Chris Elston

Ground the 24VDC Power Supply Common?

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So I was wondering what other people do... Do you ground your 24VDC common of your power supply to earth ground as well as run a common wire out? Just curious what other people do and why? Would you if you had analog devices? Would you if you had cheap VFDs?

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Back in the day when we all used linear power supplies, the common was always grounded. Since we've moved on to switch mode power supplies, we always leave the common ungrounded, with a switched power supply you have true isolation. If you use NPN inputs, I recommend you NEVER ground the common. I have not grounded the common in many years, since I work at a chemical plant, we use a LOT of analog devices, I've never had an issue. If I remember correctly, we made the switch when we quit using ExP devices and went to Intrinsically Safe Devices, some of the barriers required ungrounded DC systems. However, we do not have any cheap VFD's, ( might have a few of questionable quality, but they were not cheap) but if you use good wiring practices, then noise is almost never an issue anyway.

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Agree with Ken whole heartedly.... (One of the dumbest things I saw was an AC neutral ran into the common input for the DC SINK module, and they wondered why it did not work, and why the inputs indicators were constantly dimming and not working)

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Back in the day, when I worked for an unnamed Oriental concern we used Toshiba and Omron PLC's and Tosbac Mainframes. The DC supplies were good old linear types and we grounded 24+. Yeah I know the American Standard back then was grounding 24- but we always grounded 24+. And yes a negative voltage was a logical "1". Talk about my world turning upside down when I went to work for one of old Henry Fords suppliear and entered the AB world. If I were to ground anything now it would be the 24-.

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Speaking of cheap VFDs... I had an old EMS drive die recently. I didn't realize that the EMS drives used DC common for their inputs until I went to put in the new Control Techniques drive which used +24VDC for inputs. I had to trace all the inputs (complex 2 drive system w/ no prints, changed out both drives) to ensure that I wasn't going to blow something up by changing polarity AND then figure out by the Carnac method how they did a reverse jog with 1 input. (Yeah, it was possible thru programming on both drive types!) Anyone have a cheap VFD that they like & does closed-loop torque control? (My next quest...)

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I've recently seen a 24VDC power supply not grounded on the -ve but with a fuse on the +ve and -ve. What's that all about?

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Either the power supply does not have internal power protection (rare these days) or they simply added a voltage deregulator!

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The PSU has long since been changed for a switchmode.

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There is a publication put out by Allen-Bradley called "System Design for Control of Electrical Noise" GMC-RM001A which states:- "Grounding the DC common of the power supply attenuates common-mode noise dramatically" They recommend grounding the DC common should be standard practice. It is quite a good reference document for noise considerations when designing electrical systems.
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I always ground the DC power supply for two reasons: 1) Safety - Most people think of 24VDC as safe (including myself) and have no problem touching anything with 24V. Now lets say, the common is not grounded and some where in the circuit a DC Cmmon wire comes in contact with a high voltage like 480V, a fuse will not blow nor a breaker trip. The next time someone touches +24VDC, they will find themselves in contact with anywhere from 300-500 Volts. 2) Testing convenience - If you are at a field device and need to test for +24VDC, but do not have a DC Common wire, just use a ground for DC Common.

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As I've read this topic and thought about the question, I think we must distinguish between Controls DC and Instrumentation DC. In Cotnrols DC it makes a lot of sense to ground the 24 - for the reasons listed. In Instrumentation DC there are an equal number of arguments (isolation, noise, cross-talk) for not grounding either side of a 24 vdc supply.

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Why would grounding the common attenuate noise? If I have an isolated 24VDC power supply I can only see grounding the common as introducing noise into the system. We have our unit installed at a factory that reads our signal with an Analog input module. There is some noise on the line and i believe its because they ground the 0VDC to the PLC. Does anyone have experience with this? Thanks, Omer Peleg DTI

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What about when you have both in the same cabinet? We've had issues where level switches would not act right until the power supply was grounded, plus we had analog signals being fed off the same power supply. Also we have had problems with noise on analog signals and would cause Horner RTD modules on a GE PLC to either go out or keep getting "loss of I/O" faults. On this case we had to isolate the ground buss that the shields and power supply ground were going to from the equipment ground.

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Let me offer my 2 cents. If you take a switching power supply with no load and measure from the case to either DC output terminal you will get about 60vac or (AC input)/2. When you install this ps in a panel this voltage shifts depending on the impedances of the circuit. If you unground this power supply then you run a risk of exceeding the common mode voltage limitations of analog circuits. If you ground this power supply then you are attenuating this common voltage problem by keeping the dc- of the supply at the machine ground potential. This can cause a problem with small signal (millivolt) measurements. What I have been doing is using a linear supply for analog and a switching supply for control. I know this is controversial in some circles but if the measurements are critical then this is the solution I have found to be the most effective. I have a machine I was working on today that we decided to remove the switchmode supply and run only a linear supply. The linear supply has more heat gerneration due to the lack of efficiency but it is the superior solution given the requirements. The majority of installations will work fine with switching supplies. In answering your question: Linear supplies: ungrounded Switching supplies: grounded
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2 parts to this: I work with genertaor systems all the time. The battery supply negative is grounded - it supplies the hard wired stuff and generator controllers. Secondary supply to the PLC is through a 24VDC/24VDC switch mode power supply. Negative is NEVER grounded, for isolation purposes. Battery chargers can go high, surges damage the PLC etc. The switch mode affords protection to the PLC but if the negative is grounded there is no isolation between one side of the switch mode and the other. May as well not use one.

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Point well taken! I was addressing a typical manufacturing environment. However, we also have battery operated equipment with instrumentation. These systems are all isolated from the frame including the battery. In your case with a generator you have to deal with a whole host of issues most of us are lucky enough to avoid. For the benefit of other readers here, be careful when selecting a power supply when isolation is needed. Not all supplies provide this and simply not grounding one of the outputs does not give you isolation from the inputs. Even transformer supplies have isolation issues. If all you need is DC isolation then most transformer supplies will suffice. If you need AC isolation then look into ferro-resonant supplies. They are more expensive but if the application requires it they are a good solution.

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One other thing I might add with respect to not grounding the negative to a PLC - NPN inputs - grounding an input wire in the field can turn the input on - if it is not grounded there is no ground reference and the input will not inadvertently turn on. Typically found on Japanese machines. I normally use PNP inputs and do not really care if the output is NPN or PNP. Usually use NPN outputs.

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If I rem correctly it is required by the NEC to always ground the secondary on any power supply or transformer 120 volts and below.  My old boss who slept with the NEC code book under his pillow (Jim Grace) stressed this point with me back in the day.

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Interesting stuff, I have recently been looking into this.

With regards to what arj3090 said - if it is unsafe to leave 0VDC ungrounded  then can someone explain why it is not against regulations etc?

Edited by spitzy

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On 10/10/2008 at 7:43 AM, Chris Elston said:

So I was wondering what other people do... Do you ground your 24VDC common of your power supply to earth ground as well as run a common wire out? Just curious what other people do and why? Would you if you had analog devices? Would you if you had cheap VFDs?

Sir, there are 5 smps in PLC control panel.why?

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