Industrial control panels consist of power circuits or control circuits (or both) which provide signals that direct the performance of machinery or equipment. Industrial control panels don’t include the main power, nor do they include the controlled equipment; rather, the panel is mounted on a back panel (or subpanel) or in an enclosure, depending on the application. Industrial control panel design begins with weighing design requirements and specifications and preparing schematics, but the design process can be quite complex in order to ensure that all applicable regulatory standards and safety requirements are met.
We’ve created this guide to provide an overview of the key design considerations applicable to industrial control panel design, including schematics, relevant regulatory standards, and design considerations relating to every facet of effective control panel design for industrial equipment and machinery.
Included in this guide:
- Industrial Control Panel Design – Schematics
- Regulatory Standards Related to Industrial Control Panel Design
- Design Considerations for Industrial Control Panels
Industrial Control Panel Design – Schematics
Control panel design for industrial equipment and machinery is an important undertaking, resulting in an interface designed to control a machine or process. It’s not a simple matter of selecting an appropriate enclosure and a back panel that houses the electrical hardware. So, the proper hardware must be mounted on a back panel and properly wired and integrated into the machine – any improper configuration can result in malfunctioning machinery, but it’s also likely to pose an unnecessary risk to operators.
The process should always begin with an evaluation of the specifications, requirements, and regulatory standards. After these considerations are evaluated, drawings are created to outline the specific configuration of wiring, circuits, controls, and every other aspect of the final control panel. Good design addresses both the electrical and physical requirements. These drawings should include:
- Functional diagram
- I/O (Input/Output) diagram
- Power distribution
- Control cabinet and back panel layouts
- Bill of materials
Because there are so many elements in a proper schematic, a table of contents is also recommended. The schematic is the foundation for the subsequent development of an industrial control panel.
Regulatory Standards Related to Industrial Control Panel Design
Despite many Code cycles, many industrial control panels are still not compliant with the regulatory standards which are accepted industry-wide. Compliance, however, is critical for ensuring minimal safety risks associated with installing and operating industrial equipment and machinery.
Below is an overview of the most prominent regulations applicable to the design, manufacture, and installation of industrial control panels. Like all regulatory standards, industrial control panel standards are subject to change over time, and in fact, one of the most relevant standards, UL 508, has recently been phased out and replaced by an updated, harmonized international standard. Because of the changing nature of regulatory standards, it’s important to stay up-to-date on current requirements.
The National Electrical Code (NEC), or NFPA 70, is a widely-accepted standard for the safe installation of electrical equipment and wiring. The NEC is adopted by state or by region to standardize the enforcement of safe electrical practices. Article 409 covers industrial control panels and applies to panels intended for general use at 600 volts or less, in ordinary locations.
Article 409 specifies that industrial control panels must be evaluated and marked for their Short Circuit Current Rating (SCCR), which is established by evaluating each feeder individually as well as all branch circuits. The smallest kA value is used as the kA value for the panel as a whole. The kA value must be greater than the kA value of the incoming source in order for the panel to be installed.
NFPA (National Fire Protection Association) 79 is a section of the NEC that pertains to wiring standards for industrial machinery. The scope of this standard encompasses electrical and electronic elements of all machinery that operates at or below 600V, including injection molding machines, assembly machinery, machine tools, and material handling machinery, among others, as well as inspection and testing machinery. NFPA 79 provides safeguards for industrial machinery aimed at protecting operators, equipment, facilities, and work-in-progress from fire and electrical hazards.
Portions of NFPA 79 relate to control circuits and control functions, operator interface and control devices, location, mounting, and enclosures for control equipment, and other topics relevant to the design of industrial control panels.
UL 508 and UL 60947-4-1
- Before January 26, 2012, listed control panels were evaluated to standard UL 60947-4. If a customer specifically requested that a panel be evaluated to UL-508, this was permissible.
- From January 26, 2012 through January 26, 2017, new industrial control panels were evaluated to UL 60947-4. However, evaluating revisions to existing control panels to UL-508 was permissible if requested.
- After January 27, 2017, all listed industrial control panels are required to meet UL 60947-4-1 specifications.
The transition is meant to harmonize the standards from UL and other entities including the Canadian Standards Association (CSA) and the International Electrotechnical Commission (IEC) in Europe. It’s worth noting that UL 508 and UL 60947-4-1 are largely the same in a technical sense but do incorporate important national differences in order to harmonize and create an international standard.
The primary impacts of the transition relate to how products are tested and qualified to a standard due to differences in voltages used around the world. Industrial voltages in the United States are 480 V at 60Hz, while industrial voltages in Europe are 400 V at 50Hz.
The new UL 60947-4 standard “applies to the types of equipment listed in 1.1.1 and 1.1.2 whose main contacts are intended to be connected to circuits the rated voltage of which does not exceed 1 000 V a.c. or 1 500 V d.c.”
A variety of other standards are applicable to certain types of industrial control panels, shown in the table below:
|UL 218: Standard for Fire Pump Controllers||Fire pump controllers|
|UL 1203: Standard for Explosion-Proof and Dust-Ignition-Proof Electrical Equipment for Use in Hazardous (Classified) Locations||Equipment intended for use in hazardous locations|
|UL 698A: Standard for Industrial Control Panels Relating to Hazardous (Classified) Locations||Industrial control panels with intrinsic safety barriers intended for connection to circuits in hazardous locations|
|UL 845: Standard for Motor Control Centers||Motor control centers|
|UL 864: Standard for Control Units and Accessories for Fire Alarm Systems||Electrical control unit assemblies or equipment containing electrical control units for fire-protective signaling systems|
|UL 891: Standard for Switchboards||Freestanding assemblies of circuit breakers and related applications|
|UL 924: Standard for Emergency Lighting and Power Equipment||Any equipment necessary for supplying automatic illumination and/or power to critical areas and/or equipment essential for the safety of human life|
|UL 1563: Standard for Electric Spas, Equipment Assemblies, and Associated Equipment||Control equipment for swimming pools and spas|
|UL 1640: Standard for Portable Power-Distribution Equipment||Portable control panels for specific applications|
|UL 1741: Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources||Equipment for controlling fuel cells, photovoltaic systems, or utility interactive systems|
|UL 1773: Standard for Termination Boxes||Enclosures or pedestals that contain terminals for connecting power circuit conductors|
|UL 2017: Standard for General-Purpose Signaling Devices and Systems||Covers control panels that contain emergency alarm equipment, as well as the actual alarm equipment|
|UL 2075: Standard for Gas and Vapor Detectors and Sensors||Equipment for gas or vapor detection|
|UL 60950-1: Standard for Information Technology Equipment – Safety – Part 1: General Requirements||Covers control panels with predominantly communication equipment|
|UL 294: Standard for Access Control System Units||Control equipment for use in physical access control systems|
|UL 1037: Standard for Antitheft Alarms and Devices||Control equipment intended for theft deterrent or warning purposes|
|UL 916: Standard for Energy Management Equipment||Equipment for energizing or de-energizing electrical loads to achieve the desired use of electrical power|
Other bodies have issued applicable standards as well, such as IEC 60204-1, which relates to the safety of machinery and electrical equipment of machines. Given the many standards applicable to specific types of industrial control panels and those intended for use in specific applications, it’s imperative to identify the appropriate standards at the beginning of the design process.
Design Considerations for Industrial Control Panels
A variety of design considerations must be weighed throughout the design process in order to design a control panel that meets functional requirements, application specifications, and regulatory standards. The following design decisions represent the primary design considerations in developing an industrial control panel, although additional considerations specific to individual applications may apply.
Enclosures and Space Requirements
The intended environment largely dictates the appropriate type of enclosure for an industrial control panel. You’ll need enough space to install and wire components, and it’s also wise to plan for several receptacles to reduce the need for extension cords when it’s necessary to power test equipment or appliances.
Placement is another consideration related to enclosures. Depending on where the panel will be located, you may need to account for swing requirements of cabinet doors (if a cabinet enclosure is being used). If a control panel will be located in an area with a high ambient temperature, a ventilation fan or air conditioning may be required to keep the control panel within an ideal operating temperature range. If temperature control applies, it’s necessary to allow for easy access to air intakes and exhaust vents (which makes it easier to access and replace filters). Both NEC required clearance around power wiring and UL required clearance around heat producing devices must be considered to allow adequate room for ventilation.
Space considerations are paramount, as one of the most common NEC violations involves control panels that are too small or too confined to allow for the proper space for wiring and wire bending. It’s also wise to consider space requirements not only for the current configuration but possible future needs, as restricted space is a common challenge for retrofits.
Wire Sizing and Component Types
When it comes to wire sizing and selecting the appropriate component types, there are two important rules of thumb:
- Wires should be sized based on load current. In turn, circuit protection should be based on wire size. Selecting the appropriate wire size is essential for ensuring that the circuit is capable of delivering the required load current, while selecting the best circuit protection reduces the risk of fire by preventing wires from overheating.
- Component types should be selected based on functional requirements. Selecting components of the right size is critical, as well, as voltage and load current requirements can mandate minimum size requirements. Components must be capable of handling the voltage and load current requirements reliably – but they also must function as required.
UL standards outline specific requirements for wiring, such as wiring materials, labeling for conductors, sizing for power circuit conductors, and wire sizing.
Control Components and Circuits
Control circuits provide logic for the operation of power circuit components. These circuits are typically lower-voltage for safety, and components such as control power transformers (CPTs) and power supplies are utilized to convert the power circuit voltage to the control circuit voltage.
Supply conductors should allow for the highest possible load that may occur. This is typically calculated by determining the sum of all connected motors and apparatus (keeping in mind duty cycles and which motors and apparatus will operate simultaneously), and adding this sum to the full load current for all resistive loads plus 125% of the full load current of the highest rated motor.
Handling Overcurrent Protection
You’ll also need to consider overcurrent protection in the design phase. There are two options: overcurrent protection can be placed ahead of the panel or incorporating a single, primary protective device in the panel itself. Depending on the application, it may not be desirable to integrate the protective device in the panel to minimize the need to open an enclosure.
Operator devices include components such as push buttons, indicator lights, digital panel meters, levers, and the like, but these devices can also be more complex, touch-screen displays. Such HMIs (human-machine interfaces) allow for more complex functionality, but also require computing components and software development, which can add to the cost.
When it comes to controls, the old K.I.S.S. principle (Keep It Simple, Stupid) applies. Simpler is always better, but simpler may not always be possible. You should, however, opt for the simplest operator devices appropriate for the functional requirements to support ease of use.
All industrial control panels should include an emergency stop (which differs from a cycle stop), typically a self-latching control in a mushroom or palm type. Emergency stops, or E-stops, are also required by OSHA (Occupational Safety and Health Administration), and they cannot be buttons on an HMI; they must be a push-button design which is hard-wired into the safety circuit. Additionally, any start buttons or switches should be located directly above or to the immediate left of the associated stop button.
It’s imperative to properly label all wires, terminals, and other components in an industrial control panel. It’s a time-saving strategy that also reduces the likelihood of errors in testing, routine maintenance, and repairs. Labels should be durable enough to withstand the conditions of the target environment to avoid fading and damage that renders them unreadable.
Front Panels and Faceplates
Front panels and faceplates may seem like an afterthought, but these elements are equally as important to the overall operation of machinery as the interior functional components. Front panels should be durable enough to withstand extreme temperatures, weather conditions, cleaning products, and chemicals to protect the underlying components.
Front panels and faceplates may include metal or plastic substrates, depending on the application specifications, and can be mounted with studs, adhesives, or fasteners. Digital or screen-printed graphics provide visual guidance for operators and incorporate labeling of operator devices to ensure usability. Therefore, face plates must have the durability necessary to remain readable for the life of the asset.
There are many, many individual components and elements that go into an industrial control panel. This guide is meant to provide an overview of the primary design considerations. Given the depth and complexity of control panel design, as well as the application specifications and regulatory considerations, partnering with an expert experienced in designing and developing industrial control panels is a time-saving and cost-effective option for manufacturers.
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