ATP Power Redundancy & UPS Systems 2 — Questions and Answers
Question 1: Which UPS topology provides zero transfer time during a utility power failure?
- Standby (offline) UPS
- Line-interactive UPS
- Double-conversion (online) UPS (Correct answer)
- Delta-conversion UPS
Correct answer: Double-conversion (online) UPS
Double-conversion online UPS continuously powers the load from the inverter, so there is no transfer time when utility power fails.
Question 2: In a 2N power redundancy configuration, what happens to the total capacity when one PDU path fails?
- The load sheds 50% automatically
- The remaining path carries 100% of the load at full rated capacity (Correct answer)
- Both paths shut down to prevent damage
- The load is divided equally across backup generators
Correct answer: The remaining path carries 100% of the load at full rated capacity
In a 2N configuration, each path is sized to carry 100% of the load, so a single path failure results in the surviving path assuming the full load.
Question 3: A static transfer switch (STS) typically has a transfer time measured in:
- Milliseconds (4–8 ms) (Correct answer)
- Seconds (1–2 s)
- Minutes (1–2 min)
- Microseconds (<1 µs)
Correct answer: Milliseconds (4–8 ms)
Static transfer switches typically switch between power sources within 4–8 milliseconds, fast enough for most IT equipment.
Question 4: Which of the following best describes an N+1 UPS redundancy model?
- Every UPS module has a dedicated backup generator
- One extra UPS module is deployed beyond the minimum needed to support the load (Correct answer)
- The total UPS capacity equals twice the IT load
- All UPS modules share a single battery string
Correct answer: One extra UPS module is deployed beyond the minimum needed to support the load
N+1 redundancy means one additional UPS module is added beyond the N modules required to carry the load, allowing any single module to fail without service interruption.
Question 5: What is the primary advantage of a modular UPS over a traditional monolithic UPS in a data center?
- Lower total cost of ownership in all scenarios
- Scalability and hot-swap capability for individual power modules (Correct answer)
- Higher efficiency at all load levels
- Simpler battery replacement procedures
Correct answer: Scalability and hot-swap capability for individual power modules
Modular UPS systems allow individual power modules to be added or replaced without shutting down the system, enabling scalable and maintainable redundancy.
Question 6: When performing a UPS maintenance bypass, what is the critical safety concern the technician must verify before switching to bypass?
- That the bypass source is synchronized with the UPS output (Correct answer)
- That all batteries are fully charged
- That the generator has reached full speed
- That all PDUs are switched to their secondary feed
Correct answer: That the bypass source is synchronized with the UPS output
Before engaging maintenance bypass, the bypass source must be synchronized in phase and voltage with the UPS output to prevent a voltage step or surge to the load.
Question 7: A data center UPS is rated at 500 kVA with a 0.9 power factor. What is the maximum kW load it can support?
- 500 kW
- 450 kW (Correct answer)
- 555 kW
- 400 kW
Correct answer: 450 kW
Real power (kW) = apparent power (kVA) × power factor, so 500 kVA × 0.9 = 450 kW.
ATP Power Redundancy & UPS Systems 2
Which UPS topology provides zero transfer time during a utility power failure?