EGSA Automatic Transfer Switch Function 3 — Questions and Answers
Question 1: What type of ATS performs a 'make-before-break' switching sequence?
- Open-transition ATS
- Closed-transition ATS (Correct answer)
- Soft-load ATS
- Static ATS
Correct answer: Closed-transition ATS
A closed-transition ATS briefly parallels both sources during transfer, making the connection to the new source before breaking from the old source to achieve a seamless transfer.
Question 2: Which NFPA 110 Type designation requires the ATS to transfer load within 10 seconds?
- Type 10 (Correct answer)
- Type 60
- Type 120
- Type 0
Correct answer: Type 10
NFPA 110 Type 10 specifies that the alternate power source must supply power to the load within 10 seconds of normal source failure.
Question 3: When an ATS is described as 'switched neutral,' it means the neutral is:
- Grounded at the ATS enclosure
- Transferred simultaneously with the phase conductors (Correct answer)
- Always connected to the utility neutral regardless of source
- Monitored but not switched
Correct answer: Transferred simultaneously with the phase conductors
A switched neutral (four-pole) ATS transfers the neutral conductor along with the phase conductors, required for separately derived systems where each source has its own neutral-ground bond.
Question 4: What is the function of a 'load shed' output on an ATS controller?
- To completely disconnect all loads during a fault
- To signal non-critical loads to disconnect when the generator is supplying power (Correct answer)
- To reduce generator speed under heavy load
- To balance load between two generators
Correct answer: To signal non-critical loads to disconnect when the generator is supplying power
The load shed output signals load management panels to disconnect non-essential loads, preventing generator overload when critical and non-critical loads share the same emergency distribution system.
Question 5: A 'programmed transition' ATS differs from a standard open-transition ATS because it:
- Transfers faster than standard open-transition
- Waits a fixed time in neutral to allow motors to de-energize before reconnecting (Correct answer)
- Uses solid-state contactors instead of mechanical
- Requires a separate control power supply
Correct answer: Waits a fixed time in neutral to allow motors to de-energize before reconnecting
A programmed transition ATS adds a fixed neutral dwell time between opening one source and closing the other, reducing voltage transients caused by residual motor flux.
Question 6: What is the significance of the ATS 'withstand and closing rating' (WCR)?
- Maximum continuous current the ATS can carry
- The ATS's ability to remain closed and then close onto available fault current without damage (Correct answer)
- Minimum short-circuit current required to trip the ATS
- Maximum operating voltage for the ATS
Correct answer: The ATS's ability to remain closed and then close onto available fault current without damage
WCR defines the maximum available fault current at which the ATS can remain closed (withstand) and subsequently close (closing rating) without damage, ensuring system protection coordination.
Question 7: Which of the following best describes 'voltage window' settings in an ATS controller?
- The range of acceptable input control voltage
- Upper and lower voltage thresholds that define acceptable source quality for load connection (Correct answer)
- The voltage drop allowed across ATS contacts at full load
- Time window during which voltage transients are ignored
Correct answer: Upper and lower voltage thresholds that define acceptable source quality for load connection
Voltage window settings define the acceptable voltage band (typically 90–110% of nominal) within which a source is considered 'acceptable' for load connection or retransfer.
EGSA Automatic Transfer Switch Function 3
What type of ATS performs a 'make-before-break' switching sequence?