Phase 8: Design of Control Cabinet

In this article we discuss some options about the internal layout of the control cabinet (index of design phases).


 8-Design of the control cabinet
control panel layout

There are many factors that impact the internal design of a control cabinet and, therefore the mounting space required. There is no clear rule on what should be the size of cable ducts, how many I/O racks fit in the panel or whether we should use terminal blocks of one type or another. Here are some of the points that we must consider.

How many racks can fit in each cabinet? We must consider, in addition to the rack size, the amount of modules in each rack and I/O density.
If we use high-density modules (32 or 64 I/Os) internal cables ducts should be higher. For example, in a panel 2000 mm. high, using 16 channel digital modules and 8 channel analog modules, with a maximum of 6 or 7 modules per rack, we can mount up to 6 racks 125 mm. high and also have space to accommodate two 24 VDC power supplies and, if needed the CPU rack.

-An option to save space is to use double level terminal blocks. We get more space but not as much as we expect because the amount of wires is duplicated requiring larger ducts. We can also use the sides of the cabinet but it is advisable to leave this space for future extensions.

-Another aspect to consider is the routes of cables with different voltages. It is usual to have 24 VDC for inputs / outputs and relays, and 125/230 VAC for field outputs. If so, we should try to minimize parallel runs of cables with different voltages to avoid the risk of electrical noise, especially in the analog signals. This involves more internal space.

-We also need additional space when having a design with certain levels of redundancy, especially if we have, for example, a 2 out of 3 logic. This implies a minimum of three I/O racks and sometimes even in different cabinets depending on the application. Another typical case is when we have duplicated field elements, such as a main pump and a backup. For a good design we should not use the same I/O module for both pumps.

Customer specifications often require this type of good design practices. If end user is a big industry we can meet with demanding design requirements involving some extra space. In many projects we must separate electronics from “marshalling cabinets” (cabinets for field signals terminals).

In PLCdesign we have developed a tool in Excel to help you: Cabinet Layout.