Modes could include “Auto” (it is being controlled based on a set of process conditions) or “Manual” (it is being controlled by fixed operator input) or “Off” (device is still energized but control is presently disabled). On the other hand, Mode of Control, refers to the control strategy used to operate the device. Locations could include “Hand” (it is operated directly at the starter), “Local” (it is operated by Start/Stop contacts near the motor), or “Remote” (it is operated by a PLC or other device that is distant from the process itself) and “Off” (no location allowed). One of the best ways to determine how to designate a control switch operation is to use the terms “Location of Control” and “Mode of Control.” Location of Control is a phrase that refers to the place from which the motor or other field device is being operated. This is a recipe for confusion: if one switch is in the “Hand” position and the other is in the “Auto” position, how is the device going to be controlled?Īdditionally, when a PLC or SCADA system is involved, the opportunity for confusion increases further: does “Auto” mean controlled by the SCADA system? And if so, what term should be used if the SCADA system allows plant personnel to manually set a fixed PLC output to control the field device rather than allowing the PLC itself to automatically set the output?Īs we’ll see, perhaps the confusion is in the use of the term “HOA” itself. Undoubtedly there are many installations in which multiple control switches are needed in different areas of the plant in order to control a single field device. One of the most basic, yet often misunderstood, components of a control system is the simple HOA (Hand-Off-Auto) switch. Blog post written by SCADAware Senior Engineer, Josh Z.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |