Meeting electrical infrastructure demands in data centers

05/16/2013


Electrical equipment selection

Now that base and maintenance design requirements have been fulfilled, a data center’s electrical equipment selection will take center stage. Circuit breakers are exclusively used in data centers (except for the occasional use of medium-voltage fuses with utility switchgear at the outside the building) for their ability to reduce MTTR and aid concurrent maintainability, and relative ease of achieving selective overcurrent coordination.

Circuit breakers can be mounted in one of two ways: stationary-mounted where the breaker is bolted to the bus, or drawout-mounted where it’s connected to the bus through a finger mechanism that makes it easy to turn a crank or lever and withdraw the circuit breaker. When drawout-mounted, the circuit breaker can actually reduce MTTR and help concurrent maintainability, while all fusible switchgear is stationary mounted and therefore takes longer to replace than a drawout-mounted circuit breaker.

UL 1558 switchgear is often specified instead of UL 891 switchboards in data centers. A switchboard is rated for fault current to last no more than three cycles, which is equivalent to 0.05 sec or a bit more than 50 millisec. Switchgear, on the other hand, is rated to carry fault current for 30 cycles, or 0.5 sec. While stronger and more robust, switchgear also carries a higher price tag and often a larger space requirement. Selection of switchboards or switchgear becomes critical when performing selective overcurrent coordination.

Two techniques employed are zone selective interlocking and layering of short time breaker trips. Regardless of the technique, the switchgear or switchboard circuit breaker that clears the fault may be programmed to wait up to 0.4 sec before tripping; this is termed short-time delay. Common settings are 0.1, 0.2, 0.3, 0.4, and 0.5 sec; UL 1558 switchgear should be specified instead of UL 891 switchboards if the upstream circuit breaker has a short time trip but no instantaneous trip

The circuit breakers may have to be derated if the calculated X/R at a fault is unusually high. (This is another way of stating that the calculated power factor at a fault is unusually low.) Molded case circuit breakers are rated for various maximum X/R, depending on their interrupting rating (IR): 1.73 X/R for 10 thousand amps interrupting capacity (KAIC) IR, 3.18 X/R for 10 to 20 KAIC, and 4.9 X/R for higher than 20 KAIC. Insulated case circuit breakers are rated for 6.59 X/R. Power circuit breakers are rated for 6.59 X/R if they are unfused, but only for 4.9 X/R if they are fused. Derating can be substantial—if a fused power circuit breaker rated 200 KAIC is applied where the X/R is 19.9, the 200 KAIC interrupting rating must be derated by 17% to 166 KAIC.

These high X/R situations typically occur in data centers when the standby power plant is paralleled with the utility for a closed transition load transfer. This is the situation when the available fault current and X/R are at the maximum; it’s not unusual for a standby generator to have an X/R of 32. Ideally, the design engineer should do an analysis of the electrical system to determine how much the maximum fault current and X/R are available at each breaker to ensure that the breaker can safely interrupt the load as designed. This analysis also should include consideration of the anticipated trip unit settings of the circuit breaker. If a circuit breaker is part of a selective overcurrent coordination scheme, as data centers should be, a breaker without an instantaneous trip must be able to carry the available fault current until its short-time trip times out and it clears the fault. In this situation, the circuit breaker should be applied at its withstand rating, which is usually lower than its interrupting rating. Once this analysis is completed, the X/R rating—and therefore the interrupting and withstand ratings—of the circuit breaker needed in this location can be properly specified.

Because the data center load is both critical and constant, all circuit breakers supplying a critical load should be 100% rated because the use of 80% rated circuit breakers unnecessarily increases cabling costs. For example, if the data center has a 400 amp continuous load, a 500 amp circuit breaker applied at 80% capacity would be sufficient; however, then 500 amp wiring must be supplied downstream of the circuit breaker, costing up to 25% more than what’s actually needed with a 100% rated breaker. While more costly, the 100% rated circuit breaker will reduce TCO and the costs of over-designing.

Switchgear bus and circuit breaker terminations are routinely designed to permit conductors to operate at a 90 C rating during maintenance and emergency conditions. While a piece of wire for commercial use might be rated to operate at a peak condition of 75 C, data centers demand a higher ampere rating and conductor temperature to supply more power when needed. Often, these needs occur during an emergency or maintenance condition.

It is best practice that all circuit breakers carrying critical load (IT, network, and continuous cooling equipment) be tested to ANSI/NETA Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems during commissioning. Many circuit breakers will fail to trip or trip when they should not trip if not tested and, instead, are put directly into service. It’s rare to find the opportunity to take the circuit breaker offline for performance testing, especially in a critical facility with a continuous load, even if the electrical systems are concurrently maintainable. It’s not uncommon for engineers to see a uniform small circuit breaker failure rate of as much as 6% to 15%. So, it goes without saying that this step is crucial.



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