Clear Up Your HOA Confusion
Jane Alexander | September 19, 2016
For plant-floor personnel, the HOA (Hand-Off-Auto) switch represents one of the most misunderstood components of an industrial-control system. The problem is exacerbated by the fact that a site often employs multiple switches in different areas of the plant to control a single field device. According to Josh Niemi, P.E., of Normal, IL-based SCADAware Inc. (scadaware.com), 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?
The problem increases, he wrote in a Feb. 2016 blog post, when a PLC or SCADA system is involved. Does Auto translate as “controlled by the SCADA system?” If so, what term should be used if the SCADA system allows personnel to manually set a fixed PLC output to control the field device rather than allowing the PLC itself to automatically set the output?
Niemi believes this lingering confusion may stem from use of the single term HOA. Using the terms Location of Control and Mode of Control, he suggested, could help clear it up.
The inherent problem with the term HOA, according to Niemi, is that it combines Location of Control (the designation for hand/manual operation) with Mode of Control (the designation for automated operation) on the same switch. In older DCS (Distributed Control System) technology, this may not have been much of a problem because here may have only been a single device switch (there weren’t multiple ways to control it) and the term Hand meant Location of Control and Mode of Control. With the larger, more complex systems in use today, however, this old-school approach doesn’t work.
Distinguishing between Location of Control and Mode of Control, Niemi noted, leads to control switches that can accommodate a wide range of systems. For example, if a control switch is being used to select between controlling directly at a device or by a remote PLC or SCADA system, an LOR switch (Local-Off-Remote) or HOR switch (Hand-Off-Remote) would probably be best. But if the control switch toggles between an operator-adjustable output and a variable output based on a process condition, an MOA switch (Manual-Off-Auto) would be the better fit.
Regardless of a plant’s chosen control system(s), the most important thing to avoid is multiple switches with redundant labeling (multiple HOR switches, or mixing Location and Mode terminology on the same switch, such as a Manual-Off-Remote or Local-Off-Auto switch). Unless a very specific application demands redundant switches or mixed-terminology switches, Niemi emphasizes that it’s better to avoid them altogether.
Find Niemi’s complete blog post with detailed examples on understanding HOAs at scadaware.com/blog.
Location of Control refers to the place from which a motor or other field device is being operated. Location nomenclature includes:
• Hand: Device is operated directly at the starter.
• Local: Device is operated by Start/Stop contacts near the motor.
• Remote: Device is operated by a PLC or other device at a distance away from the process.
• Off: No location control is allowed.
Mode of Control refers to the control strategy used to operate a device. Mode nomenclature includes:
• Auto: Device is controlled based on a set of process conditions.
• Manual: Device is controlled by fixed operator input.
• Off: Device is still energized but control is currently disabled.