Centroid Community CNC Support Forum

I just received my Acorn Kit and noticed that the Meanwell PS came pre-wired with the ground connection jumpered to common. I’ve not seen that on any of the schematics I’ve looked at. This means the power supply is not floating and is hard referenced to chassis ground. I think I read somewhere that the common is tied to ground on the Acorn board so this would essentially be a redundant connection. Does anyone know why Centroid did this?

This jumper is shown on all current Acorn schematics. search "acorn" https://www.centroidcnc.com/centroid_di ... browse.php
to see all the latest Acorn Schematics.
the jumper was added to protect the Acorn and Beaglebone from damage caused by an electrical mismatch between the common and ground connections leading to unwanted current flow through the boards.

Thanks. I did notice the jumper on the latest schematics. I was looking at the bench test schematic that came in the box, which didn’t show the jumper. I’m learning to look better before I post.

Tying earth ground to power supply common has tripped up a lot of us. For decades I kept the two separate. I also kept commons separate on each side of an opto-coupler circuit. One side would have a 5VDC power supply with its common/ground and the other side would have a 12vdc or 24vdc power supply with its common/ground. I did not connect the commons together, thinking that doing so would defeat the purpose of having a opto-coupler isolation circuit.

One day I called Geckodrive and talked to the owner (Marris or Marrias). He's a great guy and extremely knowledgeable. During that conversation, he mentioned grounds, including earth grounds. He said to tie them all together IF all power supplies originated at the same breaker box. He reminded me that earth and neutral are tied together in that breaker box. He said the same method is used for DC power supplies. IF they all receive their power from the same AC breaker box, then tie AC earth and DC common together. HOWEVER, if the breaker boxes are in different locations and each has its own grounding rod driven into the ground in different locations, then code requires that the breaker boxes be wired differently. In that case, different rules apply.

There are some very informative videos on YouTube that explain what can happen if AC breaker boxes that span a wide area and that have separate grounding rods are connected together. In essence, due to the soil structure, moisture content, etc., those grounding rods might have a significant resistance between them - meaning that they would not be at the same ground potential. Not being at the same ground potential means that the source of least resistance (possibly YOU) will become the path electrical current takes during a fault condition.

So, for a typical control box that is fed from a single AC breaker box and all external switches, sensors, and accessories receive their power from within that single control box, its best practice to tie AC earth to DC common.

Richards wrote: ↑Tue Oct 19, 2021 7:25 am
Not being at the same ground potential means that the source of least resistance (possibly YOU) will become the path electrical current takes during a fault condition.

This is a good example of the issue. We also don't want ACORN to become the path of least resistance. This is why the jumper was added. The chassis and power supply ground are supposed to be about the same level, but they are not hard wired together in ACORN. Ideally, the grounds are only connected together at a single point to avoid ground loops.

eng199 wrote: ↑Tue Oct 19, 2021 9:04 am This is a good example of the issue. We also don't want ACORN to become the path of least resistance. This is why the jumper was added. The chassis and power supply ground are supposed to be about the same level, but they are not hard wired together in ACORN. Ideally, the grounds are only connected together at a single point to avoid ground loops.

There is a lot of confusion about how AC Earth should be wired. We buy a metal case that has a grounding lug. We buy a Disconnect Switch so we can easily disconnect AC power at a single point. The problem is that in the cases that I buy, the ground lug is at the botton of the case and I mount the Disconnect Switch at the top of the case. Those two points are about two-feet apart. Here's how I handle the situation. I connect the AC power leads to the Disconnect Switch (L1 and Neutral for 120VAC. L1, L2 and Neutral for 240VAC). By doing that, when the Disconnect Switch is OFF/OPEN, no AC power enters the enclosure. I mount a DIN rail close to the Disconnect Switch. On that DIN rail, I mount a DIN Grounding Terminal Block (Green/Yellow) that is electrically connected to the DIN rail. I connect the AC Earth conductor to the top terminal on that terminal block. I connect a Green/Yellow wire that is the same gauge or heavier between the bottom terminal of the DIN Grounding Terminal Block and the grounding lug at the bottom of the enclosure. I verify that the grounding lug is bare metal so that paint does not act as an insulator.

At this point, the entire metal case is at ground potential.

I connect the AC Earth conductor of ALL power supplies to that grounding lug.

If you've followed this far, you might say that the DIN Grounding Terminal Block is about two-feet from the ground lug, meaning that there are two AC Earth ground points to the case. If that bothers anyone, they can use a standard DIN Terminal Block that is not electrically connected to the DIN rail to extend the AC Earth conductor from point of entry to the ground lug. In my experience, the entire case, including the sub-panel are at the same potential, i.e., measuring any two points anywhere on the case or case to sub-panel shows zero resistance.

After all of that is done, we still have to deal with bonding AC Earth to DC ground/common. Here's how I do it. I mount several DIN terminal blocks together to act as DC ground/common, and several DIN rail terminal block together for each voltage, i.e., 5VDC, 24VDC, 75VDC. Each set of blocks is jumpered together, i.e., all the ground/common terminal blocks are connected by jumpers. All the 5VDC terminal blocks are connected by jumpers, etc. There is one more connection to make. I run a conductor from the grounding lug to the DIN ground/common terminal block array. Now, AC Earth and DC common(s) are electrically connected. Chassis ground and DC ground are electrically connected. There is no resistance (meaning that there will be no voltage and no shock hazard or arcing between AC Earth and DC ground/common anywhere in the control assembly). It also means that the Ground terminal and the Chassis/Earth terminal on the Acorn will be at the same potential as the ground lug on the case and the DC ground/common on the power supply whether you use one wire from DIN common ground terminal blocks, or two wires, one from the grounding lug and one from the DIN common ground terminal blocks.

(I didn't mean to hi-jack this thread. I'm just trying to pass along some common practices that I follow when designing, building, and installing process control equipment.)

Normal convention for 120V single phase is the neutral should not be switched.

thanks, the printed bench test 'start here" sheet that is included with the kit has been updated.

https://www.centroidcnc.com/centroid_di ... t_here.pdf

tblough wrote: ↑Tue Oct 19, 2021 3:48 pm Normal convention for 120V single phase is the neutral should not be switched.

Tom,
We agree on this, but probably for different reasons. A typical circuit breaker in a power panel switches only the Line voltage. The neutral and the earth are bonded together. In that situation, if someone wired their circuit to switch the neutral instead of switching the Line voltage, then, in their circuit, the device would always have voltage applied. If they touched the device, they would become the path to ground.

A disconnect switch is used to disconnect something completely from its power source. The disconnect switches that I buy are meant for 3-phase power. Three phase power has L1, L2, L3, and earth. There is no neutral line. Single phase 240VAC in North America has L1, L2, Neutral, and earth. L1 and L2 are each 120VAC in relation to the neutral line. L1 and L2 have opposite sine waves that create 240VAC RMS when used together. Many devices that are wired for 240VAC don't use the neutral line. They treat 240VAC as if they were in Europe or South America where L1 is 220VAC. For instance, my IPC-5 power supplies only use L1, L2, and earth. Both L1 and L2 must be switched for safe operation in North America. In that particular case, neutral is not used. Single phase 120VAC has only L1, Neutral, and earth. In a properly wired circuit, only L1 carries line voltage. In that case, if L1 is switched open, all power is cut.

I don't have 3-phase power. When I need it, it is created from 1-phase with a VFD. Given that information, at the power panel, only L1 and L2 are connected to a circuit breaker. In a 120VAC circuit, L1 is used. In a 240VAC circuit, both L1 and L2 are physically connected together such that if either circuit breaker opens, both open. That is required by code.

At the CNC controller, I wire both L1 and Neutral through the disconnect switch. If the switch is opened, all power to the enclosure is interrupted. I do that to ensure that if someone were to improperly wire the plug end of the power cable going to the wall outlet such that L1 and Neutral were reversed, that power would still be turned off at the box when the disconnect switch was opened. It's my fail-safe way of assuring that a wiring error outside of my control will not create a shock hazard.

We'll have to disagree on what's safer. NEC specifically states neutral conductors must not be switched and only gives very specific exceptions.