Panel Layout and Electrical Design
I had planned on writing a bit about my meeting with the publisher last week, but I received a comment on a post from about a year and a half ago on Controls Specifications. Doug asked me to cover a little more on the panel design process.
I am using the above picture because it contains a variety of different kinds of components in the same enclosure. Many larger enclosures allow the designer to separate similar components (i.e. voltage levels) into different enclosure bays, but when you only have one enclosure to put all of your stuff in you have to be careful about where you locate components.
The first step in laying out an enclosure is to complete the electrical design schematics. This accomplishes several things; it ensures that all major devices are accounted for (controller, disconnect and power distribution devices, power supplies, ethernet switches and fuses/circuit breakers, VFDs/motor starters/servo drives and controllers, etc.). It also allows the designer to determine how many terminal blocks and other power and signal distribution devices will be needed. Prior to drawing the schematics preliminary steps such as system concepting and creating an I/O list must be done. Much of this process was discussed in a previous post. From the electrical schematics a Bill of Material (BOM) is generated listing all of the components in the system. Advanced CAD packages such as AutoCAD Electrical will create the BOM for you. It requires the designer to account for every component in the drawings, giving it a label and part number. It also looks at all of the junction points (usually a dot joining at least two wires) and assigns terminal blocks to them. For more information on CAD and wire numbering check out this post.
If a designer is using a more limited design/drawing package they will have to create the BOM themselves. Large components are easy; just put a number by the component and start a list of labeled components in a spreadsheet. Fuses and fuseblocks fall into this category. They are also easy to account for if a good single-line drawing of power distribution is created. I/O terminal blocks are also easy to account for (if used); there should be one block for every point. I wire all spare I/O to terminal blocks. It is also common to use distributed I/O via communications or “wiring arms”, which bring a cable to the I/O card with a breakout board on the other end with I/O and power terminations on the board. These are more expensive than terminal blocks but save significantly on wiring time and space. Personally I *never* wire I/O devices directly to the card. I have seen panels wired this way to reduce the space taken by terminal blocks and the cost, but it violates 90% of the specifications I have ever seen.
Power distribution terminal blocks can be a little trickier to count. If multi-level terminal blocks are used a +DC and -DC row with jumpers is available to terminate sensor leads into, but these can be tough to wire unless fingers are small. A common method is to mount quick-disconnect I/O blocks on the machine frame; this only requires one +DC and one -DC termination inside the enclosure for a group of sensors, again it costs more but reduces labor. To account for all of the other distribution terminal blocks just count the dots on the right hand (typically -DC/Com) rail of the electrical drawings. The same can be done for Neutral wires in AC circuits. The +DC and -DC blocks are typically grouped together and jumpers are used to link the blocks together. +DC power feeds are usually fused while -DC buses are often grounded in the US. Its a good idea to plan for a few extra points than are counted.
After accounting for all of the components its time to locate them on an enclosure backplane. I generally start by placing the components in AutoCAD in a rectangle of some backplane dimension; i.e. 57″ x 33″ for a 60″ x 36″ enclosure. At this point its a rough guess until all the components have been placed. Disconnects are usually somewhere toward the right of the enclosure so I start with locating that since it’s location is usually not optional.
Its a good idea to segregate devices of different voltages within the enclosure. Mixing 24VDC and 480VAC within wiring duct can cause noise problems. Since the disconnect carries the highest voltage a distribution block is usually located close to it along with branch circuit fusing. The panel layout at the top of this post only has 24vdc and 240VAC devices in it, note that the 240VAC devices are all located on the left side. Usually in a 480VAC cabinet a transformer is used to bring 120VAC to components such as the DC power supplies, controller and other devices. These fuses or circuit breakers are generally separated from the 480VAC power and 24VDC fusing.
For some reason controllers often end up close to the upper left corner of the enclosure. There is no hard and fast rule for this , but if the power distribution is on the right and terminal blocks at the bottom this is often where it ends up. I generally try and locate all field termination points (wiring arms, terminal blocks etc.) toward the edges of the enclosure. This minimizes the amount of wiring inside of the cabinet which as can be seen below can be a considerable amount.
If the wiring arms in the center area of this panel had been located vertically on the right side or at the bottom the wire would have been much more manageable.
The red devices toward the bottom of this enclosure are guard and E-stop circuits. As safety systems have evolved over the past 20 years or so they have grown to take up more space. The guard switches, light curtains and E-stops are all brought in as field wiring and generally also require their own terminal blocks. These “dots” need to be counted and placed on the backplane also.
After all of the components, terminal blocks and other devices have been placed in the rectangle items such as wiring duct (wireway) and DIN-rail can be placed. Wireway fill percentage is another specification often included in companies’ wiring specs along with spare I/O and spare backplane space. After all of the DIN-rail mounted and other components are located the wireway can be placed between the rows of components. Efficient space utilization, heating/ventilation, separation of voltages and access to terminations for field wiring all factor into panel arrangement. I also generally place wireway all the way around the edges of the backplane. Some specifications require internal separators or even separate runs of wireway for different voltages.
Mechanical designers usually want the enclosure to be as small as possible and ideally completely hidden from sight; tucked up under the machine somewhere. Electrical designers want enough room for a lounge chair and maybe a small TV. The result is usually somewhere in-between, mostly due to specification and space requirements. Since enclosures only come in specific sizes it is usually pretty easy to round up to the next size.
After laying out a few different sizes of panel you will have the start of your own “library” of layouts. Often similar systems can use one of these saved designs as a rough template, saving time and giving the designer a running start. Hope this helps!