In a modern world of engineering, one of the successful innovations ever made is the Programmable Logic Controller or the so called PLC.
In this article, I’m going to share with you how I do the designs for PLC Control Panels based on the previous projects that I have handled.
Table of contents
1. Definition of terms
2. Essential tools
3. Review of project specifications
4. Enclosure type
5. Panel cover devices
6. Internal components
7. Auxiliary items
Definition of terms
What is PLC?
Programmable Logic Controller or PLC is a combined hardware and software tool. It function as the brain of the automated system. It monitors feedbacks and input signals by means of Input Modules. Also, it regulates the commands and output signals depending on the programmed logics using assigned softwares.
What is NEMA?
National Electrical Manufacturers Association or NEMA is used as a standard to define the types of environments where an electrical device or enclosures can be used. See common types I have used before:
- NEMA 12 – I have used this type for indoor applications (control rooms, workstations, electrical rooms). NEMA 12 enclosures and devices will protect from dust, dirt, and dripping non-corrosive liquids, oil and lubricants. Normally constructed by carbon steel material.
- NEMA 4X – I used this type for indoors and outdoors applications. Especially panels located at the field and prone to corrosion, dust, ingress of water and ice. Normally constructed by Stainless Steel (304 or 316 SS) material and more expensive in price.
What is NEC?
National Electrical Code (NEC) or NFPA 70, is an Electrical Standard for the safe installation of electrical wiring and equipment. It is imitated by other states and counties in an effort to standardize their enforcement of safe electrical practices.
Before we proceed with the designs, there are essential tool you must have acquired. Otherwise, you’ll be prone to errors and disapproval of your designs. Below are the essential tools you must be have experienced for an easy understanding as we go through the guideline.
I’m grateful that this computer application is one of our minor subjects during college. This opens opportunities for me to enter a system integration and design company. The school thought me then use of AutoCAD 2008 version and learned about 2D and 3D drawings.
This gave me knowledge about the common tools can be used for the designs. Scaling, match crop, printing, templates, etc. are the most commonly used commands I have used aside from copying and moving drawings.
When I worked as a designer, I used to generate a control panel layout, wiring diagrams, bill of materials, nameplates and markings which all require just simple autoCAD commands as it’s just 2D drawings.
Electrical Codes and Standards
You must be familiar with the standards for the country you’re working with. I have worked with U.S. based company and all the designs were generated with high regards to NFPA 70 or NEC. Having the knowledge about standards will make sure that designs you’ll do is safe to install and use.
You must be familiar with NEMA ratings to make sure that panel is safe to install in a specific environment.
Electrical Symbols is also essential based on the standard you’re following. In my previous job, we used ANSI Symbols for wiring diagrams. Others standard symbol can be refered to DIN, JIS, etc.
Designing a panel doesn’t just about generating drawings and providing documents. Different calculation will be encountered while designing a Control Panel and it should be referenced with standards that you’re using.
The common calculations that I did on panel designs are as follows:
- Protection device calculation – Fuse, Circuit Breaker Sizing, Disconnect Switches, Overload Relay Sizing
- Heat load calculation – depends on NEMA rating, panel location, ambient temperature range. This will determine your accessories either for cooling or heating.
- Wire duct sizing – to make sure that wire ducts will not overfull and should follow the NEC guidelines for wire ducting and percentage of available space inside.
- Wire sizing – normally, wire sizes are given in specs since PLC panels have low power consumption, we can easily assume the size that we can use. However, confirmation must be done and wire size rating must be larger than circuit breaker rating.
- Power supply calculation – some other clients required 24VDC supply for their PLC, OIT and other devices which should be considered for selection. Total load for 24VDC components must be calculated to select proper power supply with a clearance from standard that you’re following. Either 25% or 50% space could be mentioned from specs too.
- UPS calculation – PLC panels should have protection from sudden power interruptions. UPS sizes depends on the load and sometimes installed inside or separately from panel.
These calculations are time consuming especially when you’ll do it in every designs manually. That’s why, generating templates will help for easy calculation and recording. Microsoft Excel can be used on generating these templates as what I have experienced before. It improves manhour consumption for every projects because you’ll just have to enter specific values required for calculations.
You can also check available free softwares from the internet as I’m able to find heat load calculator online. I also use Fluidsim for control simulations to ensure that my ladder diagram will successfully run.
However, there are project specifications that mentioning client’s preferred sizes and manufacturers based on their standards and best practice. But still, confirmation is a must and always think about safety installation.
Review of Project Specifications
You must have deeper understanding about the project you are working with. To study every specifications and drawings related to the design must be a practice before you start the design. Based on my experience, the more I study the project, the more I gain confidence on starting it.
In a system integration and design, you must review documents such as P&ID or Piping and Instrumentation Diagram, Electrical Diagram, Electrical Plans, Input and Output Lists, Control Strategy, etc. Those specifications from clients should be matched and have no any discrepancies. Otherwise, request for information must be made subject for approval. This will clear things up and will help to make your designing easier and smooth.
Study it by heart, get motivation from fully learning it. It boosts your confidence level and makes your mind more focus.
There are different types of enclosures available in market according to sizes and application. Factors such as specifications, number of internal devices, heat load and location will help to identify the size and type of your enclosure. Some types are as follows:
- Free Standing
It comes with different accessories such as doors handle, legs, air vents, etc. which should be same rating as enclosure.
Panel cover devices
There are many control panel devices mounted on the panel cover that can affect your enclosure size and design. Below are the common panel devices mounted at the cover that I have used.
- Push buttons and selector switches – colors and sizes must be checked as per specifications
- Indicating lamps – colors and sizes must be checked per specifications
- OIT or Operator Interface Terminal – it’s also call HMI that is installed at the panel cover. Usually, PLC Main and Remote panel have this device for easy navigation and access from authorized personnel.
- Alarm horn and strobe – panel with the alarm signals especially those that are located in the field.
- Nameplates – identification is always present and should match the drawing especially the Panel Name and device tags.
These depends on the type of PLC panel and its controls. There are common items that should be inside the panel if you’re using a PLC. These are the common items I have used for my designs:
- PLC – Selection of PLC always depends on the specifications and control strategy. This will define the number of I/Os, type of modules, spare, power supply, communication and CPU requirements. PLC could be rack or modular type as what mentioned in specs requirements. I used sinking type of DI modules as it’s always required by clients. Isolated DO modules are also always on the specs requirements. AI and AI modules were specified too and cables could be shielded with separate fuse blocks. Clients may also require a redundant CPU and PSU which will greatly affect the size of your panel.
- Circuit/Breaker or Fuse – every PLC Panel should have protective device based on the calculated load inside the panel. I normally use 10A and 15A size single phase breakers depending on the computed load. Clients are mentioning the tripping curve class they want for their circuit breaker which you should consider on product selection. There are different types of fuse could be mentioned in specs such as AGC or GMC types that you should check.
- Surge protection device – most PLC panels should have surge protector for lightning strikes and other harmful transient voltage. Some clients want to have surge protector for each I/O signals whether digital or analog which you should check during project review.
- Fuse blocks and terminal blocks – clients always mentioning the type of block they want whether single or multiple layered blocks. Even the colors and tags must be considered too. Always be aware of the terminal’s rated voltage and physical size as it will affect your whole design aesthetics.
- Power supply unit – as mentioned above, should be properly calculated. It could be two with redundant module depending on client’s specs requirements.
- Uninterruptible power supply – UPS most of the time supplies all PLC panel components. It is installed inside the panel or or outside with a separate enclosure. Normally sized in VA or kVA and has accessories to be considered such as relay card for alarm integration.
- Ethernet switch – it could be managed or unmanaged depending on the complexity of the communication. But I normally use unmanaged because it’s a lot cheaper. Number of ports must be identified per requirements as well as the spare ports. It could be 4, 5, 8, 16, 24, or 48. It will enable the connection of PLC and OIT to other system or panels.
- Control relays – it’s commonly used to support PLC logics especially for output modules and input feedback distribution. Selection depends on how many contacts needed either SPST, SPDT, DPDT or 3PDT with proper selection of its coil voltage and contact rating.
- Convenience outlet – its essential power port to be provided for laptops for easy access to programs. Sometimes, it’s attached to the lamp or mounted on the rails.
- Panel lamp – it could be Fluorescent or LED based on specs and must have door switch that will turn the lamp when you open the door. Otherwise, manual switch should be available.
There are essential auxiliary compents a PLC panel must have depending on the specifications.
- Panel heater, fan, air conditioning unit – panels located on the field may be prone to low and high temperature in some areas and these components must me activated by thermostat to avoid damage. However, use of them only done when calculation confirmed that there are devices who’s temperature range will be exceeded by the ambient temperatures.
- Ethernet port – some PLC panels requires Ethernet ports mounted at the front if the panel. This will lessen the opening and closing of PLC Panels which are prone to change in temperatures.
Those mentioned above are the just guides before you start control panel design. The process always takes time especially on the drawing and quality checking parts.
Being familiar with different manufacturers and their products will create a big help as well.
Making a standard drawings and pages to your submittals will reduce the design time so you can focus more on the quality of the control panel.
Lastly, it’s a good experience to be in the project design field. It gave me different perspective about my career since I’m working on sites before and this is purely office. One thing I have learned from being in this field is to never stop thinking about the ways on improving your man-hour consumption on doing the designs. That’s why templates for calculation and standard drawings were made. I hope I’m able to help you through this article.