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Planning for Rack-Mounted Automatic Transfer Switch Installation

When setting out to plan for installing rack-mounted transfer switches the following is typically considered:

A) What is the rating? Typically 30A, 40A, or 50A for rack-mount static switches and 20A or 30A for rack-mount automatic transfer switches (as relay types are referred to in the market). The higher the rating the better as in most designs the enclosure sizes and internal elements of power supplies and circuit boards are the same, therefore you would get more protection per dollar when using the bigger units (“bigger” doesn't mean larger size, just more amperes protection). Be sure the rating you are looking at is a 100% continuous rating. Most published ratings are later said to be 80% derated so if you select a 30A unit for example, you will get a 30A unit but on the back plate it will say “Max Load: 24A”. All TwinSource ratings are 100% with no derating. You can run our 30A unit at 30A forever. Also consider two more things: 1) Allow a bit of room to grow in case another unplanned small load is added and 2) Allow some room to have the unit run cool. The closer you get to nameplate rating the warmer the unit runs.

Speaking of ratings, how would you feed your rack level transfer switch? Consider that all distribution circuit breakers (CBs) are 80% rated at best. Therefore if you need a 30A switch and you have 30As worth of loads then A) Get a 100% rated switch and B) Feed it with a 40A CB so 40x.8 = 32A is made available to the switch so it can deliver 30A. Many make the mistake of getting a 30A switch and feed it with a 30A CB and can only get 24As out of it. Plan ahead. With the watts/sq ft numbers going up, if you need 50A, then get a 50A RMSTS and feed it with a 60A CB so 60 x .8= 48A.

B) Power Factor - When comparing relay type rack-mounted transfer switch ratings be sure to investigate their true rating at .85 to 1.0 pf and not just at 1.0 pf. Relays have a severe drop in rating as power factor drops, by as much as 50% in some cases. Most relay switches are only rated at “resistive load” or at 1.0 pf. Check what the pf of your computer load is and see what the device can do at that pf.

C) Define how critical the mission is - Compare the cost of each outage to the cost of the protection device. You would not want to save a few thousand dollars if the outage can cost you hundreds if not millions of dollars. If MTBF is to be higher than 1M hours, then a static rack-mounted transfer switch is the only option as relays cannot deliver nearly as much reliability as SCRs due to their low overload handling capability, their internal moving parts, arcing contact issues, out of phase transfer issues, power factor derating, coil overheating and so on. In a well designed data center you may actually use both technologies: Relay types on the less critical low-end loads and static ones on the real mission critical high-end loads. If an application is budget driven and reliability is not as critical due to the use of redundant servers, then a relay type transfer switch would be a good solution.

D) MTBF - Do not let MTBF number mislead you. When you see a transfer switch has a published MTBF of 100,000 hours and you are looking to install 20 of them then consider that 20 x 8760 run time hours/yr = 175,200 and this divided by 100k hrs is 1.75 failures per year. If this is not acceptable then select a higher MTBF product.

E) Require a Bypass - Every mission critical power device in your data center has a means of bypassing it for either routine maintenance or complete replacement or repairs without having you compromise your operation by dropping the load off line. Your UPSs most certainly have maintenance bypasses and all your ATSs and STSs should have bypasses as well. Even your major switchboards have more than one way to feed them to isolate sections of them for repair access. This same philosophy should be continued with rack-mounted transfer switches (static or relay). This makes the response to any alarms non-disruptive and the fix can be performed quickly in minutes. In rack-mount applications, most static versions already come with a built-in bypass; however, most all relay types do not (but will in the future as an option).

F) Compare reliability features and technologies - If the purchase is for a large mission critical site, by all means do some testing to verify the advertised performance. This takes very little time and is a highly recommended. Above all never trust data sheets alone. They contain the data under the most ideal circumstances often removed from realities of an operational site and they also don't reveal anything that may not look good. Example - the transfer devices are often conspicuously not indicated. Transfer time is skipped. There is no indication as to how you would change preferred source. Ask how many of the transfer relay coils remain energized and if any do how does that affect the relay life. Your sales contact won't know the answer and probably won't even know if there is a relay involved. Fault rating is not indicated. Ask if the unit has protective fuses and if so where are they? Would you have to shut down to replace them? There is no indication how performance may change for out of phase conditions (which means if the data looks real good, then it is assumed your sources will always be in phase which is impractical because a failing UPS is probably not able to synchronize either). The data is often not for the full range of .8 to 1.0 pf (and it is most likely assumed that your power factor is 1.0). Contact us for comparison charts that you can fill in based on your own observations when comparing products and technologies. We would have the TwinSource side already filled in for you.

G) Fault Protection - Be sure to know where the fault protection devices are and if they can be replaced with ease. Some designs have built-in fuses and some rely on upstream breakers for protection. If fuses are provided then they should be readily available on the outside of the unit enclosure so they can be replaced with ease quickly. Designs that incorporate fuses deep inside the electronic module power section that can only be replaced at a factory should be avoided. You should not have to send the electronic module of a system back to the factory just because of a small short that blew a fuse. The fuse should be replaceable in 1-2 minutes so you can go back on line and not be without a switch while the insides of the unit is at the factory far away for repairs. On units that rely on an upstream breaker be sure to get a list of breakers approved to power the device. This information is normally found in the operator's manual of the product.

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