CDCP Data Center Power Infrastructure 4 ā Questions and Answers
Question 1: What is the primary purpose of a static transfer switch (STS) in a data center power system?
- To step down voltage from utility feed
- To transfer load between two power sources with sub-cycle switching speed (Correct answer)
- To filter harmonic distortion from PDUs
- To regulate output frequency of generators
Correct answer: To transfer load between two power sources with sub-cycle switching speed
An STS transfers load between two independent power sources in less than one AC cycle (~4ms) to maintain continuous power to critical loads.
Question 2: In a 2N power architecture, if one complete power path fails, what percentage of IT load capacity remains available?
- 25%
- 50%
- 75%
- 100% (Correct answer)
Correct answer: 100%
In a 2N architecture, each path is sized to carry 100% of the load, so failure of one entire path leaves the other path fully capable of supporting all IT equipment.
Question 3: Which harmonic order is typically the dominant harmonic produced by six-pulse rectifiers commonly used in UPS systems?
- 3rd harmonic (180 Hz)
- 5th harmonic (300 Hz) (Correct answer)
- 7th harmonic (420 Hz)
- 11th harmonic (660 Hz)
Correct answer: 5th harmonic (300 Hz)
Six-pulse rectifiers predominantly generate 5th harmonic (300 Hz at 60 Hz systems) as the lowest-order characteristic harmonic, which is the most problematic for power quality.
Question 4: A data center operator installs a 1500 kVA transformer with a 0.85 power factor rating. What is the maximum real power (kW) the transformer can deliver?
- 1,000 kW
- 1,275 kW (Correct answer)
- 1,500 kW
- 1,765 kW
Correct answer: 1,275 kW
Real power (kW) = kVA Ć power factor = 1500 Ć 0.85 = 1,275 kW.
Question 5: What does the term 'power stranding' refer to in data center power planning?
- Exposed electrical conductors posing a safety hazard
- Unusable power capacity due to imbalanced phase loading or circuit limits (Correct answer)
- Power loss during transmission over long cable runs
- Backup power that cannot be transferred to the primary bus
Correct answer: Unusable power capacity due to imbalanced phase loading or circuit limits
Power stranding occurs when available electrical capacity cannot be used because of imbalanced phases, circuit breaker limits, or cooling constraints, leaving capacity installed but inaccessible.
Question 6: Which standard defines the test procedures and performance requirements for UPS systems used in data centers?
- ANSI/TIA-942
- IEC 62040-3 (Correct answer)
- NFPA 110
- IEEE 1100
Correct answer: IEC 62040-3
IEC 62040-3 specifies the method of specifying the performance and test requirements for UPS systems, defining classifications like VFI, VI, and VFD.
Question 7: In data center power distribution, what is the recommended maximum loading percentage for a branch circuit to avoid nuisance tripping under continuous loads?
- 100%
- 90%
- 80% (Correct answer)
- 70%
Correct answer: 80%
The NEC requires continuous loads (lasting 3 hours or more) to not exceed 80% of branch circuit breaker rating to prevent thermal tripping.
CDCP Data Center Power Infrastructure 4
What is the primary purpose of a static transfer switch (STS) in a data center power system?