Control Panel LayoutAnd Wiring Best Practices.

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Large Control Panel Wiring Example.What are some good practices? What could be improved?Click to enlarge

The quality of the wiring methods used in an industrial control panel can vary quite widely. This article summarizes what this author believes are some best practice when it comes to control panel layout and wiring.

The goal is to produce a panel that is logically arranged and easy to maintain for the life of control panel.

I leave it to the reader to use these suggestedbest practices outlined below to evaluate and improve upon the control panel designs that they encounter or are part of producing.

BASIC WIRING PRACTICES.

1. * Wire:Use all 600V90 Deg C rated wire. Use stranded wire. Use MTW type wire. Note any exceptions so these can be added to the drawings or design notes.

2. * Wiring across a hinged door or panel. U loop, as long as possible, facing down anchored on each side of the hinge with screws or bolts (no adhesive). Place sleeve or spiral wrap over the wires running over the hinge between the anchor points.

3. * Spacing between wired devices and wirewayor other obstructions:2 minimum; 2 1/2 3 preferred for 120VACand less. 4 for 480 volt (enough to insert a closed fist between the device and the wireway or obstruction.

4. * Minimize the use of cable/wire ties if wire duct is used. They get cut off when troubleshooting and are rarely replaced. A good wire management system should not require any wire ties. Make it a goal to use no wire ties except temporarily while wiring.

5. Leaving Slack: Generally, leave only hidden slack. Leave service loops as the wires leave or enter the device or terminal. Run wires in the wirewayso they enter and run to the middle or far side of the wirewayor duct. Take all corners in a wiring duct as wide as possible. Run wires in horizontal and vertical lines. This also adds further slack and improves the appearance. Avoid looping wires in the wirewayunless the wireway is designed for this.

6. * General Wire Routing: Run wires in horizontal and vertical lines, no diagonal runs. Train the wire by bending it to make neat vertical and horizontal lines. Delicate wire will require training by bending and forming the bend gradually. Wire in wire duct should be run so they do not cross each other excessively. Wire entering or leaving a wire duct should be brought to the front of the duct before entering/exiting where possible. Leave service loops and run wires in the wirewayso they enter and run to the middle or far side of the wirewayor duct and take all corners as wide as possible. Do not run wire over other devices, including the wireway. Elevate the duct and go under the duct with wires if needed. Review needed exceptions.

7. * Wiring Power And Motor Wiring: Place Pig tail loops between devices that are spaced such that it makes it easier to remove wiring if the pig tail is added. Consider using High Flex power wires such as Railroad Wire or high strand count wire. Train the wire by bending it in the direction you want it to go or lay in the duct,rather than just trying to lay it in a wire duct and hope it stays down in the duct. See also General Wire Routing.

8. * Wiring Signal and Shielded Cables:Use 18 AWGshielded, twisted pair (or Triad) type cables rated at 600Vas the default signal wire type. Unless specifically required strip off a generous amount of the jacket so that each conductor can be easily accessed for removal, testing, and replacement. Also remove the jacket as it exits a wire duct, keeping the twists where the cable otherwise creates unwanted wire congestion. Examples: going to Analog I/O modules, or routing to elevated side terminals. Terminate all shields. Terminate all shields close to the signal wires. Consider using 2, 3, or even 4 high terminal blocks with jumper slots for signal wiring depending on the wiring needed. This allows busing the power supply voltages for a cleaner installation. Option: Place heat shrink tubing 1/2 over the cut end of the cable jacket and 1/2 over the exposed wires.

9. * Wiring Control Wires:Use 14 AWG600VMTW(stranded) wire for 120VACwire. Use 16 or 18 AWG600VMTW(stranded) wire for 24VDC wire for up to 10 and 5 amps respectively. Use General Wire Routing recommendations found elsewhere in this document.

10. * Terminations:leave some bare wire showing to allow visual inspection and to avoid screwing down on the insulation. Wires should exit the terminal straight. Do not bend the wire at the point of termination. Instead loop or bend wires on the insulation that do not go straight to the wireway.

11. * Terminals: Screw Terminals: Use tubular, pressure plate type screw terminals that minimize wire distortions or damage when terminating. Position Terminals to allow visual inspection of the recessed connections. Elevate Control Terminals to allow wiring under the terminals if needed. Keep it stiff using a heavy-dutyDIN rail or Hoffman Terminal Straps or equivalent. Angle and elevate terminals mounted on the side panel for wiring ease and to allow visual inspection of wiring in the terminals.

12. * Grounding Principle: Wire all grounds to the incoming ground lug either directly or with a wire to the other ground bus bars. Add a main ground lug and/or a ground bus bar for each grounded power supply. A number of busbarscan be utilized but should all be wired together and then to the incoming ground lug to at least 1 point if not two (2). This is in addition to the ground established through the panel. Use 2 ground wires from opposite ends of the bus or chain of ground bars if the ground is isolated. Wire the ground on all doors and subpanels and the cabinet itself to a ground bar terminated at the main ground lug. Wire all equipment and chassis grounds to the ground bar(s) which is terminated at the main ground lug. For additional details on grounding and bonding see the Grounding And Bonding post dedicated to just this subject.

PANEL LAYOUT CONSIDERATIONS:

Example of good spacing between the terminalsand the wireway.

1. * Optimize the Space. Place PLC I/O racks in the bay created by the wiring duct to allow room for the high density of wires going to them from the duct. Dont leave space where there is no wiring, typically the top of the I/O rack. Place similar sized devices in their own bay where possible. Consider the routing of all of the wires and how the various voltages will be kept separated.

2. * Spacing between wired devices and wirewayor other obstructions:2 minimum; 2 1/2 3 preferred for 120VACand less. 4 for 480 volt (enough to insert a closed fist between the device and the wireway, another device, or obstruction.

Control Panel DesignApproaches

How are electrical (and pneumatic) control panels for industry designed and built? I see that two methods are still largely in use. In this article 2 approachesare compared to each other noting the strengths and weaknesses I see in each of them. I leave for another discussion how engineers are adding value to either of these approaches, as well as what I have been calling a data driven engineering approach, to not just panel design, but the entire capital equipment intensive design project. For now lets just compare these two common approaches to panel designs.

Panel Design Foundations

Traditionally Designed Control Panel Example

A bill of material (BOM) is often the basis for each control panel design. This is usually prepared by an engineer or experienced designer who can take the available design information (P&IDs, layout drawings, similar equipment, equipment list(s),and/ or other basis for the new design) and convert this into control panel designs needed to house all the required electrical and/or pneumatic control components.

Electrical (and Pneumatic) schematic (elementary) drawings are also needed so the panel shop can wire up the devices correctly. To show where terminals should be added or other special wiring, these can be shown on the schematic drawing as well turning a logic drawing into a hybrid drawing containing wiring information as well.

Panel Design Approaches

Traditional approach: Scaled Layout and Schematic Drawings using a CAD tool are prepared for the panel shop. The BOM is included (usually hand entered) on the CAD layout drawing, balloon all the items to match the physical depiction of the layout with the BOM.

Pros: Well recognized method. Very clear to the panel fabricator what to do.

Cons: labor intensive from a design view: requiring manual checking to verify part information is entered correctly, spatial relationships not easy to visualize often requiring the actual panel to be modified during assembly, drawings difficult to maintain. BOM often exists on the drawing only. Data from the drawing is often impossible to extract for analysis or to make part changes across the project. Not kept up to date after initial build.

Value Engineered Panel (Click to see enlarged view)

Value approach: Prepare a BOM on a spreadsheet, make a sketch only to verify that all the parts will fit in the box if needed. Panel shop must estimate the amount of wiring needed from the BOM. If intermediate or special wiring is required describe this in the Request for Quote (RFQ) or Purchase Order (PO). Schematic drawings are supplied when wiring begins.

Pros: One spreadsheet can be generated for all the parts for all the panels and field equipment on the entire project. Issuing this spreadsheet to a panel shop directly requires only a day of prep typically since no drawings are needed to bid. This allows more time to collect information and/or allows panels to be built quicker. Layout drawings or sketches could follow later as a guide to assembly once all parts are received.

Changes to the BOM are easier to manage on a spreadsheet then on drawings typically.

Cons: Drawings are usually filed in a manner that makes them retrievable by anyone within the company. Spreadsheets are typically project specific and are often lost when the original engineering team has moved on.

In both the above approaches the design is then sent to the Panel Shop(s) to Quote and/or order parts and assemble.

Panel Layout Review and Checkout

Layout Review:Optionally the engineer/designer can show up when all the parts are received to modify the layout based on actual parts received and other changes that might require last minute modifications. Also it is much easier to visualize what the finished control panel will look like when the engineer, designer, and assemblers can layout the actual parts to optimize the panel for spare space, wiring access, to review wire routing, and the like. There is much more that can be said for the value this step can add to the finished product. In the two approaches discussed here this step is often (if not usually) omitted.

In the traditional approach the designer/engineer do not show up at the panel shop till it is time to checkout the panel unless called by the panel shop.

Sometimes when the schedule demands it, the panels will be shipped and checked after installation in the field. This increases the risk of adding cost to the checkout when modifications are needed since the resources of the panel shop are not very handy and electricians earn significantly more and are usually not as efficient at doing control panel modifications/corrections. It can also add to the checkout time

The value approach also places value on using a panel shop that can make reasonable decisions about how to layout the panel for themselves. The value approach also qualifies panel shops to do all of the required checkout making sure that all the components are functional or at least appear to be powering up correctly and verifying that the wiring is correct per the elementary drawings and functioning as expected.

Sometimes in both these approaches additional checkout is required and performed. This includes loading programs, configuring drives, setting up communications as applicable, testing I/O from the programming environment to test operations end to end. In addition some functional testing might occur for higher risk items such as running a new servo drive system when a motor is available to testing some new I/O device. This high risk testing will allow any problems to be identified as early as possible allowing extra time to address special problems. Also when successful the potential risks can now be reduced.

Conclusion

Many companies have abandoned the traditional approach to panel design in favor of some form of the value approach for control panel fabrications. This has led to the reduction of CAD design support for these companies.

Those that continue to use the traditional approachto panel design tend to have larger CAD design groups in house.

Both approaches require answering the question what is the cost/benefit of each approach. Increasingly companies are realizing that the value of detailed assembly drawings, especially once already built, is continuing to decrease. Both approaches do not value keeping the assembly drawings up to date. For new projects, a set of good digital photos with or without a parts list is often all that is needed to check, modify, or add to an existing panel.

Earlier in my career about half of the CAD design effort was spent with the control panel designs. Now, Id estimate that perhaps this is down to between 5 20% of the CAD design effort depending on whether assembly drawings are required at all.

What is left for another discussion is how to layout panels for optimal space utilization, for good access, for maintainability, for field wiring, for cooling or thermal management, for arc flash protection and working on a panel hot, for good power and signal wiring separation, for good grounding and shielding for analog signals, and the like.

What is also left out is how to determine the number and type of panels and the location of panels in the first place.

Grounding And Bonding: Best Practice: Wired Star Grounding

There is a lot that can be said about grounding and bonding and providing an effective fault current path. This article describes what this author believes is a best practice when it comes to establishing a safety ground. The safety ground is one that ensures enough ground fault current to quickly trip the circuit breaker connected to the faulted wire.

Typical Service Grounding/Bonding "Back to the Source". Click to enlarge.

Technically what this article is focusing on is more correctly called bonding. In control panel designs this is normally established by a green or green with yellow stripped wire and is part of what is often called the grounding system. However there is nothing we do that actually ties anything to the earth. Instead the control panel bonding/grounding point is tied to a green wire that at some point is tied to the return path of some power source. For this article the word grounding and bondingare used interchangeably to conform with typical usage.

The focus is also on the safety aspects of the grounding system and does not deal with the EMI and shielding aspects associated with bonding. However in all but the most special cases this method will provide good EMI and equipment shielding unless high frequency generating devices (such as variable frequency drives) are located inside the control panel. In this case, this method is still likely a best practice, but additional measures may need to be taken. __________________________________________________________________________________

Over the years I have almost unwittingly developed my own form of grounding and bonding best practices as it relates to grounding/bonding of control panels and componentsin establishing a clear ground path back to the source. Recently I started becoming aware that it does not appear to be a common practice and am trying to get input on what people in the field think of this idea. Ill call the idea the Wired Star Grounding Method. The practice puts a wire on everything that needs to be grounded and does not just rely on the cabinet and/or subpanelconnections to establish the ground path. I implement this using ground bus bars (typically the kind used in panelboardsto connect the neutrals together) bolted to the subpanelof each panel in a system. The main ground wire that is run with the power source wires will feed the first ground bar. From there, power fed to other external devices and panels will get their own ground wire of suitable size to each panel or device tied to the same ground bar. In each panel each device requiring a ground wire, the subpanel, the enclosure body, the door will have separate ground/bonding wires connected to the panel ground bus bar. The ground bus bars are tied together from panel to panel in a star (rather than series) connection as much as possible. In this way there will be a wired ground/bond path back to the source that does not count on the metal parts of the panel to provide the fault current path.

Star Grounding ExampleClick To Enlarge

The reason I prefer this is that it is much easier to inspect for proper connection. Metal to metal connections sometimes are not adequate ground fault paths to trip circuit breakers in the event of a ground fault. Since we typically cannot test for this, it becomes difficult to verify that the ground/bond is solid. We typically check the ground with an ohm meter, but that draws almost no current so it is not possible to tell how solid the ground/bond connection is. Also over time metal to metal connections can foul. Finally they can just be installed improperly and this is not always inspectable. A wired connection is easier to inspect with a screwdriver or a good tug on the wire to tell if the connection is solid.

A couple of stories will help illuminate the problem. 3M thermionically welds the building steel together for all of its manufacturing facilities just to establish the building ground grid because they have had problems with the bolted connections in typical steel building frames providing a reliable ground/bond at all points in the building.

Another example is from my most recent project where due to a communication problem the vendor asked us to make sure the grounds are solidly connected. During the first check everything looked good using an ohmeterand a long spool of wire to check resistance from one panel to each of the other panels on the site. But when the problem persisted the ground/bonding at one panel was checked again and this time a 5 and then 10 ohm impedancewas measure to the subpanelwhen it was less then2 ohms before. It was found that the ground wire going back to the source was connected to a conduit hub ground screw that was loose and it could not be tightened.

My belief is that wiring the grounds back to the source ground/bond via a wired star grounding method avoids problems, is easier to inspect and verify, and provides another path to ground, thereby increasing reliability.

How to Power Up a Control Panel for the first time,

without tripping breakers or blowing things up if things are wired wrong

Small Control PanelPowering Up an IndustrialControl Panel for the first time either after fabrication or after all the field wiring is connected and ready for power up verification, has some risks. The main risk is that there are short circuits in the wiring that will cause the breakers to trip or the fuses to blow. However there is also a risk that voltages can be mixed up that can cause damage to the control components. This article examines this authors recommendation for powering up control panels or other equipment for the first time after major wiring changes have been made.There are basically three (3) different ways to power up a panel for the first time, at least that I have seen: 1. Method 1. Just turn on the main breaker and begin the checkout. 2. Method 2. Turn on one circuit at a time and check out once circuit at a time. 3. Method 3. Turn off the main circuit breaker or disconnecting device, turn on all downstream circuit breakers, fuses, and disconnecting means, and ohm check each phase or the load side of the main breaker phase to phase and phase to ground.Method 1 is quickest method but only if there are no shorts. Method 2 is really no better since no checks are performed till after each circuit is turned on. Also this method takes the most time to find all of the shorts.The above 2 methods are widelypracticed but rely on everything being wired reasonablycorrectly in the first place. However when a control panel is first turned on, it is for the purpose of verifying that the wiring and the functionality of the controls is correct. Therefore some checks should be performed before powering up the panel for the first time.The method I prefer is Method 3. In this method all the downstream circuits except the main disconnect or circuit breakerare closed. (Verify that there are no voltages present on the load side before proceeding and follow lock out procedures as required by your facility). In closing all the downstream fuses, breakers, and switches, any shorts in most if not all of the wiring will reveal itself when using an ohm meter on the disconnected loadside of the main disconnecting meansand checking phase to phase and phase (or circuit)to ground impedance. If a short is found then open 1/2 of the switches, fuses, and breakers in repeated steps until the location of the short(s) is(are) isolated.

When checking for impedance to ground, Im looking for at least 1 Mega Ohm. When looking for phase to phase shorts I am often only looking for more than 2 ohms typically if there are coils in the circuit such as fans, as these look like near shorts but are perfectly normal. If the phase to phase impedance is less than 2 ohms, Ill start to disconnect the fans or other coils from the circuit till the number comes up. Once its above 2-3 ohms then Im usually satisfied that the panel is ready to power up. If there are no coils in the circuit then there should be a higher impedance. However the impedance may not be much more, depending on the expected loading. For example if the panel draws 20 amps at 120VAC, thats just 6 ohms of impedance. If the panel is a 200 Amp 480 VAC panel, then it could be much lower. In this case it might be worthwhile to check sections of the load so the Phase to Phase impedance is above 2 ohms or is otherwise reasonable for the given connected load. Most of the time motors are turned off so this does not end up being in circuit for the ground/fault test.There is one morething however that would be good to check prior to powering up the panel. If there are multiple voltages in the panel, check the impedance between each of the different power sources. The Impedance should be greater than 1 Mega Ohm between any 2 power sources such as 120VACand 24VDCor 5VDC. Also check each power supply to ground, again making sure the impedance is high.Once the phase to phase and ground checks are performed, then I close the door (for arch flash protection) and turn on the main breaker to turn on everything all at once. I have never seen a reason to turn things on one at time. If the ohm checks are good then all should be good. Also, this method avoids putting on the Arc Flash Suit to power up the panel as this is perhaps the most likely time that such an event will occur.This entirecheck takes anywhere from 15 minutes to maybe an hour to perform depending on the complexity of the panel if there are no shorts found.Disclaimer: While I consider this a best practice, I cannot take responsibility for things going wrong. There is inherent risk in performing electrical work that cannot be completely eliminated..Panel Layout and Electrical Design

Posted on November 18, 2012 by Frank 4 Comments

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 its 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!