The post AutoQuiz: How to Interpret Relay Logic for a Pump Control Switch first appeared on the ISA Interchange blog site.
Today’s automation industry quiz question comes from the ISA Certified Automation Professional certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional’s skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.
Given the process shown in the post graphic, 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
e) none of the above
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.
The correct answer is D. The storage tank low level switch LSL103 or the pressure tank high level switch LSH101 can stop the pump.
Want to learn more? Check out these ISA reference sources:
Control Systems Engineering Exam Reference Manual: A Practical Study Guide, by Bryon Lewis, CSE, P.E., ISA Press.
Programmable Controllers, 4th Edition, by Thomas A. Hughes, ISA Press.
Source: ISA News