# Grounding Control Power



## macmikeman (Jan 23, 2007)

I tried a quick search thru my nec looking for a statement about not grounding the secondary side of small class 2 and 3 transformers but couldn't find it now. 


Famous Saying: "I know it's in there someplace"


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## GatorIE (Mar 22, 2020)

I'm assuming you're working with industrial equipment--I usually therefore reference NFPA 79 for questions like this. 



> 8.3 Control Circuits. Control circuits shall be permitted to be
> grounded or ungrounded.
> 8.3.1 Where grounding is provided, that side of the circuit
> common to the coils shall be grounded at the secondary winding
> ...


The important thing to consider is the reference to Section 6.4, Protection by the Use of Protective Extra Low Voltage (PELV) or Class 2 Circuits. My interpretation of that section leads me to believe that all DC commons in low-voltage machinery should be grounded, which I execute in all my work. In most cases, this is the 0VDC (-) side of the power supply, assuming sinking inputs and sourcing outputs. One curious case is the use of sourcing inputs and sinking outputs, in which the 24VDC (+) side of the supply should be grounded, as it is the side of the circuit common to all control coils.

Another exception I often see made (and for which I'm not sure a code basis exists) is when a separate power supply is used for networking devices on weld cells. The DC common is frequently left floating on these cells, because it is believed to reduce damage to networking equipment.


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## MikeFL (Apr 16, 2016)

GatorIE said:


> ... The DC common is frequently left floating on these cells, because it is believed to reduce damage to networking equipment.


To reduce noise.


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## Going_Commando (Oct 1, 2011)

I don't do much with 24v control power, but I've worked in multiple places where they didnt ground the 120v control and its a b*tch trying to troubleshoot push buttons, and stuff when they aren't grounded and there's no neutral nearby.


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## joe-nwt (Mar 28, 2019)

Going_Commando said:


> I don't do much with 24v control power, but I've worked in multiple places where they didnt ground the 120v control and its a b*tch trying to troubleshoot push buttons, and stuff when they aren't grounded and there's no neutral nearby.


There would be no neutral at all but you could temporarily make one with an alligator clip jumper.


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## gpop (May 14, 2018)

I was taught that if you have more than one dc power supply you need to connect all of the DC- together. I tend not to ground the dc- as you are basically making a ground path which is the whole reason you avoid landing both sides of the shield. 

As its Florida both + and - are going to have a bunch of suppressors to avoid either side exceeding 30v to ground (grounding negative would make that kinda pointless on the negative wires)

I have had engineers insist on negative being grounded and it doesn't bother me enough to put up a fight so i ground it out when its requested. I also will add a jumper when im troubleshooting just to make life a little easier.


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## didntdoit (Jun 18, 2020)

Okay, back to the question, I don't believe lightning would be a problem grounded vs ungrounded. Not so sure about power surges, but the lightning bit got me asking about localized, not wearing anti-static gear before touching electronics (yeah we never do that).
The chance that a static charge could go to ground rather than though the equipment maybe I re- think the ungrounded setups.


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## gpop (May 14, 2018)

splatz said:


> I had a system where the 24vdc control power was floating, but I had what appears to be a ground loop issue on the communications between the sensors, which went away when I bonded the 0vdc terminals of the control power supplies to ground. I'll try and go back and connect the dots and figure out what was happening there but I am not sure I'll find any other way to fix it.


Ive come across a few problems where a bad HPS light would affect the sensor reading. Trend is butter smooth then the outside lights come on and suddenly the trend jumps then stays smooth but out of calibration until the lights turn off. Fixing the lights and cleaning the grounds took care of the problem.

I have also had a rtd - loop powered hockey puck go nuts when any 120v starter pulled in (clean power supply etc). we tried everything including complete isolation on a different dc power supply, grounding. etc but some thing around 30-40khz would mess with the sensor but it would not mess with the fluke on source. We ended up changing the sensor (same problem) then added 0.3 seconds dampening and the problem went away.


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## cuba_pete (Dec 8, 2011)

I had an issue a few years back where I was getting artifacts in my data. The artifacts were from the singing the SMPS was doing in a VME chassis from time to time, right around the primary switching frequency, but in a sub-harmonic range.

Anyway...I contacted the mfr. to verify I could ground the -VDC to the VME backplane from the internal power supply and did so, then physically insulated the SMPS metal casing from the VME chassis. This gave the chassis the effect of shielding to the DC output side. I added an extra ground bar to the 19" rack and extended the EGC to the main ground and the DC ground to the same plate on its own yellow wire ground. Voila, I achieved about 6-10 dBV of quieting of the SMPS noise, which was more than I needed to remove the artifacts from my data.

As it turned out a majority of the noise came from the 200 kHz timing function from a high-speed data bus daughter card on one of the main boards. The image plane was carrying and radiating the clock rate since the entire card nest used the same ground reference.

There was a little more to it, measurements and readings and whatnot, trial and error in equipment bonding locations and such, all in keeping with primarily NEC and applicable MIL STD and IEEE references.

Lightning protection is taken care of external to the cabinet and conforms to NFPA 780, MIL STD and other guidance, and surge protection is provided in each PDU for the cabinets. Each PDU typically uses MOVs but rarely see duty.


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## Bird dog (Oct 27, 2015)

cuba_pete said:


> I had an issue a few years back where I was getting artifacts in my data. The artifacts were from the singing the SMPS was doing in a VME chassis from time to time, right around the primary switching frequency, but in a sub-harmonic range.
> 
> Anyway...I contacted the mfr. to verify I could ground the -VDC to the VME backplane from the internal power supply and did so, then physically insulated the SMPS metal casing from the VME chassis. This gave the chassis the effect of shielding to the DC output side. I added an extra ground bar to the 19" rack and extended the EGC to the main ground and the DC ground to the same plate on its own yellow wire ground. Voila, I achieved about 6-10 dBV of quieting of the SMPS noise, which was more than I needed to remove the artifacts from my data.
> 
> ...


Starting to sound like the problems, workings, design etc of radio ckts and equipment. :surprise:


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## splatz (May 23, 2015)

gpop said:


> I was taught that if you have more than one dc power supply you need to connect all of the DC- together.


Now that you mention it this might be what fixed this particular issue. With IO coming from devices on identical but separate power supplies, you have to get a common reference voltage for the IO. Is there a palm to the forehead smiley?


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## GrayHair (Jan 14, 2013)

splatz said:


> ... Is there a palm to the forehead smiley?


I've looked for one often. :vs_laugh:


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## Bird dog (Oct 27, 2015)

....


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## didntdoit (Jun 18, 2020)

GrayHair said:


> I've looked for one often. :vs_laugh:


 Anyone ever wonder when a simple fix isn't working and then realize the fuses are still in your pocket? :surprise:


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