4-20 ma Current Loops – Assembling the DC Power Supply Panel

Well, I finally got the drawings finished that I needed for this post and am ready to pick up where I left off in my last post in this series where I gave you the materials list for the DC Power Panel. In this post, I’ll provide the wiring information and a few assembly tips.

The picture at the top of the post is of the assembled panel. As you can see, there is really not much to it. In general terms, the steps are as follows.

Install the Cord and Cable Connectors

The cord and cable connectors provide protected points of entry for the current loop cables and ac power cord used by the power supply. They also provide a measure of strain relief and dust and moisture protection. The picture below shows a typical connector disassembled with a cable passing through it.

From left to right, the components are:

  • A knurled compression nut that threads onto the body of the connector and compresses the grommet.
  • A white plastic washer that transfers the force from the compression nut to the grommet while providing a smooth surface (relative to the grommet) for the compression nut to work against.
  • A black rubber grommet, which is compressed between the compression nut and body of the connector, thereby gripping the cord that passes through it.
  • A machined body threaded on the left side to engage the compression nut and on the right side to engage a standard 1/2″ electrical lock nut.

There are two common approaches to making the hole in the panel box to mount the connector. One is to use and appropriately sized hole saw and the other is to use a knock-out punch. Both tools are illustrated in the picture below.

The hole saw has the advantage of being more readily available in your typical hardware store and of costing less than the punch.  One with good quality steel (lasts longer) in the size you would need for this project will probably cost $10 or less. Lower quality steel versions can be had for under $5.

But, the standard sizes that are available in most hardware stores do not exactly match the diameter of the hole required for the connector and it is easy to end up with a hole that is a little to big.  This is especially true if you try to drill the hole by mounting the hole saw in a hand-held electric drill rather than a drill press because things are not held as rigidly during the drilling operation. The cutting operation also tends to leave a burr that you need to remove before you can install the connector.

In contrast, the knock-out punch will make a hole that is more closely sized to the actual requirement of the cord and cable connector.  You use it by drilling a small (3/8″) pilot hole on the center line of the the location where you want to installed the cord and cable connector.   Then you slip the female part of the cutter over the draw stud, slip the draw stud through the hole, thread the male part of the cutter onto the end of the draw stud, and use a wrench to crank the nut until the cutters pull together through the enclosure’s wall.

The result is a clean hole more closely matched to the diameter you really want.  But, a punch for a 1/2″ cord and cable connector will probably cost in the range of $20 – $30; worth it if you are going to be doing this more than once, but perhaps hard to justify for a one time operation (unless you just happen to like tools and having exactly the right tool for the job).

Assemble the Panel Hardware

Some panels will come with the studs that hold the sub panel and provide a grounding point spot welded in place while others will have a bag of parts and holes. Assembling everything is pretty straightforward and the only thing that you probably need to think about a bit is how much space you want between the back of the sub panel and the enclosure. Usually, this is simply a matter of how long the screws are that you will be using to mount things on the sub panel.

The spacing is typically controlled by how far you thread a nut down the sub panel mounting studs. In other words, once the stud is installed, a nut is typically threaded down the stud to the point where you want the sub panel to sit. Then you slide the sub panel on and thread another nut down and clamp the sub panel between the two nuts. Some manufactures provide a third nut that is installed behind the first nut as a jamb nut to lock it into position.

Mount the Power Supply and Terminal Blocks to the Rail

Mounting the power supply and terminal blocks is typically pretty straight forward. After cutting the mounting rail to length with a hack saw, you can either slide them onto the rail from the end or snap them over the rail. Different folks will have different preferences for the order that you install the terminals in. This drawing shows the arrangement I used.

The resolution I have to use to load images on to the blog website may make the image above and the wiring diagram I insert later in the post difficult to read.  For those who are interested, you can find higher resolution versions of both the terminal strip layout and the wiring diagram in my Google shared photos.

My arrangement uses the grounding terminals to visually separate different functions. For me, that makes it easier to figure out what is what and it also makes grounding a shield wire a little cleaner as the grounding location is immediately adjacent to a terminal that will be used to land a wire in the cable the shield serves. Bottom line though is that there is really no “right answer” other than what works best for you.

Larger components like the power supply often have a small spring loaded clip that you need to pull out with the tip of a small screw driver to get them to snap in place as illustrated in this picture.

The grounding terminals actually clamp to the mounting rail to establish a good ground. This picture illustrates a typical grounding terminal and its clamping mechanism.

By turning the mounting screw, you can open the clamp up sufficiently to allow the terminal to be installed with out sliding it onto the rail. Or, you can loosen it just a bit and slide the terminal onto the rail from the end in sequence with the other terminals.

Most of the other terminals have enough flex in the plastic and the way that it is formed to allow them to snap on or off of the rail in addition to sliding on from the end. I’m pointing at the part of the terminal I used that snaps over the rail in this picture.

By comparing the shape at the tip of my finger with the close-up of the mounting rail below, you can probably see how everything fits together.

Once you have the terminals and power supply mounted to the rail, you can mount the rail to the sub panel. Using self drilling “Tech Screws” makes short work of this task and gets you ready for making the wiring connections.

Wiring the Panel

This drawing is the wiring diagram for the panel, including field wiring for both two wire or loop powered 4-20 milliamp transmitters as well as three-wire transmitters.

Two wire transmitters, which derive the power they need from the current loop itself, don’t require a separate DC common connection and are frequently encountered out in the field.

Three wire transmitters require a separate DC common to reference the electronics in the transmitter and have a seperate connection that provides 4-20 milli-amp output as well as a separate connection for the incoming 24 vdc. The transmitters that the panel is serving in the pictures in the post that started this series are three wire transmitters.

When wiring up your panel, you want to be sure that you get the wires inserted into the terminals properly. The picture below is of a terminal clamping assembly that has been removed from the plastic terminal housing.

My terminals (and most other terminals of the type and class we are talking about) are technically termed “tubular screw” clamping terminals. That’s because the clamping assembly is a squared off tube (circle A) combined with a flat plate (circle B) that is used to secure the wire in the tube by virtue of a screw (circle C) that forces the flat plate against the wire, which is inserted between it and the tube (circle D).

The picture below is taken looking at a typical terminal from the end.

Most terminals are shaped to naturally feed the wire into the clamping mechanism properly as you can see from the picture. But it is possible to get on the wrong side of the clamping plate. Because the plate is held stationary by the plastic terminal housing, the tube moves up and down relative to the plate and housing (versus the plate moving relative to the tube).

Referring to the picture below, if you start out with the terminal tightened up with the bottom of the tube against the top of the plate, its possible to insert a wire, especially a small wire into the terminal between the outside bottom of the tube and the housing (circle A, area shaded red) rather than between the plate and the tube. As a result, when you turn the screw, the tube (circle B) moves away from the plate (circle C) and captures the wire between the bottom of the tube and the housing.

This is a poor connection at best, even though it might feel secure if you tug on it. The clue that you did it wrong is that you are turning the screw counter clockwise and its tightening the connection. Normally, you tighten a terminal by turning the screw clockwise and loosen it by turning it counter clockwise.

My approach to avoid this problem is to start out by turning the terminal screw counterclockwise until it bottoms out. Usually, I feel/hear the little “click” sound associated with the last thread hitting the little piece of metal that “captures” the screw in the terminal. Once I am at that point, I insert the wire and tighten the screw by turning it clockwise. After its tight, I give the wire a firm pull to be sure its fully engaged by the terminal.

Final Assembly

Once you have completed wiring the terminal strip, all that is left to do is to insert the sub panel into the enclosure over the mounting studs, bolt it in place by threading nuts, washers, and lock nuts onto the studs and tightening them down, and inserting the power cord and connecting it to the incoming line connections on the power supply. Don’t forget to connect the green grounding wire of the power cord to the grounding stud in the control panel.

To mount and connect the logger, I simply attached the adhesive backed magnetic square to the back of the logger, plugged the resistor cables into the 4 input jacks on the logger, and then “stick” the logger to an open area on the sub panel.

As a finishing touch, you may want to put a label on the cover, and maybe your business card so folks give you a call and find out what you are up to if they discover the panel in the field logging data.

In the next post, I’ll show you how I deployed the panel in the field and then share some of the data and insights I gleaned from my data logging experience.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Click here for an index to previous posts

This entry was posted in 4-20 ma Current Loops, Data Logging. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s