Tying 24VDC- to Ground

[Lancie1]

I don't see how tieing 0V to the ground is wrong.

if in the panel wires are labeled 0VDC and +24 (or -24VDC),
i expect to see difference of 24V (result could be positive or negative value depending on reference
and meter polarity).
But if the terminals or wires are labeled +24 AND -24V, the difference should be 48V (+48V or -48V).

Technically, the two leads on a 24 vdc power supply are not +24 volts and 0 volts, unless it is a 3-wire supply with a common 0 point. One of the leads ONLY becomes 0 volts if you tie it to a 0 volts reference point, such as a "ground". Then of course you have +24 and 0 volt busses. That is why many of you refer to the minus power lead as 0 VDC, because you always tie it to ground. But what if you do not tie it to ground? Then you cannot correctly refer to it as "0 volts".

These semantic problems are why Panic Mode thinks it is incorrect to label 24 vdc system wires "+24" and "-24". I think that is no more incorrect than referring to an ungrounded 24 volt system as "+24" and "0". Neither method exactly tells the whole story, but also, neither labeling method is likely to lead to any shutdowns.

As for the +/-24 vdc labeling, I always thought that the 24 referred to the system level (24 volts) and the + and - then told you the polarity. Unlike Panic Mode, I have never took these labels as an attempt to show the actual voltage between wires. As RSDoran pointed out, in an ungrounded 24 vdc system, the actual voltage of each wire would technically be +12 volts DC and -12 volts DC (with a voltage difference between wires of 24 volts), so none of the methods discussed would be literally correct. Then, if we follow Panic Mode's reasnoning, we would have to label to show the actual voltage DIFFERENCE, so we would have "+12 vdc" and "-12 vdc" wires. Wait, would not that confuse everybody that has become used to seeing "+24" and "-24"? Now they will think the plant has suddenly reduced its DC power to 12 volts!

The question is, does any of this matter a hill of beans?

 

[panic mode]

As RSDoran pointed out, in an ungrounded 24 vdc system, the actual voltage of each wire would technically be +12 volts DC and -12 volts DC (with a voltage difference between wires of 24 volts

sorry, you lost me. where is all this comming from? how can one put number on something that has not been referenced? Why +/-12 and not +2 and -22? how is +/-12 technically correct?

if floating PSU output is necessary, what is the problem to pick reference point and label it COM?

 

[Tom Jenkins]

"COM" is very misleading. A common can be 120 VAC if it is the common on a limit switch. Neutral can be common on AC powered devices. We usually label "24+" and "-".

 

[Lancie1]

how can one put number on something that has not been referenced? Why +/-12 and not +2 and -22? how is +/-12 technically correct?

V = I R
where V is the potential difference between two points which include a resistance R. I is the current flowing through the resistance.

So, the potential difference between the two legs of our 24 vdc power supply is 24 volts.

If we set up a way to measure the voltage across each leg of our power supply circuit with a meter, then the total current would be I, and the resistance RT, so 24V= I x RT, or RT = 24/I. Becasue we assume that both legs are the same length and same resistance, then one leg's resistance would be 1/2 of the total, or RT/2 = 24/2I. Because the resistance is 1/2 of the total circuit resistance, then the voltage across that resistance must also be 1/2 of the total, Vleg = I x RT/2 = 12 volts.

In reality, there will be small differences in the length and resitances of the legs, but does it matter when it comes to labeling the wires? All I am saying is that your argument for using labels "+24" and "0" needs to be recognized for what it is, an obsession that is no more correct than any other commonly used method.

 

[John Hawkins]

zero volts

+ - "0" volts

many op amps use a bi-polar ps therefore I make my case of "0v"
0v is the reference for the potential difference "period"
we plc people are just switches & lites. In high gain analog ckts. the most important thing is WHERE 0v is connected. IE: at the input or at the output. As an analog designer I have to have 0v connected to chassis. In the case of switches & lites how can it matter!

 

[brucechase]

Sorry Lancie1, I don't get it. Where are you measuring the voltage at? You need a potential between 2 points to measure voltage. When you say across each leg, what does that mean. There is no way to guarantee 12 volts across each leg. If you are measure to a powersupply that has a common point - OK, but if you are measuring an "UNGROUNDED" powersupply to ground, there is no guarantee what you will get. If you want to talk about capacitive coupling of the wires to the grounded plane, then you will still not always get a divided voltage.

 

[Lancie1]

Like I said, I am measuring across 1/2 of the total load resistance. The actual load can always be replaced with two resistances equal to 1/2 of the total, so the RT/2 will be the sum of 1/2 of the load + the wire resistance, and no, I am not talking about relative to "ground". We are talking theoretical voltages here. Your point and my point are really the same: It is no more correct to label power supply wires "+24" and "0 volts" than it is to label them "+24" and "-24".

 

[brucechase]

Lancie1 - Got it! I really don't label my either way. I take the wire number from the line on the print reference and that becomes my + and -. For example, if the PSU is on line 130 then the + is 1301 and the - becomes 1302. There are many of my cabinets that have more than 1 power supply so this way makes sense to me. If I had to label it at something just for the sake of this long drawn out thread, I would label it as +24 and -24 because that is what it is marked on the terminal of the powersupply. If the electrician troubleshooting the system expects to see 48 VDC from the + to -, then we have much bigger problems then what to call the wires.

By the way, I don't ground the powersupply. (back to the original post)

 

[panic mode]

As Tom pointed out common can be confusing. This is why on rare occasion of ungrounded PSU I was using labels like DC_COM and 5COM. I do like the idea of using sign after value.


Lancie1
You can label your wiring any way you like or any way you find common or more appropriate.
Between "+24, -24" and "24+, 24-" I would prefer later one for a single output, nongrounded
24V PSU (obsession I guess...), while our service department will rather know if it was first one.

Btw. I didn't see your opinion about original question (grounding PSU).

 

[MarkTTU]

Just to beat a dead horse a bit...

As an analog designer I have to have 0v connected to chassis.

0v measured from chassis to where?

 

[John Hawkins]

op-amp connected to a +15 "0" -15 vdc ps. "0" v is connected to the chassis at the output. The point of the circuit that consumes the most current. otherwise the input stage will amplify any voltage drop across the chassis even if it is in the microvolt range IE: the first stage may have a voltage gain of 1000 and therefore the amp will have an output of 1 mV -60 dB, when it should be - 80 to -100 dB

 

[MarkTTU]

Fair enough; like I said I knew I was beating a dead horse, but when speaking of voltages I've found its always good to be very explicit with defining at least two points...

 

[agarb]

Do you ground? Do you use PanelView+?

Well, here we normally gound the low supply of our DC supplies.

But, at the last place I worked, we didn't.

----

I was just reading A/B publication #2711-TD001, "Wiring and Grounding Guidelines for PanelView Plus and VersaView CE devices."

On page 19, it states, "Most PanelView Plus terminals have dc- connected to earth ground internally in the product. Circuits powering these terminals must not have another dc- point connected to earth ground. Otherwise, the dc- current will flow through the earth-ground wiring and earth-ground structure. Current flowing through earth ground can damage the terminal."

Sure enough, I popped the wires off the PanelView+ 600 at my desk. My Fluke shows very little resistence between the dc- and earth ground terminal.

Hmm, so lets assume one is in the habit of grounding the low side of the DC power supply at the supply. Other than adding a separate, isolated, power supply is there any thing else a person can do?

Everybody always talks about how power flowing through a ground loop is bad, but in this case how likely is it to damage a PV+?

Aaron






​

 

[John Hawkins]

Grounding 0 volts

Agarb

The most important thing is only one device is to be connected to chassis-frame-ground. If the ps unit has it's - terminal connected to it's chassis then when it is fastened in the ordinary way, - is grounded This would be correct as it is the OUTPUT that is grounded. Now the output via 2 wires is connected to unit A which is in turn connected to unit B which in turn is connected to unit C which ............... NO other unit should should have the - terminal connected to chassis-frame-ground. Another person " +&-
that is how the terminals on the ps are marked". However on all prints, wire markers &so on in my opinion should be referenced to as "0 VOLTS", not -

 

[vpaul]

May I ask about a sceneio?

I know this is an old thread. I am hoping to get proper information about my project. This thread is the closest to the information I have searched for on the internet.

This is an outdoor project. Please tell me if I am out of place here.
I am putting in a wind sensor that runs off of a 24 VDC power supply and rated at 25mA.
I wish to Not run additional wires from my front to back yard. I have a spare wire (just one) in my sprinkler watering valve box that may be handy to use. The sprinkler valves run off of this box controller rated at 25.5 VAC (AC) and the documentation states 24 VAC.
I am hoping to use the one existing wire to supply the voltage to the wind sensor but I am unsure about what to do for a ground (return path). I can plug in the 24VDC power supply and connect it conveniently in the same box as the sprinkler controller as the is a wall outlet available.

So I guess the delema for me is what to do for a ground and not blow up the entire systems.
Questions....
1. Is it possible to use the ground for the sprinkler valves 25.5 or 24 VAC to ground the wind sensor 24 VDC? What if a valve is operating at the same time?

2. Can I some how use Earth ground right inside of the sprinkler valve box? It's a pretty damp area 100% of the time.

3. If I can use Earth ground for the 24 or 25.5 VDC, Is there a fairly simple explanation of how that works to further my understanding? I know that the house power is earth grounded and it seems to me to be 120V and 240 V. My imagination leads me to thing 120 or 240 V ground is much different than 24 V.

THANKS so much in advance for any assistance and answers.