The art of designing the cabinet with maintenance in mind

Aug. 28, 2017
A properly designed control panel should be fit to hang in the Art Institute

When you open the door of a well-designed control panel, it’s like seeing a fine piece of art. OK, that is a little overboard, but, to an engineer, the layout of the control panel is pleasing to look at. All of the like components are grouped together; numbering makes sense; and everything looks like it belongs where it is placed.

There’s no set of rules to follow when designing a panel layout, other than what might be in a company specification book, but there are guidelines to follow. NFPA 79, NEC Article 409 and IEC 61439 have specific recommendations, but here are some basic strategies that may not be covered.

Many new design engineers and some experienced ones, as well, make the mistake of using a panel sized so everything will just fit. It is true that a larger panel and enclosure is more expensive than a smaller one; trying to fit every thing into the smallest area is only required when space is at a premium. Keep the maintainability of the equipment and future expansion in mind. If a maintenance person must use needle-nose pliers to place a new wire in a terminal, the spacing is too close. If a new component, such as an Ethernet switch or current monitor, must be mounted outside of the enclosure, the control panel is too crowded. If the size of the enclosure is limited, consider using space-saving devices. IEC-rated components are smaller than a NEMA-rated device. IEC devices do require more thought during the design phase because they typically have a narrower operation specification than NEMA devices.

Space saving may also be achieved using busbar devices. A busbar system does away with device-to-device interconnection wiring and replaces it with a set of electrical bus ways that the components are mounted to. If the design includes a PLC or PAC, consider using high-density I/O modules. A 32-point or 64-point input or output module uses the same space as one 16-point card. Additionally, many of manufacturers offer a cable/breakout strip combination to be used with the high-density I/O cards. The person building the panel will appreciate wiring 64 I/O points in a matter of minutes using a cable, as opposed to individual wires. Moving I/O out of the enclosure by using fieldbus devices is also an option. Configuring an Ethernet/IP or DeviceNet system is much easier than it has been in the past.

All heat-producing devices, transformers, power supplies, variable frequency drives (VFDs) and so forth, should be mounted at the top of the enclosure. I am amazed at how many systems I have worked with that have transformers mounted low, perhaps because of their weight. Be sure to do a heat-load calculation to determine if ventilation or cooling will be required.

The ac and dc wiring must be kept separate. Very often this is accomplished by routing the wires on opposite sides of the enclosure. Devices that produce electromagnetic interference (EMI) should be addressed in the panel design phase. Variable frequency drives are especially good at creating noise that affects microprocessor-controlled devices. This noise can be mitigated using line reactors on the VFD and line filters on the microprocessor.

Label everything with the same device name or wire number shown on the electrical drawings. Arrange the devices on the panel in some logical order left to right, or right to left. Hunting for a particular terminal or circuit breaker is very difficult when the numbering is not sequential. Using color-coded terminal blocks that match the wiring color code is helpful when a maintenance person is troubleshooting a problem. Group terminal blocks that have like functions together, DC+, DC-, PLC inputs, PLC outputs, digital signals, analog signals, safety circuits and interlock circuits should be placed in separate groups. Add a number of additional unused terminals within each group to allow additional connections when the system requires modification. Choose the wireway for terminal strips carefully. Thin terminal blocks require wireway that has narrowly spaced slots, so the wires exit the wireway at 90° angles. Make sure the wireway is large enough to hold the number of wires placed inside. Most manufacturers have calculators on their websites to determine how many wires of specific gauge will fit inside the different sizes. To ease troubleshooting, use wider wireway instead of taller wireway.

When designing panels with programmable devices, allow for an external programming port. Be sure the programming port is tested before the control panel is placed into service. Arc-flash guidelines may not allow a programming device to be connected while the enclosure door is open without the use of proper personal protective equipment (PPE) to mitigate the risks of arc-flash hazard. Typing on a laptop keyboard is very difficult wearing arc-flash gloves. If the system includes an HMI, have as much diagnostic information as possible contained in the HMI’s diagnostic screens. Troubleshooting a control system without opening the panel door will be very helpful to the maintenance personnel who work on it.

If the control system has 100–120 Vac available, installation of service receptacles both inside and outside the enclosure is also helpful for maintenance personnel. Interior lighting of the enclosure should be considered, even with smaller systems. Industrial plants often place lighting to benefit the production area where people are working. This area is seldom where control enclosures are physically located.

Finally, if designing a control panel for a customer, request feedback from the client often. It is an enormous waste of time designing a system that does not meet your customer’s specifications, expectations, or their preferred way of doing things.

About the author
Thomas Stevic is a controls engineer at Star Manufacturing, an engineering and production company in Cincinnati. Contact him at [email protected].

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