Comparing Availability of Various Rack Power Redundancy Configurations

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1 Compaing Availability of Vaious Rack Powe Redundancy Configuations White Pape 48 Revision by Victo Avela > Executive summay Tansfe switches and dual-path powe distibution to IT equipment ae used to enhance the availability of computing systems. Statistical availability analysis techniques suggest lage diffeences in availability ae expected between the vaious methods commonly employed. This pape examines vaious electical achitectues fo edundancy that ae implemented in today s mission-citical envionments. The availability analyses of these vaious scenaios ae then pefomed and the esults ae pesented. The analysis identifies which appoach povides the best oveall pefomance, and how altenatives compae in pefomance and value. Contents Click on a section to jump to it Intoduction 2 Appoaches to distibuting powe to acks Availability analysis appoach 8 Results 9 Conclusion 2 Resouces 3 Appendix 4 2 white papes ae now pat of the Schneide Electic white pape libay poduced by Schneide Electic s Data Cente Science Cente

2 Intoduction Equipment with edundant powe supplies is also efeed to as dual-coded equipment, having edundant powe supplies, each with its own cod. The use of dual-coded equipment is a best pactice that helps maintain optimal powe availability fo the IT equipment and povides the necessay edundancy to pevent downtime fom a single failue within the powe distibution system. This added edundancy also facilitates powe system maintenance. Unfotunately, the majoity of today s mission citical envionments do not fully benefit fom this best pactice. This pape pesents vaious electical achitectue scenaios that may be implemented in today s data cente. The availability analyses of these vaious scenaios ae then pefomed and the esults ae pesented. Appoaches to distibuting powe to acks The following illustations povide an oveview of vaious appoaches fo inceasing availability to ack-mounted equipment but can also be applied to stand-alone equipment as well. The diffeent appoaches ae typically selected with the objective of achieving a desied level of availability, with the moe costly appoaches pesumably poviding a highe level of availability. Figues and 2 show how powe is often distibuted within a data cente ack today. Monito Figue (left) Typical ack-mount powe Figue 2 (ight) Typical centalized powe Monito Keyboad Seve Seve Keyboad Seve Seve P o w e S t i p Stoage Stoage Rackmount UPS Powe fom lage 3 phase UPS Figue shows a typical ack powe distibution configuation used in small o medium size data centes and wiing closets. This configuation allows fo easily moved acks with intenal UPS battey backup and suge potection. In data centes whee dozens o hundeds of acks ae used, Figue 2 with a lage centalized uninteuptible powe supply (UPS) is a moe common configuation. Thee is no powe edundancy in the powe distibution to the ack in eithe case. Othe electical achitectues use devices to switch fom a pimay powe souce to a seconday powe souce. Two such devices ae a static tansfe switch (STS) and an automatic tansfe switch (ATS). Both of these units ange in size fom about kw to ove Schneide Electic Data Cente Science Cente White Pape 48 Rev 2

3 Link to esouce White Pape 2 Poweing Single Coded Equipment in a Dual Path Envionment MW. These devices ae discussed in detail in White Pape 2: Poweing Single Coded Equipment in a Dual Path Envionment. Examples of both of these switches ae shown in Figue 3. Rack-mount 3-phase kva ATS Figue 3 Example of an automatic tansfe switch (ATS) and a static tansfe switch (STS) 3-phase 300 kva STS Figues 4 and 5 demonstate how powe is sometimes distibuted in lage, mission citical facilities. In both cases thee ae two edundant paths leading to an STS, howeve the utility souces feeding the UPS may o may not be edundant, depending on factos such as cost and substation availability fom the utility company. The only diffeence between the two scenaios is that Figue 4 uses a single tansfome downsteam of the static switch while Figue 5 uses edundant tansfomes upsteam of the static switch. Howeve, in both cases, the STS, downsteam subpanel, and associated wiing ae potential single points of failue. These methods povide some edundancy, but the emaining components that have no edundancy pesent failue isk hazads and potential maintenance difficulty. Schneide Electic Data Cente Science Cente White Pape 48 Rev 3

4 Figue 4 Redundancy to the load with STS Pimay powe path UPS UPS 2 Static Tansfe Switch PDU with STS Step Down Tansfome P o w e S t i p Monito Keyboad Seve Seve Stoage Backup powe path Figue 5 Redundancy to the load with STS (edundant tansfomes) Pimay powe path UPS UPS 2 Tansfome Tansfome 2 PDU with STS Static Tansfe Switch P o w e S t i p Monito Keyboad Seve Seve Stoage Backup powe path Figues 4 and 5 ae an impovement ove the data cente configuations shown in Figues and 2, but they still do not offe full edundancy to the ack. Although a edundant UPS and tansfome ae added, the static switch, subpanel and thei associated wiing ae single points of failue. Figue addesses the single points of failue limitation found in Figues 4 and 5 by pushing edundancy towads the load. This solution emoves the STS and adds an exta subpanel theeby pushing the edundancy benefits close to the load by means of a ack automatic tansfe switch (ATS). Any maintenance upsteam of the ack ATS can now be completed without taking down the load. Although this scenaio exhibits fewe non-edundant components than that of Figues 4 and 5, the ack ATS emains a single point of failue, as does the equipment s own powe supply. Schneide Electic Data Cente Science Cente White Pape 48 Rev 4

5 Monito P o w e Keyboad Seve Figue Redundancy to the load with ack ATS Pimay powe path PDU S t i p Seve Stoage Rackmount Tansfe Switch UPS Tansfome Backup powe path UPS 2 Tansfome 2 PDU 2 Figue 7 shows how full edundancy to the load can be achieved using dual-coded equipment with edundant powe supplies. This scenaio has two impotant changes to Figue : the ack ATS is taken out, and dual-coded equipment is used. Full edundancy is now bought staight though to the load. Notice also that an exta powe stip is used to maintain edundancy. This solution is highly available compaed to those discussed thus fa; howeve, it is also the most expensive solution and can only be used with dual coded equipment expessly designed fo this use. Schneide Electic Data Cente Science Cente White Pape 48 Rev 5

6 Figue 7 Redundancy to the load with dual-coded equipment P o w e S t i p Monito Keyboad Seve Seve Stoage P o w e S t i p 2 Pimay powe path UPS Tansfome PDU Backup powe path UPS 2 Tansfome 2 PDU 2 The achitectue in Figue 8 combines the achitectues of Figues and 7, and shows an altenate solution that accommodates both single and dual-coded loads. This solution employs a hybid combination of peviously eviewed designs. Full powe edundancy is maintained fo the dual-coded compute equipment. Fo the single-coded equipment, edundancy is maintained up to the ack ATS, howeve, the switch and equipment powe supplies ae now single points of failue. Figue 8 also shows added physical sepaation. This is often efeed to as "compatmentalization," whee vaious subsystems within the powe distibution and backup system ae physically sepaated. Physical sepaation, if implemented popely, can pevent an event as seious as a mechanical collapse in one path fom affecting the second path (common cause failue). Schneide Electic Data Cente Science Cente White Pape 48 Rev

7 Equipment with 2 powe cods P o w e S t i p Monito Keyboad Seve Seve Stoage P o w e S t i p 2 Souce UPS Tansfome Figue 8 Physical Sepaation Redundant achitectue fo single and dualcoded loads Souce 2 UPS 2 Tansfome 2 2 Equipment with powe cod P o w e S t i p Monito Keyboad Seve Seve Stoage Rackmount Tansfe Switch The achitectues discussed in Figues 4, 5, and 8 incopoate tansfe switches. With a lage tansfe switch, one failue can bing down an extemely lage potion of equipment, wheeas failue in a smalle switch will bing down only one ack. Fo some uses, a failue of any one ack has equivalent business consequences as the failue of 50 acks; while fo othes the isolation of a failue to a single ack is an advantage. Fo uses of the latte type, the ack ATS povides an added availability advantage of fault isolation. Anothe facto to conside is the time needed to epai these switches. A small tansfe switch will not be epaied but eplaced, and can be kept as a spae pat fo a vey quick swap out. In addition, it can be quickly bypassed if needed. A lage switch will need to be epaied and depending on location, will take a few hous to get a epai peson on site. Additional time will be needed to diagnose and epai the system, and if the technician does not have the equied pat, even moe time is lost. Thus, when evaluating some of these Schneide Electic Data Cente Science Cente White Pape 48 Rev 7

8 moe advanced designs, a vaiety of issues should be evaluated to make an optimal decision. Repai time is consideed in the statistical availability model descibed in the next section. In geneal, equipment with only one powe cod can be a significant liability when tying to develop a high availability mission citical envionment. This is tue not only fo ack-mounted equipment but also fo any mission-citical equipment. Even with the best possible constuction, any single point of failue will fail eventually and esult in downtime. If a tue high availability envionment is equied, single points of failue in the powe distibution must be minimized as much as possible, if not emoved completely. Availability analysis appoach An availability analysis is done in ode to quantify the impact of having single vs. dual-coded devices. Five availability analyses ae pefomed: Case Single-coded load fom Figue 2 Case 2 Single-coded load with static tansfe switch fom Figue 4 (single tansfome) Case 3 Single-coded load with static tansfe switch fom Figue 5 (edundant tansfomes) Case 4 Single-coded load with ack ATS fom Figue Case 5 Dual-coded load fom Figue 7 Linea combinatoial analysis, also efeed to as eliability block diagams (RBD), is used to illustate the powe availability at the outlet fo these five configuations. This method of system modeling is the most diect, and woks well fo systems whee thee ae few state tansitions. Linea combinatoial analysis woks by using defined eliability data and then developing a system model that epesents the configuation being analyzed. Because this analysis focuses only on the diffeences between the configuations, it is assumed that eveything upsteam of the UPS system is pefect including utility powe. Theefoe the availabilities pesented hee will be highe then what is expected in an actual installation. The details of the analysis ae povided in the Appendix. Data used in the analysis Most of the data used to model the components is fom thid paty souces. Data fo the ack ATS is based on field data fo Schneide Electic s ack ATS poduct, which has been on the maket fo appoximately 5 yeas and has a significant installed base. In this analysis the following key components ae included:. 2. beakes 3. UPS systems 4. PDU 5. Static tansfe switch (STS). Rack ATS The PDU is boken down into thee basic subcomponents: cicuit beakes, step-down tansfome and teminations. The subpanel is evaluated based on one main beake, one banch cicuit beake and teminations all in seies. The ack ATS component is used in the fouth case only. The Appendix includes the values and souces of failue ate Schneide Electic Data Cente Science Cente White Pape 48 Rev 8

9 MTTF and ecovey ate MTTR mean time to failue and MTTR is mean time to ecove. data fo each subcomponent, whee MTTF is the The failue ates and epai ates used fo the analysis ae povided in the Appendix. Assumptions used in the analysis As with any availability analysis, assumptions must be made to ceate a valid model. Table lists the basic assumptions used in this analysis. Table Assumptions of analysis Assumption Failue ates of components Repai teams Desciption All components in the analysis exhibit a constant failue ate. This is the best assumption, given that the equipment will be used only fo its designed useful life peiod. If poducts wee used beyond thei useful life, then non-lineaity would need to be built into the failue ate. Fo n components in seies it is assumed that n epaipesons ae available. System components emain opeating All components within the system ae assumed to emain opeating while failed components ae epaied. Independence of failues Failue ate of wiing Human eo These models assume constuction of the descibed achitectues in accodance with Industy Best Pactices. These esult in a vey low likelihood of common cause failues and popagation because of physical and electical isolation. Wiing between the components within the achitectues has not been included in the calculations because wiing has a failue ate too low to pedict with cetainty and statistical elevance. Also pevious wok has shown that such a low failue ate minimally affects the oveall availability. Majo teminations have still been accounted fo. Downtime due to human eo has not been accounted fo in this analysis. Although this is a significant cause of data cente downtime, the focus of these models is to compae powe infastuctue achitectues, and to identify physical weaknesses within those achitectues. In addition, thee exists a lack of data elating to how human eo affects the availability. Powe availability is the key measue This analysis povides infomation elated to powe availability. The availability of the business pocess will typically be lowe because the etun of powe does not immediately esult in the etun of business availability. The IT systems typically have a estat time which adds unavailability that is not counted in this analysis No benefit of fault isolation The failue of any ack is consideed a failue and equivalent to the failue of all acks at once. This assumption undestates the advantage of the Cases 4 and 5. Fo some businesses, the failue of a single ack is of less business consequence than the failue of all acks. Results It is impotant to undestand that the objective of this analysis is to compae the theoetical availabilities between cases. Since all components in all five cases shae the same failue ate data the only diffeences between each case ae the quantity, MTTR, and placement of Schneide Electic Data Cente Science Cente White Pape 48 Rev 9

10 the components. This method povides a vey effective demonstation of availability effectiveness of one achitectue when compaed with anothe. Availability is measued with espect to the outlet(s) supplying powe to the citical load. In evey case, the same component eliability data is used. In case, the failue of any one component in that chain would cause the load to dop. This is a baseline case. In both cases 2 and 3, any one component fom each edundant path would have to fail simultaneously fo a dopped load to occu. Howeve the failue of any single component downsteam of the STS, including the STS, would also dop the load. The emakable esult in this case is how little the installation of the STS inceases system availability. The eason is that the STS is not significantly moe eliable than the upsteam UPS, and the STS is still a single point of failue. Note futhe that in case 2 the tansfome MTTR minimizes any benefit of the STS. In case 4, any one component fom each edundant path would have to fail simultaneously fo a dopped load to occu. Despite being a single point of failue, the MTTR of the ack ATS is small due to the fact that it can quickly be eplaced if a spae is available. The key finding hee is that although the ack ATS is not necessaily moe eliable than the lage STS, the much lowe MTTR gives it a vey lage availability advantage. Table 2 Summay of availability esults In case 5, any one component fom each edundant path would have to fail simultaneously fo a dopped load to occu. Table 2 is an oveview of the esults of the five availability calculations. Case Configuation Availability Numbe of 9 s Case Single-coded load % 3.8 Case 2 Single-coded load with STS (single tansfome) % 3.85 Case 3 Single-coded load with STS (edundant tansfomes) % 4.5 Case 4 Single-coded load with ack ATS %.2 Case 5 Dual-coded load % 7. This analysis illustates the significance of dual-coded equipment in attaining high availability within a double-ended electical achitectue. The benefits of such an elaboate design ae not fully ealized with single-coded equipment but come close by implementing a ack ATS. Fom the esults pesented above it is quite clea that binging edundancy to the load impoves availability. Figue 9 demonstates that even if the eliability (MTTF) of a poduct is inceased 0 fold, it still does not povide the same availability as does using a edundant set at a lowe eliability level. The edundant system is poviding nea 00% availability, o a lage numbe of 9 s. Schneide Electic Data Cente Science Cente White Pape 48 Rev 0

11 Figue 9 Availability vs. MTTF Oveall System Availability Stand Alone System Redundant System ,000 07,000 4,000 22, , , , ,000 50,000 53,000 20,000 77, ,000 79,000 MTTF (hous) Schneide Electic Data Cente Science Cente White Pape 48 Rev

12 Conclusion When implementing a high availability achitectue, powe distibution to the ack needs to be caefully consideed. The typical types of powe distibution descibed in this pape vay by a facto of 0,000 in the magnitude of the downtime they ceate. This analysis demonstates vey clealy the impotance of using dual-coded equipment in a citical data cente. The analysis pesented hee suggests a full dual path achitectue can povide up to 0,000 times less down time than a single path design. The common pactice of using tansfe switches to incease the availability of single coded loads povides highly vaiable esults depending on how it is implemented. In some cases, the analysis suggests almost no advantage fom the use of a lage STS. By contast, when the tansfe switch is moved to the ack, the system downtime caused by the powe distibution system deceases by a facto of 250. In addition, the ack based tansfe switch povides additional fault localization since a failue takes out only a single ack. Futhemoe, the ack based tansfe switch can be deployed as needed and whee needed in a dual path envionment. This data suggests that the common pactice of using lage STS systems to powe single coded loads should be e-evaluated, and that ack-based tansfe switches have significant advantages fo appoximately simila costs. In geneal, the analysis suggests a geneal pinciple of binging edundancy close to the loads to impove availability. Caeful analysis should always be a peequisite to investing in any high availability system. How much money a custome can justify spending to einfoce thei electical infastuctue detemines which solution to select. A custome must have a clea undestanding of thei business pocesses so that the cost of downtime can be calculated. This cost is what should ultimately dive any investments in availability. About the autho Victo Avela is a Senio Reseach Analyst at Schneide Electic. He is esponsible fo data cente design and opeations eseach, and consults with clients on isk assessment and design pactices to optimize the availability and efficiency of thei data cente envionments. Victo holds a Bachelo s degee in Mechanical Engineeing fom Rensselae Polytechnic Institute and an MBA fom Babson College. He is a membe of AFCOM and the Ameican Society fo Quality. Schneide Electic Data Cente Science Cente White Pape 48 Rev 2

13 Resouces Click on icon to link to esouce Poweing Single Coded Equipment in a Dual Path Envionment White Pape 2 White Pape Libay whitepapes.apc.com TadeOff Tools tools.apc.com Contact us Fo feedback and comments about the content of this white pape: Data Cente Science Cente If you ae a custome and have questions specific to you data cente poject: Contact you Schneide Electic epesentative Schneide Electic Data Cente Science Cente White Pape 48 Rev 3

14 Appendix Table A Components and values Component Failue ate Recovey ate Souce of data Comments UPS 75 kw / 750 kva E Failue Rate is fom Powe Quality Magazine, Recovey Rate data is based on assumption of 4 hous fo sevice peson to aive, and 4 hous to epai system Used to supply uninteupted 480 VAC powe to the PDU. Static tansfe switch (STS) 4.00E Godon Associates - Raleigh, NC Includes contols Step-down tansfome 7.077E beake E E E UPS 75 kw / 750 kva E MTBF is fom IEEE Gold Book Std , Page 40, MTTR is aveage given by Macus Tansfome Data IEEE Gold Book Std , Page 40 x IEEE value Computed fom value by IEEE Gold Book Std , Page 4 8 x IEEE value Computed fom value by IEEE Gold Book Std , Page 4 Failue Rate is fom Powe Quality Magazine, Recovey Rate data is based on assumption of 4 hous fo sevice peson to aive, and 4 hous to epai system Used to step down the 480 VAC input to 208 VAC outputs, which is equied fo 20 VAC loads. Used to isolate components fom electical powe fo maintenance o fault containment. Upsteam of the tansfome, one temination exists pe conducto. Since thee ae 2 sets of teminations between components a total of six teminations ae used. Downsteam of the tansfome, one temination exists pe conducto plus the neutal. Since thee ae 2 sets of teminations between components a total of eight teminations ae used. Used to supply uninteupted 480 VAC powe to the PDU. Availability of a single-coded load [Case ] The availability fo a single-coded load, fom Figue 2, is calculated based on the following RBD. Figue 0 epesents the top laye of the RBD, which calculates the steady state availability based on the seies components. This RBD incopoates "expandable" blocks fo the Tansfome Pats" and the " Pats". An expandable block means that thee s a lowe level RBD that defines its sub-components. Laying the RBD out in this manne facilitates availability calculations. The subpanel is used to distibute powe diectly to the citical equipment. The contents of these blocks ae shown in Figues and 2. Schneide Electic Data Cente Science Cente White Pape 48 Rev 4

15 Figue 0 Single-coded load ë =8.988e-008 ë =8.988e-008 ë =4e-00 ì = kW UPS ë =8.988e-008 ë =8.988e-008 Pats Tansfome Pats Figue Tansfome pats ë =8.988e-008 ë =7.077e-007 ì =0.004 Step down Tansfome ë =.598e Figue 2 Sub-panel pats ë =.598e Table A2 Single-coded load availability [Case] Based on the RBD pesented above, the availability of the single-coded system is shown below. Model name Availability Unavailability MTTR (hous) MTTF (hous) Annual downtime (hous) Single-coded load %.502E ,5.358 UPS system % E Tansfome pats %.205E , pats % 2.987E-0 2.4,092, Schneide Electic Data Cente Science Cente White Pape 48 Rev 5

16 Because the analysis is caied out using data with five significant digits, unavailability is anothe way to expess the esults. Unavailability is simply calculated as ( Availability). Availability of a single-coded load with static tansfe switch (single tansfome) [Case 2] Figue 3 Single-coded load with STS The distibution method fom Figue 4 uses an STS and adds edundancy to eveything upsteam of it except fo the tansfome, which is placed downsteam. The availability fo this scenaio is calculated based on 7 RBD stings that ae boken out fo claity. Figue 3 epesents the top laye of the RBD. The "UPS system" block is a out of 2 block, meaning that all components within that block ae edundant. Figue 4 shows the contents of the "UPS system" block. out of 2 UPS system STS & Distibution Figue 4 UPS system ë =8.988e-008 ë =8.988e-008 ë =4e-00 ì =0.25 ë =8.988e kW UPS ë =8.988e-008 Eveything upsteam of the STS is edundant; howeve, eveything inside the STS & Distibution block, shown in Figue 3 is a single point of failue. The STS & Distibution block contains the STS system, tansfome pats and the subpanel pats as illustated in Figue 5. The STS system is what allows the use of the upsteam edundant components. This system incopoates cicuit beakes, teminations and most impotantly, the static tansfe switch. The RBD fo the STS system is shown in Figue. Figue 5 STS & distibution STS system Tansfome Pats Pats Schneide Electic Data Cente Science Cente White Pape 48 Rev

17 Figue STS system ë =4.e-00 ì =0.7 Static Switch ë =8.988e-008 The contents of the Tansfome Pats block and Pats block of Figue 5 ae boken down futhe in Figues 7 and 8 Figue 7 Tansfome pats ë =8.988e-008 ë =7.077e-007 ì =0.004 Step down Tansfome ë =.598e Figue 8 Sub-panel pats ë =.598e Table A3 Single-coded load with STS availability (single tansfome) [Case 2] Based on the RBD diagams pesented above, the availability of the single-coded system with STS and single tansfome is shown in below. Model name Availability Unavailability Singled-coded load with STS ( Tansfome) MTTR (hous) MTTF (hous) Annual downtime (hous) %.404E , UPS system %.2930E ,025,25, Single UPS % E , STS & distibution %.404E , STS system % 2.4E , Tansfome pats %.205E , pats % 2.987E-0 2.4,092, Schneide Electic Data Cente Science Cente White Pape 48 Rev 7

18 Availability of a single-coded load with static tansfe switch (edundant tansfomes) [Case 3] The distibution method fom Figue 5 uses an STS and adds edundancy to eveything upsteam of it including the tansfome. The availability fo this scenaio is calculated based on 7 RBD stings simila to the pevious analysis. Figue 9 epesents the top laye of the RBD. The "UPS system & Tansfome" block is a out of 2 block, meaning that all components within that block ae edundant. Figue 20 shows the contents of the "UPS system & Tansfome" block. The Tansfome Pats block is composed of the same pats as those of Figue 7. Up to this point evey component has been edundant; howeve, eveything inside the STS & Distibution block, shown in Figue 9, is a single point of failue. Figue 9 Single-coded load with STS out of 2 UPS system & Tansfome STS & Distibution Figue 20 UPS system & tansfome ë =8.988e-008 ë =8.988e-008 ë =4e-00 ì = kW UPS ë =8.988e-008 ë =8.988e-008 Tansfome Pats In this case the contents of the STS & Distibution block, Figue 2, contains only the STS system and the subpanel pats because the tansfome is pushed upsteam as a edundant component. The STS system in this scenaio is identical to that of Figue 7 except that thee ae 8 teminations athe then as illustated in Figue 22. The components of the Pats block ae identical to those of Figue 8. Figue 2 STS & distibution STS system Pats Figue 22 STS system ë =4.e-00 ì =0.7 Static Switch ë =.598e Schneide Electic Data Cente Science Cente White Pape 48 Rev 8

19 Table A4 Single-coded load with STS availability (edundant tansfomes) [Case 3] Based on the RBD diagams pesented above, the availability of the single-coded system with STS and edundant tansfomes is shown below. Model name Availability Unavailability Singled-coded load with STS (2 tansfomes) MTTR (hous) MTTF (hous) Annual downtime (hous) % E , UPS system & tansfome % 2.90E ,22, UPS system % E , Tansfome pats %.205E , STS & distibution % E , STS system % 2.274E , pats % 2.987E-0 2.4,092, Availability of a single-coded load with ack ATS [Case 4] The analysis fo a single-coded load with a ack ATS, fom Figue, is calculated based on the RBD in Figue 23, which epesents the top laye of the RBD. This model now povides edundancy to the ack howeve the ack ATS becomes the single point of failue. Figue 24 shows the components of the UPS system & Distibution block. The contents of the Tansfome Pats and Pats blocks ae identical to those of Figues 7 and 8 espectively. Figue 23 Single-cod loaded with ack ATS Figue 24 UPS system & distibution out of 2 UPS system & Distibution ë =2e-00 ì =3 POU Switch ë =8.988e-008 ë =8.988e-008 ë =4e-00 ì = kW UPS ë =8.988e-008 ë =8.988e-008 Pats Tansfome Pats Schneide Electic Data Cente Science Cente White Pape 48 Rev 9

20 Based on these RBD, the availability of the single-coded system with ack ATS is shown below. Table A5 Single-coded load with ack ATS availability [Case 4] Model name Availability Unavailability Single-coded load with ack ATS MTTR (hous) MTTF (hous) Annual downtime (hous) % E , UPS system & distibution % 2.252E ,898, Tansfome pats %.205E , pats % 2.987E-0 2.4,092, Rack ATS % E , In this case, by simply adding anothe PDU, the availability has dastically impoved. Howeve, the ack ATS is the single point of failue in this system, limiting the oveall availability to six "9 s". Because of this, a ack ATS should always be chosen based on its eliability and spaes should always be kept on site to minimize MTTR. Availability of a dual-coded load [Case 5] The analysis fo a dual-coded load, fom Figue 7, is calculated based on the RBD in Figue 25, which again epesents the top laye. Like the system with ack ATS, this RBD calculates the steady state availability based on the oveall UPS and PDU failue and ecovey ates, howeve, it does not include the ack ATS because the load is dual coded and can fully utilize the edundant paths. Only of the 2 paths must be opeational to maintain the citical loads. Thee ae no single points of failue in this system. As a matte of fact, even the citical load powe supplies ae edundant. Figue 25 Dual-coded load out of 2 UPS system & Distibution The lowe level RBD that the "UPS system & Distibution" block is composed of ae identical to those in Figues 0 2. Based on these blocks, the availability of the dual-coded system is shown below. Schneide Electic Data Cente Science Cente White Pape 48 Rev 20

21 Table A Dual-coded load availability [Case 5] Model name Availability Unavailability MTTR (hous) MTTF (hous) Annual downtime (hous) Dual-coded load % 2.252E ,898, UPS system & distibution % 2.252E ,898, Tansfome pats %.205E , pats % 2.987E-0 2.4,092, In this final case, the UPS system & Distibution availability is identical to the pevious case yet oveall availability has inceased to seven 9 s. The majo diffeence is that the ack ATS is no longe needed when using dual-coded equipment. As is shown in the last system, the ack ATS is a single point of failue and limited the availability to six 9 s. Schneide Electic Data Cente Science Cente White Pape 48 Rev 2

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