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ISA Certified Automation Professional (CAP) program
Given the process shown below, with the relay logic shown, which of the following statements is true?
A. When the pump control switch is in auto, and the pump is stopped, a low pressure in the pressure tank will start the pump.
B. When the pump control switch is in auto, and the pump is running, a low pressure in the pressure tank will stop the pump.
C. When the pump control switch is in auto, and the pump is running, a low level in the pressure tank will stop the pump.
D. When the pump control switch is in auto, and the pump is running, a low level in the storage tank will stop the pump.
The correct answer is D. The storage tank low level switch LSL103 or the pressure tank high level switch LSH101 can stop the pump.
The pressure tank low pressure switch PSL103 is not in the motor control circuit, so a high pressure or low pressure does not have an impact on the pump status. The pressure storage tank low level switch LSL102 is involved in starting the pump, but does not stop the pump since the motor seal-in circuit is in parallel.
References: Bryon Lewis, CSE, P.E.; Control Systems Engineering Exam Reference Manual: A Practical Study Guide, ISA Press, 2007. Thomas A. Hughes; Programmable Controllers, 4th Edition, ISA Press, 2005.