1 November 2005
Integrate codes, standards, and practices
Operators find windows on enclosures worthwhile.
Although PLCs are industrial equipment, they require some protection from the contaminants found in the industrial environment.
In addition, people get protection from the voltages involved. There is also a certain amount of other equipment that the PLC controls that needs protection or from which people also must get protection.
Wired and mounted PLC equipment
The electrical codes do not require approvals for equipment operating at 30 volts or less except for hazardous areas. For this reason, there has been a large movement away from 120-volt control towards 24-volt DC control.
This low voltage approach can reduce the need for conduits and junction boxes in many types of equipment (packaging, discrete part manufacture, etc).The use of busses such as Device Net, Fieldbus, and others also means much more equipment may be located in the field or on the machine instead of in the control panel.
The system layout includes integrating all the above into a system that satisfies the needs of the user, the operator, and the maintenance personnel. In large systems, there may be further constraints due to maximum shipping dimensions and weight.
The various enclosure types usually hide the PLC from view unless the door is open. In some cases, this leads to folks leaving panel doors open a lot thus exposing workers to possible shock from contact with energized components and allowing dust, dirt, liquids, and other contaminants to get into the panel. To avoid this problem, provide windows to allow visual inspection of all the indicator lights on the I/O racks and on the processor, as well as key switches or other devices that may be of interest. Quite often, it is necessary only to observe the status of the I/O lights for a point or series of points to know what is wrong with the equipment. This makes it easy to keep the panel doors closed and to maintain the PLC in the desired environment.
The PLC manufacturer will usually provide a recommended layout for various models and configurations and even drilling templates for the racks and other devices. Remember, these are minimum requirements, and one may want to be more generous. Here is a minimum spacing drawing. This is for equipment mounted on a flat plate in an industrial cabinet.
Another type of mounting is in racks used originally in the electronics industry but now common to the process industries, especially for computer or control room use. In rack mounting, the equipment is mounted one above the other in standard racks, usually a nominal 19 in wide. Wiring then runs to terminal blocks elsewhere.
Heat dissipation is a concern in panel layout. Generally, the heat-producing components mount near the top, the other components farther down. In many cases, there is considerable heat produced in the racks themselves due either to rack-mounted power supplies or to the power handled by the output modules. One must consider the space occupied by wire ways in allowing adequate room for ventilation; minimum spacing's may have to increase as a result. In some cases, fans may be required to circulate air inside the cabinet. Fans with filters sometimes work to bring clean ambient air into the enclosure. When using this approach, make sure you include both intakes, exhaust fans, and filters on both. (You need filters on the exhaust to keep dust from getting in when the fan is stopped). Air conditioners may be necessary, especially in areas with a high ambient temperature or where the air contains particulates or contaminants.
In planning the layout, consider wire routing for various signal levels and types.
While noise doesn't usually affect 120-volt AC signals, they themselves can generate noise that affects other low-level signals, like analog inputs and outputs, high-speed, low-level DC signals, and so on.
These constraints will affect the I/O configuration, so give them due consideration when configuring the system.
Proper grounding is an essential requirement for both safety and good operation. Be careful to observe the manufacturer's instructions about which terminals should go to ground and which should not. The routing of the ground wires and the connection points are very important.
Remember to leave room for cables and accessories. Cables are often a problem, especially if there is not enough room left to plug and unplug them. Some manufacturers give good information on the space required for this, while others are remiss or make the information hard to find. In any case, always leave adequate space for the cables connecting the various parts of the system and route them away from sources of interference or noise.
Leave space for future additions of racks, power supplies, terminal blocks, and so forth. Do not forget the space for wiring these items as well as the distribution equipment they will require. Consider drilling and tapping the mounting holes for these future devices to avoid a possible shutdown in the future. It is also a good way to remind others that the space is already reserved for equipment.
All electronic equipment needs clean, reliable power. PLCs, because of their industrial design, should be less demanding than other electronic equipment, but there are limits to everything. Since most industrial equipment operates on 220 , 440 , or 575 volt three phase power, and PLCs tend to operate on 120 volt, single phase power, some transformation is necessary. There are many ideas on how to distribute power to the PLC and its I/Os. Some prefer to use a single transformer for everything, saying the same quality of power will be distributed everywhere. Others suggest using separate transformers for the PLC power supplies and the I/Os to avoid noise and load problems being communicated from the output devices to the processor electronics. When using multiple voltage levels and both AC and DC power, it is no longer possible to have a common supply. Problems could occur in one power supply but not in the others. You do have to be careful about inadvertently connecting the different power sources together especially through the commons of the I/O cards. IC
Nicholas Sheble edits PLC feature articles. This material came from Fundamentals of Industrial Control , D.A. Coggan, editor, ISA Press 2005.
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