Cabinet Power Distribution Units (CDUs)
Features & Benefits of Implementing Server Technology’s Cabinet Power Distribution Units (CDUs) in Your Data Center
BENEFITS and FEATURES of CDU IMPLEMENTATIONIntroduction |
| With the high cost of data center floor space and current advances in technology, new installations with denser cabinets that require more power continues to be the trend. The required power depends on the equipment, how dense the cabinet is and whether redundancy is needed. These new demands have led to new and innovative solutions for providing cabinet level power utilizing CDUs (Cabinet Distribution Units). Surveys show that when asked about their top 3 concerns; Heat/Power Density is the number one concern of Data Center Management today. |
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Chart 1 |
| Reference: Data Center User’s Group Conference, The adaptive Data Center: Managing Dynamic Technologies |
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The focus of this paper is to investigate the features and benefits of implementing Server Technology’s Cabinet Power Distribution Units (CDUs) in your data center. Metered, Smart and Switched CDUs provide many solutions to common data center problems including cable management, temperature and humidity monitoring, circuit overload prevention and remote reboot capabilities, just to mention a few of the benefits. These solutions are accomplished with advanced interface techniques, protocols and security; providing safe and simple operation.
The term Cabinet Power Distribution Units (CDUs) is used in this document so that there is no confusion between a large Power Distribution Unit (PDU) that would be installed on the data center floor and a Cabinet Power Distribution Unit (CDU) that is installed in a cabinet and distributes power to all of the devices within that cabinet.
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Current Monitoring Using a Metered CDU |
| Utilizing a CDU that provides the current load or “metered” reading for the overall CDU or in some cases the load on each branch circuit provides valuable information to the user. |
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Figure 1
Current Meter indicating the current load of the CDU |
| Circuit/Cabinet Maximization and Safety Factors |
With a CDU that has a current meter, the operator always knows how close they are to exceeding the overall current rating. This allows the cabinet to be operated with maximum efficiency and ensures that the data center is best utilizing their assets, as well as expensive cabinet space. Without current monitoring, the operator has no idea how much current is being drawn and therefore how many more devices can be added to a particular CDU or cabinet. They also have no idea how close they are to exceeding the current capacity of the circuit or providing any safety factor within their installation. The standard safety factor for de-rating power capacity is typically 80%.
Also, in many Co-Location facilities, the user is billed per each power whip or drop no matter how much of that power they are actually using. Without current monitoring, the user might be paying for power that they are not using or do not need.
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| True RMS Current Monitoring |
Server Technology Inc, utilizes “True RMS” measuring when monitoring and reporting the current draw within our Sentry™ CDUs. True RMS current measuring is the best and most accurate way to measure dynamic waveforms. True RMS measuring must be used for non-sinusoidal (distored) current waveforms otherwise an overload condition may not be detected until a problem occurs.
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| Three Phase Load Balancing |
Current meters also allow for a three phase power load to be balanced, which is required for proper operation of three phase circuits. Load balancing will make the most efficient use of 3-phase power and will reduce the neutral current to zero, reducing heating and other unwanted effects from unbalanced circuits. Using three separate current meters rather than just one which requires the user to press a button to cycle through the readings makes this a simple process without any confusion and provides an at-a-glance viewing if there is a problem.
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Branch Circuit Protection |
Over current protection is driven by the safety standard for Information Technology UL60950-1, Clause 2.7 which states that “standard supply outlets and receptacles shall be protected by an overcurrent protective device in either the equipment or the branch circuit, rated not more than the
outlet or receptacle. Branch circuit protection is required on all 30A rated CDUs. The overcurrent protective device shall be of a type that is suitable for branch circuit protection in accordance with the National Electrical Code (NEC) ANSI/NFPA 70…” Branch circuit protection ensures that if there is a short circuit or other overcurrent condition that the only outlets that will be lost are the ones associated with the particular branch circuit that had problems.
There are several ways to provide branch circuit protection within a CDU, but by far the most common ways are either with fuses or circuit breakers. Server Technology has chosen to implement fuses for over current protection within our CDUs due to their simplicity, operational benefits and superior protection of which some are discussed below.
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| Fuses vs Circuit Breakers for Branch Circuit Protection |
| SELECTIVE COORDINATION |
| Selective coordination is the act of isolating a faulted circuit from the remainder of the electrical system, while maintaining uninterrupted power to the unaffected circuits. The faulted circuit is isolated by the selective operation of only that Over Current Protection Device (OCPD) closest to the over current condition. Fuses open the circuit when they ‘see’ a specific level of current passing through the fuse. Lower amperage fuses have a narrow range of operation and do not overlap from one amperage rating to another. Thus, fuses are easy to coordinate. Circuit breakers require a coordination study to ensure selective coordination. Inherent overlap of circuit breaker trip curves between the upstream and downstream devices often results in simultaneous operation of both breakers. This will clear the fault condition and open the circuit, but it will also remove power to all of the adjacent and/or upstream loads being served by the CDU. Proper selective coordination eliminates unnecessary power outages and reduces costly downtime. Figure 2 demonstrates selective coordination and how the upstream devices can be affected if it is not achieved. |
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Figure 2 |
Without Selective Coordination
(using circuit breakers) |
With Selective Coordination
(using fused protection) |
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| COMPONENT PROTECTION |
According to NEC 110.10, overcurrent protection devices shall be selected to permit the OCPD to clear a fault without damage to the electrical components of the circuit. Fuse operation is based on a simple thermal principle; the internal fuse element will rapidly melt/vaporize at a very specific level of energy. This amount of energy is well below the total amount of energy potential available during a specific fault. The resultant clearing time and the subsequent peak let-through current is significantly reduced which results in less energy than a downstream component is required to withstand. Per UL248 testing, fuses are required to meet maximum allowable energy let through values, which allows for very accurate fuse sizing.
Thermal magnetic circuit breakers are not current limiting. They do not interrupt short circuit currents in less than a ½ cycle, and typically require a full cycle to clear a fault condition. This means that the full peak current and energy of the first cycle of the fault will be let-through. Per UL489 testing, standard thermal magnetic circuit breakers are not tested to limit the maximum amount of energy let-through to downstream components.
By reducing the amount of energy which passes through to the protected device, you decrease the damage, which reduces repair and downtime. In order to successfully protect sensitive equipment, the upstream overcurrent protective device needs to be able to operate in a very short amount of time, and consistently limit the amount of peak current/energy which passes through to the downstream devices.
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| MAINTENANCE |
Proper maintenance of over current protection devices, as specified by the device manufacturer, is critical to effectively and consistently operate within their manufacturing specifications in the event of an overcurrent condition. Fuses do not require maintenance. Molded case circuit breakers require periodic inspection and manual operation as part of their prescribed maintenance procedures. Failure to manually exercise the mechanism can cause the internal lubricants to thicken, and cause the breaker to open slower than specified. Most manufacturers, as well as NFPA 70B, recommend that if a molded-case circuit breaker has not been operated, opened or closed, within six months time, it should be removed from service and manually exercised. Because of the highly engineered yet simple design, fuses ship from the factory calibrated to a very specific set of operating parameters. This ensures that the fuse will operate as specified without maintenance and upkeep concerns.
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| INTERRUPTING RATING |
According to NEC 110.9 “Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.” Failure to comply can result in catastrophic failure of the overcurrent protective device, which will require replacement of the entire CDU, and an immediate loss of power. Worst case examples could result in a fire and/or explosion. All modern fuses employ a simple and reliable method of current limiting and are able to easily achieve interrupting ratings of 100,000 amps or higher. Standard UL489 Circuit Breakers typically tested to safely interrupt much lower levels of fault current, and are not inherently current limiting.
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| PHYSICAL ATTRIBUTES |
The fuses utilized in Server Technology’s CDUs have a very specific physical footprint and rejection style fuse holder that prevents the wrong fuse from being installed. This prevents unqualified personnel from replacing the blown device with a different device that may not provide the correct level of protection. CDU suppliers such as Server Technology utilize a Class G fuse, UL specifications file #E42730, which provides a very high degree of current limitation. After a fault occurs, fuses are replaced assuring the same level of protection that existed previous to the fault. This ensures a high level of protection and reliability, without concern for maintenance and potential mechanical damage inherent to re-settable OCPD’s.
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| RESETTABILITY |
There are several misconceptions concerning the suitability for using re-settable devices for reliable overcurrent protection. Per OSHA 1910.334(b)(2), after a circuit has been de-energized by the operation of a circuit protective device, the circuit may not be reenergized until it has been determined that the circuit can be safely energized. A qualified person is required to determine the cause of the overcurrent condition, and in the event of a short circuit, fix the problem prior to reenergizing the circuit. Circuit breakers that have interrupted a fault approaching their listed ratings shall be inspected and tested to the manufacturer’s instructions, according to NFPA70E 225.3. After a circuit breaker safely interrupts one short circuit fault, the breaker needs to be evaluated to determine if it can safely be put back into service, and it may need to be tested in order to determine if it will safely interrupt a short circuit in the required amount of time. This testing can involve taking the CDU out of service and taking the breaker out of the CDU. In some cases the breaker may need to be discarded and replaced.
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Redundancy with A In-Feed and B In-Feed Input Circuits |
For Mission Critical Applications, or for providing redundancy to single power cord devices two CDUs should be provided or a Fail Safe Transfer Switch (FSTS) is used. Many servers and other devices today come with multiple power input feeds. These provide both redundant power to the server, and the ability to compensate should one of the power supplies within the server fail. These safety measures are designed to ensure reliability and proper up-time. Server Technology’s CDUs have dual power in-feeds either for power redundancy, automatic fail over if one source fails or to meet the high power demands of today’s environments.
For devices that come with a single power cord, a Sentry Fail Safe Transfer Switch provides dual power in-feeds from separate power sources (A in-feed and a B in-feed). Should one of the in-feed sources fail half of the load is transferred to the remaining power in-feed without interruption to the connected devices (see figure 4). This is a result of STI’s patent pending power in-feed sharing, where each in-feed supports half of the loads, ensuring reduced wear on the FSTS as the loads being switched are much smaller, versus switching the entire load as some competitive products do. Another unique feature of the FSTS is our patented arc suppression technology which uses a combination of solid state and electromechanical relays to ensure high current transfer capability along with increased isolation between the in-feed sources. This results in less wear and longer life of the relay contacts as arcing is prevented which is due to large dv/dt voltages that can occur with in-feeds that are not phased synchronized.
Dual input CDUs or multiple CDUs within a cabinet with an A feed and B feed power source are another way to provide redundancy within a cabinet, though they don’t provide the FSTS feature of switching the load upon a power loss. |
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Figure 3
FSTS Arc Suppression Technology |
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The Technology Behind Smart AND Switched CDUS |
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