining units.

  15. Tomi Engdahl says:

    Inverter transfer switch

    This is a load priority transfer switch. It is intended for applications where a load is being supplied by both an inverter and a generator. In order to minimize the load on the battery bank supplying the inverter, the load receptacle on the switch is supplied by the generator whenever it is running regardless of the operating status of the inverter.

    DIY: Automatic Load Transfer Switch(ATS) for Solar Power Inverter or Generator

    It is automatic. When I switch off the breaker for that circuit, the relay automatically transfers power to the inverter. This is the same thing that would happen if the power went out, all loads would be switched to the inverter.

  16. dharwendra says:

    primary input work but secondary input is not working .please send solution APC automatic transfer switch

  17. Tomi Engdahl says:

    An On-Line AC power source selector

    This device switches between two 230V 50/60 Hz AC power sources within a period (less than 20 ms). Application: off-grid solar systems

    This device allows fast and reliable switching between the Solar power and Grid power in my home made Off-Grid Photovoltaic System.

    It is able to switch up to 6 KVA of power within 20 ms. This allows nearly imperceptible switching between two power sources like Solar power and Grid or Genset power.

    When switching occurs, it does not impact, neither your PC nor any other appliances like TV sets and washing machines. You may just observe a faint blink on LED and neon lighting at this very moment.

    The main components of the source selector are two Opto Triacs wired in a SPDT (Single Pole, Dual Throw) switch fashion (Blue and Red boxes).

    In case both Triacs are ON at the same time, a source paralleling condition occurs. This means that the Inverter output and the Mains are connected together. This would create a severe short circuit because the inverter is not synchronized with the grid. The result in a damaged inverter.

    To avoid source paralleling, an interlock circuit (pink box) is placed between the processor and the triacs. This circuit based on CMOS logic inhibits simultaneous activation of both triacs.

  18. Tomi Engdahl says:

    For DC power Oring diodes can be used:

    Back to Basics: What is Active ORing?

    The uninterrupted availability of server, communications and telecoms equipment is frequently critical to its client applications, so this equipment typically uses two or more power sources in a redundant power architecture. These sources need mutual protection, otherwise a short-circuit fault in one could quickly overload and damage the others.

    Each diode allows current to flow in a forward direction only, while preventing either supply from drawing short-circuit current. Therefore the system can continue to function if one supply fails. Although simple and fast, this arrangement has a drawback due to the diodes’ high forward voltage in their normal state of forward conduction. This creates high power and heat dissipation and an unwelcome need for thermal management and extra board space.

    Active ORing offers a better alternative; it comprises a power MOSFET and controller IC. The MOSFET has an on-state resistance RDS(ON) which, multiplied by the square of the current through the device, creates an internal power loss. However this loss can be substantially lower than that of a Schottky diode for the same current; a ten times efficiency improvement is typically achieved

    For example, for a 20 A application, a Zener diode with a .45 V forward voltage drop would dissipate 9 Watts of power, while a MOSFET with a 2 mOHM RDS(ON) on would dissipate only 0.8W, a more than 10x reduction in power loss.

    Overview for OR-ing and Smart Diodes

    OR-ing and Smart Diode controllers make FETs act like ideal diodes, significantly reducing the energy normally lost across the forward voltage drop of a diode. While Texas Instruments’ OR-ing and smart diode controllers reduce power loss by acting like ideal diode controllers, they also protect power sources and loads against reverse polarity conditions that could damage or reduce system reliability. TI’s controllers provide several advantages over traditional Silicon and Schottky diodes. When paired with the right MOSFETs, they reduce power loss, provide system feedback, and protect systems against voltage and current transients.

  19. Tomi Engdahl says:

    Reliable system availability in case of short-circuit
    Efficient redundancy for power supplies

    Redundancy modules guarantee reliable system availability, even if
    a power supply fails. However, the decoupling diodes in the module
    lead to high power losses in form of heat and a large voltage drop.

    Therefore, PULS is replacing the diodes with efficient MOSFETs.
    The redundancy modules also receive further useful features.

    With MOSFETs as a decoupling element,
    even voltage drops can be minimised.
    Diodes in standard redundancy modules
    cause a voltage drop of 500mV bet-
    ween the input and output. Thanks to
    the MOSFET redundancy modules, this
    situation could be improved drastically.
    For instance, in the redundancy module
    YR80.241, the voltage drop at 40A out-
    put current is lower than 50mV between
    the input and output.

    Hot swapping – replacement without voltage interruptions

    With „hot swapping“
    one understands the replacement of a
    power supply or a redundancy module
    while the system is running. To enable
    this, the critical connections have plug
    connectors with short-circuit protec-
    tion. If the predetermined sequence is
    observed when replacing the defective
    device, it can be replaced without volta-
    ge interruptions.

  20. Tomi Engdahl says:

    Ethernet switch series with redundant power supply inputs

    Wieland Electric Inc.’s UMS 5-W, UMS 8, LMS 16-W wienet series of Ethernet switches have redundant power supply inputs and a wide temperature operating range.


  21. Tomi Engdahl says:

    Transfer switches: Which configuration is right for your system?

    When it comes to picking the right transfer switch for a facility, engineers need to consider many aspects such as system installation, operation modes, and switching mechanisms to help prevent downtime in the event of a power outage.

    Many commercial and industrial facilities require continuous uptime to maintain business continuity in the event of a power outage. For this reason, these facilities rely on electrical distribution equipment such as transfer switches to safely transition electrical power between normal and emergency power sources.

    Not all transfer switches are alike, however. The sheer number of available options and configuration modes can be daunting for an engineer while designing a system. Because of that, engineers need to understand the configurations available to determine what is correct for the application’s needs when implementing transfer switch technology.

    Determine the right configuration for a facility

    Transfer switches support multiple operation modes and transition types, and feature a range of different switching mechanisms. To determine the best fit for their facility, engineers should take the time to understand all the different aspects of transfer switch configurations and make their choices based on unique application needs.

    By understanding the configurations available and choosing the right switch for the specific requirements, control engineers can help keep a facility up and running in the event of an outage or power loss, positioning them as essential contributors to the business’ bottom line.

  22. Tomi Engdahl says:

    CUI Global says new switch can unlock data center power capacity

    CUI Global Inc. (NASDAQ:CUI) says its digital power monitoring subsidiary has started production of its ICE switch – aimed at unlocking capacity in data centers. “The ICE Switch is the next step in unlocking underutilized power capacity in data centers,”

    The device is a power monitoring and switching system that connects to Virtual Power Systems’ (VPS) ICE (Intelligent Control of Energy) software to provide dynamic redundancy inside a data center.

    The company notes that, usually in a data center, around half of the power infrastructure is untapped to provide redundancy to be available in the event of a failure. The ICE switch unlocks additional power by allowing facilities to use that stranded power for non-critical servers, and can still provide redundancy if a failure occurs. The ICE software combines telemetry and machine learning predictions to distribute redundant power for optimal use.


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