This may just be me not having a degree in electrical engineering or electronics, but the whole notion of "ground" and "earth", when used in electrical circuit diagrams (especially integrated circuits), is extremely confusing. I guess the whole notion of current "comming from" the positive terminal (which is often how current seems to be described) seems backwards and misleading to me, given the quantum mechanical description of electrical current as the flow of electrons. So, I'd just like to clear up my understanding of things.

First things first...to make sure my understanding of voltage and current is correct. Assuming a direct current context (I understand things are more complex when using alternating current, and I understand that it is possible to have ground at a positive terminal in some systems and things like that.)

A. The positive terminal in a circuit is what creates voltage. Voltage is a potential, so given that it is the positive ions in, say, a battery, which are generally fixed in place, it makes sense that the + terminal in a circuit would create voltage.

B. The negative terminal in a circuit is what provides current. Current is the flow of electrons, and that flow is towards the terminal that is creating the potential for current.

Assuming these statements are true...then why is the term "ground" (primarily) or sometimes the symbol for "earth" used so extensively in electrical circuit diagrams? Why is it ground or earth, rather than just a negative terminal, or a 0V terminal, or maybe just a "common" terminal? The use of ground or the earth symbol, particularly in IC circuit diagrams (which are not necessarily used in circuitry that is even remotely capable of being "grounded" to the earth...such as in an airplane or a spacecraft, or even any number of isolated, insulated systems that cannot be directly connected to the earth), is extremely confusing to me.

Is this just some old convention that hasn't ever been broken? Is ground (the GND terminal) or the earth symbol in a circuit diagram just a thing that's done, because that is always how it's done? Because that's how it's always been taught? Does it really just mean a negative terminal, or a terminal from which electrons flow? When is the use of a literal ground, a point where a circuit actually connects to the literal earth, actually required? It seems clear that not every circuit, like an IC, does not actually need a literal connection to the earth in order to function.

Well, sorry if this is an odd question, however as I play more and more with electronics, and since I'm powering most of my little projects with batteries, this whole concept seems odd and confusing to me...there is no literal "ground" or "earth" involved in the circuit. Only the battery terminals and electronic parts.

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    \$\begingroup\$ Very similar question (almost duplicate): Understanding ground symbol. \$\endgroup\$ Commented Aug 3, 2014 at 23:37
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    \$\begingroup\$ Yeah, I found that question before...it doesn't really answer my question, though. It just states that ground (whatever that is...which is the heart of my question) can be moved around a circuit and still perform the same job. \$\endgroup\$
    – jrista
    Commented Aug 3, 2014 at 23:42
  • \$\begingroup\$ You can erase the ground symbol from (-) side of a battery and re-draw in on the (+) side. The electrons will keep flowing in the same way. The circuit will function in the same way. In most cases, the notion of ground is just an engineering shorthand. It's a very common shorthand, and a useful one, and everybody is used to it. (Your question does not deal with electric shock hazard and such. Rather, you're grappling with fundamental understanding of the ground symbol, I think. In the mains AC wiring, "earth" has a specific physical meaning. But I'm not going to go there.) \$\endgroup\$ Commented Aug 3, 2014 at 23:57

6 Answers 6



First, currents don't "come from" the positive terminal. That's a very common misconception, an error called the "sequential fallacy" appearing widely in grade-school electricity textbooks. The basic problem is that wires are not like empty pipes. And, power supplies don't fill them up. Instead, wires are already pre-filled with charge, so that currents always appear everywhere in a circuit, all at the same time. ("Current" means charge-flow. When a circle of movable charges starts flowing, "current" appears in the entire ring. That's the basic circuit rule.)

In other words, electric circuits behave like flywheels and drive-belts. In the same way, the metal of a bike-chain doesn't "come from" a particular location on the sprocket. It doesn't "start out" at one point. Instead, the entire circle is made of chain, just like the entire circuit is made of movable electrons. Also, all the chain was there before any power supply existed. With bike chains, when a force is applied, the whole thing turns. With circuits, when a potential difference is applied, all the movable charges inside the ring (inside the circuit,) they all start moving as a unit, like a solid chain going in a complete circle. But those charges were already inside the wires before any battery was connected. Wires are like water-filled hoses.

Second, electric potential can only exist between two points, and one single spot on a circuit never "has a voltage." This is true because voltage is a bit like altitude: an object cannot "have an altitude," since height can only be measured between two points. It's meaningless to discuss the height or elevation or altitude of an object. Altitude above what? Above the floor? Above the ground outside the building? Altitude above Earth's center? Any object will have infinitely many altitudes at the same time!

Voltage has exactly the same problem: one terminal can only "have a voltage" when compared to another terminal. Voltage acts like distance: voltage and distance are double-ended measurements. Or in other words, one terminal in a circuit always has many different voltages at the same time, depending on where we place the other meter lead.

Third, in circuits the driving force is provided by the positive and the negative power supply terminals, both at the same time. And, most important: the path for current is through the power supply. Power supplies are short circuits. An ideal power supply acts like a zero-ohm resistor. Think about it: in a dynamo coil, the charges pass through the coil and back out again. The wire has a very low resistance. Same thing with batteries: the path for current is through the battery and back out again. The battery plates are shorted out by very conductive electrolyte.


  • Here's a correct description of a flashlight. The charges "start out" inside the tungsten filament. When the switch is closed and the circuit is complete, one end of the filament gets charged positive, the other negative. This forces the filament's own charges to start flowing. The charges move out of the filament and into one wire, while at the same time, more charges are coming into the other end of the filament. These charges are supplied by the metal wires (and, before the switch was turned on, all the conductors were already full of movable charges.) Continuing, the charges that were in the filament will flow out into one wire, move slowly to the battery (takes minutes or hours to get there,) then flow through the battery and back out again. They exit from the battery's other terminal, flow back to the other end of the filament, finally ending up where they started. A "complete circuit." The charges are like an endless drive-belt, or like a rotating flywheel or a bike-chain. The battery pushes the charges, but it doesn't supply the charges. The copper and the tungsten supply the charges which flow in the flashlight circuit. The density of charges throughout the circuit is constant (like a water-filled pipe with no bubbles.) Charges move quite slowly, but since they all start moving at the same time, the light bulb lights up instantly, even if the wires are quite long.

Fourth: any positive ions inside a battery are extremely movable. They're certainly not locked in place. If they were, then batteries would be insulators, and wouldn't work. Some batteries are based on the flow of positive ions in one direction and negative ions in the other. Lead-acid batteries are different. In the acid, only the protons are flowing. Acids are proton-conductors.

But beware: batteries give added complexity which can derail an explanation.

Instead, replace your flashlight battery with a big coil, and a supermagnet. Connect it to the light bulb. Shove the supermagnet into the coil, and the light bulb flashes briefly. Where did the charges come from? How can a moving magnet create charges? IT DOESN'T. Dynamos and batteries are charge-pumps. The moving magnet forces the wire's own charges to start moving. (A pump does not supply the stuff being pumped!) The moving magnet causes a current, because it applies an EM pumping-force to the movable charges already inside the metal.

Bad conductor. Bad!
Here's a clarification. Many intro textbooks provide the wrong definition of "conductor;" totally wrong, and extremely misleading. They'll teach you that conductors "let charges pass through" (or, that 'electricity' passes through, or 'current.') Nope. Conductors aren't like hollow pipes. Conductors aren't transparent to electricity. Instead, the word "conductor" actually means "a material which is full of mobile charges." Conductors are like tanks full of water. They're like aquariums, or like pre-filled pipes. Conductors obey ohm's law: whenever we apply a voltage-difference to the ends of a wire, the flow of the conductor's own charges depends on the wire resistance: I = V/R. It is the wire's mobile charge which does the flowing. Think about it, vacuum is an insulator. How can vacuum block the flow of charges? Vacuum does not need to, as there are no movable charges present in a vacuum. That is what makes it an insulator.

All of this leads to an important concept. Whenever we take a piece of wire and hook the ends together to form a closed loop, we've created an "invisible drive-belt," a loop of movable charge inside the non-moving wire. Thrust a magnet-pole into the metal loop, and all the charges of the wire will move as one, rotating like a flywheel. It's a ring-shaped swimming pool, and if we push on the water, we can get all the water turning like a flywheel, while the swimming pool itself remains still.

FIFTH, currents aren't backwards, because electric currents aren't flows of electrons.

Specifically, the polarity of the flowing charges depends on the type of conductor. Yes, in solid metals, the movable charges are electrons. But there are large numbers of conductors where no electrons can move. The closest ones are your brain and nervous system: simultaneous flows of positive and negative atoms in opposite directions: moving ions, with no electron-flows at all. The "Electrolytes," salt water including the damp ground and the oceans; these are not electron-conductors.

Weirder example: acids are conductive because they're full of +H positive hydrogen ions. Another name for an +H ion is... "the proton." When we put some amperes through acid, the current is a flow of protons. (Heh, if there's any ground-currents in the dirt, and also the dirt is acidic rather than salty, then those underground currents are proton-flows! )

In other words, "amperes" can be electrons flowing, or protons flowing, or positive sodium passing through negative chloride going the other way. Or, fast electrons going one way in a spark, while slow nitrogen ions go forward or back depending on whether they're pos- or neg-ionized. And in p-type semiconductors, the current is a flow of valance-band electrons, the "lattice vacancies" in the crystal! (Each vacancy exposes an excess silicon proton, so the vacancies each carry a genuine positive charge. "Holes" move by electron-transfer, yet each hole really is positively charged.)

With all the above complexity, how can we possibly describe what's happening inside circuits? Easy: it's already done for us. We conceal the moving charges and ignore them. We ignore their flow-speed, and their quantity. We ignore their polarity. Instead we add up all the various charges which might be inside any conductor, calculate the total flow-rate, and call this "amperes." Is your conductor a hose full of salt water? Put a clamp-on ammeter around it, and read off the amperes. The ion density doesn't matter. The ion speed doesn't matter, and it could even be an acid-hose full of protons, instead of a seawater-hose.

Amps is amps.

Amperes are also called "conventional current," or just "electric current."

Very important: amperes are not charge-flow. A conductor might have one amp, but this doesn't tell us anything about the charges inside. There could be a few charges flowing fast, or lots of charges flowing slow. There could be positive charges going forwards, or neg going backwards, or both at the same time (as with human bodies receiving DC electric shock.) All that stuff is covered up, and all we have left is the amperes ...amperes of conventional current.

OK, back to GND versus COM versus EARTH.

"Ground" is confusing because the word is nearly always used incorrectly.

In circuits, we almost always choose one power supply terminal to be the "common," and we connect one voltmeter-lead to it. It's not grounded, so we really shouldn't call it "ground" (it's not connected to a metal stake driven into dirt!) Instead the "common" is just the traditional point for making voltage readings. We never explicitly explain this fact (it's a silent agreement!) Since voltages are complicated double-ended measurements, things are simplified if we pretend that they're single-ended. So, hook your black voltmeter lead to the "circuit common," then ignore it.

Now pretend that the red-colored probe on your voltmeter can actually measure the voltage OF A TERMINAL. But terminals can't "have a voltage!" Yes, exactly right. But we silently pretend that they do. Any point on the circuit can have a voltage ...in relation to another circuit point. If we were talking about altitudes, we could always make our measurements in relation to sea-level. Next, never mention sea-level, then finally pretend that objects and locations can "have an altitude," when actually that's impossible, since altitude is a length; a distance and not a location.

So, all the new students typically get confused when we discuss the "voltage of a terminal." Actually we meant "the voltage that appears between a terminal and the Circuit Common." But that's too much to repeat all the time. We're silently saying "voltage between, voltage between," while we actually say "voltage at this spot," or at that other spot over there. Then all the new students start thinking that one single terminal can have a voltage, even though voltage doesn't work like that.

Is our negative supply terminal the Circuit Common? Yes, usually. I've seen very old radios with PNP transistors, and a negative main supply with "positive ground." The positive battery terminal is the Circuit Common. All the measurements in that schematic were negative voltages. Besides 1950s transistor radios, the same thing happens in old VW Beetles, and in some motorbikes. The positive battery terminal is connected to the chassis, so the "supply terminal" is the negative one. Don't install a normal car-radio in an old VW, because it will short out or catch fire when you turn on the ignition. Power supply was backwards.

All we gotta do is get rid of all collectible 1950s Japanese PNP-transistor radios, VW beetles, and positive-grounded motorcycles, and then Circuit Common will always and forever be the negative supply terminal! Well, unless it's some weird, electrically-floating industrial sensor system with a mix of AC power and virtual-ground op-amp circuits.

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    \$\begingroup\$ One of the best explanations of voltage and current I've seen. Thanks, there is so much great information here. \$\endgroup\$
    – Cthutu
    Commented Jul 26, 2018 at 13:51
  • \$\begingroup\$ Great explaination, thank you! But I guess a 6th point is missing (well, it's mixed up with the the 5th). 5th should focus (in my opinion) on: "it's not only electron that are flowing". The 6th should focus on: "what is amp"? [I did not understood it] (And then, it'll be more clear to have a 7th title for the "GND versus COM versus EARTH") \$\endgroup\$
    – JinSnow
    Commented Jan 2, 2019 at 8:32
  • \$\begingroup\$ Ampere: "Think of electricity in a wire as water in a pipe. Amps is the flow rate, volts is the pressure drop from one end of the pipe to the other, and watts is the power needed to move the water - or the power produced by moving the water as in a hydroelectric generator." electronics.stackexchange.com/a/267900/60167 \$\endgroup\$
    – JinSnow
    Commented Jan 2, 2019 at 8:42
  • \$\begingroup\$ Granpa's Austin 8 was also +ve chassis, so you may want to add that to your list! \$\endgroup\$
    – Indraneel
    Commented Jan 6, 2019 at 18:29
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    \$\begingroup\$ @Hörmann wrong on all counts. Electrons in metals are already dislodged, otherwise metals wouldn't be conductors. Electric current is defined as a flow of charge and this flowing charge comes from the copper, not from the battery. The charge in a battery never varies, only the stored energy varies (coulombs are not joules, while amperes are a flow of coulombs.) The charge (electrons) in metals IS the drive-belt. Engineers learn all this in 2nd-semester undergrad physics. Wires are full of charge (mobile electrons,) not full of net-charge (that's why even uncharged wires are conductive.) \$\endgroup\$
    – wbeaty
    Commented Sep 20, 2019 at 0:16

A voltage source has both negative and positive terminals, and produces a voltage (or potential difference) between those terminals.

In The Beginning, the early scientists studying electricity had no means of determining what, if anything, comprised an electric current, so they somewhat arbitrarily declared that current was a flow of positive charge,flowing from the positive terminal of the voltage source, through the external circuit, and returning to the negative terminal. We now call this concept "Conventional Current", and scientists and engineers generally use this concept when discussing current flow.

We now know that, in most materials, current is actually carried by negatively charged electrons. When vacuum tubes were developed, many technicians were taught using electron current, as the internal operation of a vacuum tube can't readily be described using Conventional Current. Unfortunately, electron current lives on in many places, causing students to be confused between Conventional Current and Electron Current. I think it is best to stick with Conventional Current, as that is what most of the technical and scientific community uses.

"Ground" is a severely misused term in electronics.

In AC power distribution and some radio antenna systems, "Ground" really does mean "a connection to the Earth".

However, in most electronics, "Ground" is merely a label we stick on a point in the circuit that we wish to consider "Zero volts" (where we put the black meter lead when measuring voltages elsewhere). It would be better to call this point "reference" or "common", but the use of "ground" is so well established that we're stuck with it. This "ground/common" has no magical powers - it is not an infinite sink for electrons - it is just another point in the circuit.

These days, "ground/common" is usually the most negative point in the circuit, but it may sometimes be the most positive point (one logic family is intended to operate from -5 volts - there the ground is positive). In many audio circuits, "ground/common" is the midpoint of the power supply, and we find both positive and negative voltages in the circuit.

  • \$\begingroup\$ Hmm. For me, electron current makes far more sense, as that seems to describe what is actually happening. Unless I am mistaken, positive ions are usually a part of the materials that make up wires, resistors, capacitors, etc. So they aren't freely flowing through a circuit. It's the electrons that are usually flowing "backwards" through a circuit to create what we describe as current, right? Positive charge doesn't flow, negative charge flows? I guess my problem with electronics is it's so steeped in old notions that were created before we understood what was actually occurring... \$\endgroup\$
    – jrista
    Commented Aug 4, 2014 at 1:51
  • \$\begingroup\$ ...that now we have things like "Conventional Current" which described a, at least in my understanding, non-existent "flow" of positive charge. Or is that just wrong...do positively charged ions actually flow through electrical circuits? \$\endgroup\$
    – jrista
    Commented Aug 4, 2014 at 1:53
  • \$\begingroup\$ Another question I have, related to the Ground issue. I've found a number of circuit diagrams only seem to have a positive voltage source, and a ground. I often cannot find a negative terminal or anything like that. In such a circuit, is ground the same as the negative terminal on a battery? Because of the difference between Conventional Current and Electron Current, I'm not really sure how to read such a diagram...I don't know how to complete the circuit, unless it's just completed at the ground point. \$\endgroup\$
    – jrista
    Commented Aug 4, 2014 at 1:55
  • \$\begingroup\$ @jrista: Yes, "ground" is usually the negative terminal of the power supply. In many circuits, you will see ground symbols scattered around the drawing - these should all be connected together. Using ground symbols like that is intended to reduce congestion in the drawing. Often, you will also see isolated "Vcc" symbols - these also are all connected together, and to the positive terminal of the power supply. \$\endgroup\$ Commented Aug 4, 2014 at 16:01
  • \$\begingroup\$ Yeah, that's exactly what I've seen, particularly in IC schematics. Ground symbols scattered all over the place, and at least one Vcc. Thanks for the info. \$\endgroup\$
    – jrista
    Commented Aug 4, 2014 at 16:06

First, your A and B are simply wrong. Given a voltage between points A and B, neither is privileged as a "source" of current or a "source" of voltage. All you can say is that if a conductor is used to connect A and B, current will flow between A and B. If the voltage between A and B is positive, in a metal this will take the form of electrons flowing from B to A. In semiconductors such as transistors the second part is not (necessarily) true, as current can be caused either by electrons or by absences of electrons (holes, which flow in the other direction).

In large part, the identification of "ground" with "earth" is indeed a historical accident, and arises from practices used by early power distribution companies. In current American terminology, ground is a reference point for measuring voltage and current in a circuit, while earth is an actual connection to a rod pounded into the ground.

The more general use of ground is descended from this practice, and it's actually still important in systems using any large amount of power. For low power systems, especially battery-powered systems, ground can be completely detached from any connection (physical or otherwise) to physical earth. But any electrical or electronic circuit, whether it's in a plane, or a car, or even in outer space, needs a reference point to start from in describing voltages and currents, and that reference point is generally referred to as ground.

It is perfectly possible to produce a power system with voltage which is consistently negative with respect to ground (and earth). While not used much any more, in the 70s and 80s the highest-speed logic family was ECL, which used -5.2 volts as its base voltage. Cray computers were, for a while, the fastest supercomputers around, and they used almost exclusively ECL, and drew a whole lot of current - produced by - 5.2 volt supplies.

So, when is the connection of ground and earth necessary? Well, basically whenever you're talking about systems connected to the AC power grid. If you don't pay attention to that, you risk killing yourself if you accidentally provide an inadvertent path for current to flow. Power lines have to be referenced to earth to provide things like lightning protection, and so such considerations have to be taken into account.

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    \$\begingroup\$ I'm curious about the whole notion of "holes". Do holes actually flow, like electrons, through a circuit? If so, what exactly is a "hole"? Or is that, again, just another abstract concept? From a quantum mechanical level, the only thing I think can actually flow through an electrical circuit made up of metals and semiconductors is electrons themselves. I think plasmas would be different, as in a plasma both ions and electrons are free to flow...but I was pretty specific about my question being about DC electronics applications. \$\endgroup\$
    – jrista
    Commented Aug 4, 2014 at 1:48
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    \$\begingroup\$ Think of holes in terms of a Chinese checkers board. There are a bunch of depressions on a regular grid, each of which holds a marble in place. However, the marbles can move if they have somewhere to go and are given a nudge. Now take one marble out. This leaves a hole in the array of marbles. If you tilt the boards slightly and jiggle it, marbles will slowly move down to fill the hole, but in doing so will leave a hole themselves. You can think of the net marble movement as a slow movement of many marbles down, or the single hole moving up. \$\endgroup\$ Commented Aug 4, 2014 at 2:02
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    \$\begingroup\$ I see. So it is a bit of an abstract concept...electrons are still moving, but they are affecting charge in more than one way as they move through the circuit. Interesting... \$\endgroup\$
    – jrista
    Commented Aug 4, 2014 at 2:08
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    \$\begingroup\$ Yeah. In some respects it's a matter of convenience. If you concentrate on the behavior of electrons, it gets rather messy, as you deal with the jostling of the marbles in the grid, and you have to deal with the bulk statistical behavior of a lot of them. Dealing with holes allows the same net charge transfer, but in terms of a single "virtual" particle, the hole, which moves more slowly than the many individual electrons. (Which is why p-type MOSFETs have higher resistance than equivalent n-types). \$\endgroup\$ Commented Aug 4, 2014 at 2:14
  • \$\begingroup\$ Alright, so I understand that. But just to get back to the basics, in a simple circuit of say a capacitor, and inductor, and a resistor. If I encounter a circuit diagram that has a +5V connection in one corner, and the Earth symbol in another (I think use of the earth symbol would be invalid, but I've seen it on quite a few occasions, hence the reason I asked the question :P)...the earth symbol, that's the ground...and is it also the negative terminal? In other words, connect "ground/earth" to the - terminal on a battery to complete the circuit and actually make it work? \$\endgroup\$
    – jrista
    Commented Aug 4, 2014 at 2:28

Voltage and Current

In electricity there are positive charges (usually protons) and negative charges (usually electrons.

When one object is positively charged, and another is negatively charged, then there exists an electrostatic field. This is the voltage, or the potential for charge to be able to be moved by the electrostatic field.

If some sort of conductor is put between the two, a current will flow. This will either be electrons toward protons (as in a wire connected to a battery), or protons towards electrons (as inside fluorescent lights), or both flowing in both directions (as in some batteries).

Ground / Earth / 0V / Common

Ground and earth mainly come from AC electricity. They are used interchangably today. In AC power distribution you literally connect one side of the circuit to the ground/earth/terra.

0V came into use because its simple. If you have a 6V battery what do you name each terminal if you want the names to also contain the voltage? +6V and 0V seems the simplest way. +(6V) and -(6V) could also be used as the positive and negative side of a 6V potential difference - but that would be confusing and people might think that the potential between them is 12V, or that the potential from one to earth is 6V and the other -6V etc.

Common is different again and came into meaning with communications. If you are sending a signal over one wire, then anyone reading that signal needs to measure the voltage between the wire and an agreed 'common' point voltage reference.


I am not an EE. From what i understand: Voltage is the bias in potential between two terminals which generates electron flow through conductor, semiconductor or load. The electrons will flow from the most negative to most positive terminals. The term GND, COM is a relative term and is not always the same as 0Vdc

Let say the circuit has terminals: A) +5Vdc B) 0Vdc C)+10Vdc D) +24Vdc
So the ground for all terminals definitely is A) 0Vdc, electron will flow from B to A (5v) and B to C (10v) and B to D (24v). But +5Vdc can be considered common terminal for both C and D : Because electron can flow from A to C (5v) and A to D (19v)

Some circuit has this terminals (eg ATX PSU) A) -5vdc B) -12vdc C) 5vdc D) 12vdc. edit: E) 0vdc Any of the lower voltage terminals can be called ground for any higher voltage terminals.

  • \$\begingroup\$ I'm not sure that this is a valid answer. 0Vdc is not a magical actually existing thing, it's always a point defined (and most likely labelled GND or COM), so your ATX PSU also has a 0Vdc terminal, otherwise you wouldn't be able to measure any of the other voltages. \$\endgroup\$
    – Arsenal
    Commented Sep 4, 2015 at 8:28
  • \$\begingroup\$ I've encountered circuit that has both GND and COM. In the manual of the device COM is mention exactly (common ground for terminal X1) which is a sourcing input of 24v. And when measured via multimeter GND connected to COM(as ref) there is potential of 12v. And when GND(as ref) connected to X1 there potential of 12v. And when COM(as ref) connected to X1 there is potential of 24v. So concluded. \$\endgroup\$ Commented Sep 5, 2015 at 0:48
  • \$\begingroup\$ So concluded that GND is 0Vdc, COM is -12Vdc and X1 is 12Vdc while the signal input from X1 via external relay switch is 24Vdc. Initially my thoughts are 0Vdc as absolute and always GND or COM and they are the same. But since i saw that circuit i change my understanding. \$\endgroup\$ Commented Sep 5, 2015 at 0:53
  • \$\begingroup\$ And in the manual, they specifically mention to isolate GND and COM, i am not sure is it because to isolate noise or because there is current between GND and COM. \$\endgroup\$ Commented Sep 5, 2015 at 0:57
  • \$\begingroup\$ correction : or because there is POTENTIAL between GND and COM. \$\endgroup\$ Commented Sep 7, 2015 at 3:35

I always isolate my dc Psu 0v referance from my ac ground/earth to avoid any ac noise in the dc circuit. I then protect both the + and -dc using bridges back to the ac incase of ac being inadvertantly reintroduced to dc unprotected by ground/earth. It's a failsafe method that protects pnp,npn,people and devices. No smoke or bangs, just a protective device that that will keep tripping unless the fault has been rectified. I then monitor the complete system via volt free aux/no/nc to determine if it's in the logic or wiring and determine if it occurs at a logical or physical event. I then blame my programmers or my engineers. Nine times out of ten i have to go and put it right myself.


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