EGSA Automatic Transfer Switch Function 5 — Questions and Answers
Question 1: What type of overcurrent protection is typically required ahead of an ATS in a distribution system?
- Current-limiting fuses only
- Coordinated overcurrent protection sized per NEC 230.95 and ATS WCR rating (Correct answer)
- Ground fault circuit interrupters (GFCI)
- Arc flash detection relays only
Correct answer: Coordinated overcurrent protection sized per NEC 230.95 and ATS WCR rating
Overcurrent protection upstream of the ATS must be coordinated with the ATS withstand and closing rating to ensure the protective device clears faults before the ATS is damaged.
Question 2: Which NFPA standard governs the testing and maintenance intervals for ATS equipment in health care facilities?
- NFPA 70 (NEC)
- NFPA 99 (Correct answer)
- NFPA 101
- NFPA 72
Correct answer: NFPA 99
NFPA 99 (Health Care Facilities Code) establishes inspection, testing, and maintenance requirements for ATS equipment in hospitals and other health care occupancies.
Question 3: What is the purpose of 'peak shaving' mode in an advanced ATS controller?
- To limit generator output during peak summer temperatures
- To transfer critical loads to the generator during utility peak demand periods to reduce utility demand charges (Correct answer)
- To shed non-critical loads when utility voltage drops
- To cap generator fuel consumption during extended outages
Correct answer: To transfer critical loads to the generator during utility peak demand periods to reduce utility demand charges
Peak shaving mode uses the ATS to transfer loads to the generator during utility peak demand windows, reducing the facility's metered peak demand and lowering demand charges on the utility bill.
Question 4: What is a 'static transfer switch' (STS) and how does it differ from a mechanical ATS?
- An STS uses electromechanical relays for faster switching
- An STS uses solid-state thyristors (SCRs) to transfer load in less than one electrical cycle with no moving parts (Correct answer)
- An STS is a manually operated transfer switch with a static display
- An STS uses magnetic contactors with no time delay circuitry
Correct answer: An STS uses solid-state thyristors (SCRs) to transfer load in less than one electrical cycle with no moving parts
A static transfer switch uses silicon-controlled rectifiers (SCRs) to switch between sources in under one AC cycle (< 4 ms at 60 Hz), far faster than mechanical ATS devices, with no physical contact wear.
Question 5: When sizing an ATS, which load characteristic most critically affects the ATS interrupting capability requirement?
- Total connected load kVA
- Available short-circuit current at the ATS line terminals (Correct answer)
- Load power factor
- Harmonic distortion level of the load
Correct answer: Available short-circuit current at the ATS line terminals
The available short-circuit current at the ATS terminals determines the required interrupting and withstand ratings; the ATS must withstand fault current until upstream overcurrent devices operate.
Question 6: In a system with an uninterruptible power supply (UPS) and an ATS, the ATS is typically located:
- Between the UPS output and the critical load
- On the input (utility/generator) side upstream of the UPS (Correct answer)
- Inside the UPS bypass cabinet
- Between the PDU and the rack distribution
Correct answer: On the input (utility/generator) side upstream of the UPS
The ATS is positioned on the upstream (source) side of the UPS to switch the UPS input between utility and generator power, while the UPS itself provides seamless power to the load during the ATS transfer transient.
Question 7: What is the effect of operating an ATS beyond its continuous current rating?
- The ATS will automatically reduce load by shedding non-critical circuits
- Overheating of contacts and conductors, leading to insulation damage, contact welding, and potential failure to transfer (Correct answer)
- The ATS controller will reduce transfer speed to protect the mechanism
- Increased contact bounce during transfer operations only
Correct answer: Overheating of contacts and conductors, leading to insulation damage, contact welding, and potential failure to transfer
Exceeding the continuous current rating causes resistive heating at the ATS contacts and bus, degrading insulation, potentially welding contacts closed, and ultimately causing the ATS to fail to transfer during an emergency.
EGSA Automatic Transfer Switch Function 5
What type of overcurrent protection is typically required ahead of an ATS in a distribution system?