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Blog post written by SCADAware Senior Engineer, Josh Z. Niemi

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One of the most basic, yet often misunderstood, components of a control system is the simple HOA (Hand-Off-Auto) switch. 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. 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?

Additionally, when a PLC or SCADA system is involved, the opportunity for confusion increases further: does 'Auto' mean controlled by the SCADA system? Chatty for facebook 2 0. 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?

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Hands Off 3 2 64

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As we'll see, perhaps the confusion is in the use of the term 'HOA' itself. 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. 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).

On the other hand, Mode of Control, refers to the control strategy used to operate the device. 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). Thus, an inherent problem with the term 'HOA' is that it combines Location of Control ('Hand' designation) with Mode of Control ('Auto' designation) on the same switch.

In older Distributed Control Systems (DCS), this may not have been much of a problem. In many older systems, there was only a single switch for the device, there weren't multiple ways to control it, and the term 'Hand' could serve the dual function meaning both Location of Control and Mode of Control. But in larger and more modern systems in which there are multiple places and means of operating a device, the term HOA is often insufficient. Thus, by distinguishing between these two aspects (Location of Control and Mode of Control), we can determine how to implement a control switch (or switches) that can accommodate a wider range of modern control systems.

If a control switch is being used to select between controlling directly at a device versus control by a remote PLC or SCADA system, an 'LOR' switch (Local-Off-Remote) or 'HOR' switch (Hand-Off-Remote) would probably be ideal. 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 what is decided, the most important thing to avoid is having multiple switches with redundant labeling (i.e. multiple 'HOR' switches) or mixing Location and Mode terminology on the same switch (i.e. a 'Manual-Off-Remote' or 'Local-Off-Auto' switch). Unless a very specific application demands redundant switches or mixed-terminology switches, it is better to avoid them altogether.

Consider the following example:
A motor has two control switches that control its operation: the first is located in the Motor Control Center (MCC) and the second is located on a control station directly near the motor. Both are labeled 'HOA.' What happens if the motor is in 'Hand' at the MCC but in 'Auto' at the motor? Or in 'Auto' at the MCC but in 'Off' at the motor? How are these switches wired to control the motor? Perhaps more importantly, how are plant personnel going to correctly remember and understand the various switch combinations?

Multiple uses of the 'HOA' designation for the same device is a recipe for confusion – both switches are labeled with the same function which makes it unclear exactly how the motor is going to be started. Instead, properly understanding Location of Control and Mode of Control can solve the issue.

Typically, the control switch at the MCC determines where the motor will be control – thus, this switch would be the Location of Control. It could be labeled as 'Local-Off-Remote,' with Local referring to operating the motor right there in the MCC via Start/Stop pushbuttons and Remote referring to operating the motor elsewhere (i.e. out at the motor control station).

The second switch, out at the control station itself, could then be labeled as 'Hand-Off-Auto' which would designate the Mode of Control. In 'Hand' the motor would run continuously and in 'Auto' the motor would start and stop by a PLC or other controller.

Consider a second example:

A modulating valve has a control switch labeled 'Hand-Off-Auto' located near the valve. In 'Hand,' the valve can be jogged open or close right at the switch. In 'Auto,' the valve position is set by a PLC analog output remotely. However, when the valve control switch is put into 'Auto' mode, plant personnel then have two options to control it: they can use their SCADA interface to manually set a fixed valve open percentage in a register located in the PLC, or they can allow the PLC to modulate the valve position based on the plant process. Thus, when the valve control switch is in 'Auto,' that doesn't truly mean that the valve will operate automatically – it could be that the plan personnel has decided to manually enter a fixed valve position through SCADA.

Hands Off 3 2 64gb

Instead, a proper understanding of Location of Control will lead to better design of the control switch. Rather than labeling the switch 'Hand-Off-Auto,' the switch should be labeled either 'Hand-Off-Remote' or 'Local-Off-Remote.' The Remote designation would indicate to plant staff that the valve is being controlled by a remote device – in this case, a PLC. Then, in SCADA, the Mode of Control for the valve would either be 'Manual' (fixed valve percentage set by plant personnel) or 'Auto' (real-time modulation by the PLC based on process conditions).

Conclusion

Regardless of your particular installation, recognizing the difference between Location of Control and Mode of Control can alleviate much of the confusion that arises when either multiple control switches are needed or a SCADA interface adds another layer of control situations. Keeping the terms separate and identifying the true function of the control switch will not only streamline design for the engineer, but also help avoid unnecessary headaches for plant personnel as they operate the system.

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• Care Notes

WHAT YOU NEED TO KNOW:

What is Raynaud disease?

Raynaud disease is a disorder that affects blood circulation, usually in the hands and feet. The arteries (blood vessels) that carry blood to your fingers, toes, ears, or nose tighten. This is often triggered by cold or emotional stress. The decrease in blood flow causes a lack of oxygen and changes in skin color. Over time, ulcers or gangrene (tissue death) may develop if frequent or severe attacks are not prevented.

What causes Raynaud disease?

• Primary Raynaud: The cause of primary Raynaud disease is unknown. This form usually affects both hands and feet. It is more common and is often milder than secondary Raynaud. It often affects women and first appears before the age of 30.
• Secondary Raynaud: This form is also known as Raynaud phenomenon. Nicotine, alcohol, caffeine, and exposure to certain chemicals, such as vinyl chloride, can increase your risk for secondary Raynaud. The following are some common causes:
  • Inflammatory and autoimmune diseases: Secondary Raynaud may be caused by certain diseases, such as scleroderma, lupus, Sjogren's syndrome, rheumatoid arthritis, and carpal tunnel syndrome.
  • Medicines or illegal drugs: Medicines used to treat high blood pressure, headaches, cancer, or colds may cause Raynaud disease. Use of illegal street drugs, such as amphetamines or cocaine, and some herbs may also cause Raynaud.
  • Trauma or injuries: Long-term use of vibrating tools, such as chain saws, grinders, or drills, may hurt nerves or blood vessels. Injuries to the hands or feet, such as a wrist fracture, surgery, or frostbite may also cause damage.

What are the signs and symptoms of Raynaud disease?

Your fingers or toes may first turn pale when you are exposed to cold or stressful situations. Due to the decrease in blood supply, your fingers or toes may then turn blue and may feel cold and numb. As blood supply returns to your fingers or toes, they become bright red. You may feel tingling, throbbing, or pain in your fingers or toes. Additional signs and symptoms may include the following:

• Primary Raynaud: The color changes usually affect both hands or feet in the same way and at the same time. You may develop thick or tight skin and brittle nails over time. Signs and symptoms are usually mild.
• Secondary Raynaud: The color changes usually do not affect both hands or feet in the same way or at the same time. You may develop thick or tight skin and brittle nails over time. You may also develop skin ulcers. Your skin may develop gangrene if your fingers or toes do not get enough blood for a long period of time. Signs and symptoms are generally more severe.

How is Raynaud disease diagnosed?

• Nail fold capillary test: Your healthcare provider may put a drop of oil on your nail folds (skin at the base of the fingernail). The capillaries (tiny blood vessels) will then be checked under a microscope.
• Blood tests: You may need blood taken to give healthcare providers information about how your body is working. The blood may be taken from your hand, arm, or IV.
• Angiography: This test looks for problems with your arteries in your hands, arms, feet, and legs. Before the x-ray, a dye is put into a thin tube through a small cut in your groin. The dye helps the arteries show up better on these x-ray pictures. Tell the healthcare provider if you have ever had an allergic reaction to contrast dye.
• Arterial doppler: An arterial doppler test is done to check blood flow through an artery. A small metal disc with gel on it is placed on your skin over the artery. You can hear a 'whooshing' sound when the blood is flowing through the artery. An 'X' may be marked on your skin where healthcare providers feel or hear the blood flowing best. Healthcare providers may need to check blood flow more than once.
• X-rays: Pictures of the bones, soft tissues, and other parts of your body may be taken. X-rays can show changes that will help healthcare providers learn if you have other diseases that may be causing Raynaud disease.

How is Raynaud disease treated?

Healthcare providers may tell you to avoid things or situations that could trigger an attack. If your daily activities are affected and symptoms are hard to control, you may need any of the following:

• Medicines:
  • Alpha blockers: These medicines work by stopping a hormone that tightens blood vessels.
  • Antithrombotics: These are medicines that break apart clots and restore blood flow.
  • Calcium channel blockers: These medicines relax and open up small blood vessels in your hands and feet. They may also help heal skin ulcers on your fingers or toes.
  • Vasodilators: These medicines relax and widen the walls of the arteries. This may also help heal skin ulcers.
• Surgery: A surgery called sympathectomy may be done to cut sympathetic nerves. Sympathetic nerves in your hands and feet control the opening and narrowing of blood vessels in your skin. Surgery may also need to be done if affected parts have developed gangrene.

What can I do to care for my skin if I have Raynaud disease?

• Avoid putting too much pressure on your fingertips, such as typing or playing the piano. This kind of pressure may cause your blood vessels to narrow and trigger an attack.
• Check your feet and hands daily for numb areas, thinning or thickening skin, black spots, cracks, brittle nails, or ulcers.
• Keep your skin clean and dry to prevent an infection. Use lotion that contains lanolin on your hands and feet to keep the skin from drying or cracking.

What can I do to prevent a Raynaud disease attack?

• Avoid cold temperatures when possible: Wear gloves, scarves, or other winter garments during the winter months or before you go into cold rooms.
• Limit alcohol and caffeine: Men should limit alcohol to 2 drinks a day. Women should limit alcohol to 1 drink a day. A drink is 12 ounces of beer, 5 ounces of wine, or 1½ ounces of liquor. Try drinking decaffeinated coffee, tea, or soda. Ask your healthcare provider for more information about alcohol and caffeine.
• Use caution with medicines: Talk to your healthcare provider before you use medicines that may trigger an attack. These include certain medicines used for treating high blood pressure, headaches, cancer, or colds.
• Exercise regularly: This prevents narrowing of the blood vessels and increases blood flow in your body.
• Learn to manage stress: Stress may trigger an attack. Try new ways to relax, such as deep breathing, meditation, or biofeedback. Biofeedback is a way to control how your body reacts to stress or pain.
• Stop smoking: If you smoke, it is never too late to quit. Smoking causes your blood vessels to narrow and may trigger an attack. Ask your healthcare provider for information if you need help quitting.

What should I do during a Raynaud disease attack?

• Get inside to warm yourself.
• Wiggle your fingers or toes, or swing your arms around to increase circulation. Massage the affected parts.
• Place your hands under your armpits or run warm water over the affected area. Do not place the affected part in direct contact with hot water or a hot water bottle. The affected parts may be injured if they are not able to feel that the water is hot.
• Get yourself out of stressful situations if possible. Deep breathing, meditation, or biofeedback may help decrease stress.

Where can I find more information?

• National Institute of Arthritis and Musculoskeletal and Skin Disease
  Information Clearinghouse
  National Institutes of Health
  1 AMS Circle
  Bethesda , MD 20892-3675
  Phone: 1- 301 - 495-4484
  Phone: 1- 877 - 226-4267
  Web Address: http://www.niams.nih.gov
  

• National Heart, Lung and Blood Institute
  Health Information Center
  P.O. Box 30105
  Bethesda , MD 20824-0105
  Phone: 1- 301 - 592-8573
  Web Address: http://www.nhlbi.nih.gov/health/infoctr/index.htm
  

When should I contact my healthcare provider?

• You have new symptoms since your last appointment.
• Your symptoms prevent you from doing your daily activities.
• You need help to quit smoking.
• You have questions or concerns about your condition or care.

When should I seek immediate care?

• You have many attacks even if you prevent cold, stress, or other triggers.
• You have pain in your fingers or toes that does not go away or gets worse.
• You have sores or ulcers on the tips of your fingers or toes that do not heal.
• You have black spots on your fingers or toes.
• Your hands or feet remain cold or discolored even after you warm them.

Care Agreement

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