Automatic Transfer Switch Function Flashcards
7 cards from real EGSA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Automatic Transfer Switch Function flashcards as text
What type of overcurrent protection is typically required ahead of an ATS in a distribution system?
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.
Which NFPA standard governs the testing and maintenance intervals for ATS equipment in health care facilities?
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.
What is the purpose of 'peak shaving' mode in an advanced ATS controller?
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.
What is a 'static transfer switch' (STS) and how does it differ from a mechanical ATS?
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.
When sizing an ATS, which load characteristic most critically affects the ATS interrupting capability requirement?
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.
In a system with an uninterruptible power supply (UPS) and an ATS, the ATS is typically located:
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.
What is the effect of operating an ATS beyond its continuous current rating?
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.