Ungrounded DC Systems

you do have a point mr johnDG but

Wouldn't it be affected like making the positive or negative side dropped to zero

See #5, by Paulusgnome. Conceptually, if you ground the negative side, -V is '0V' and +V is 'above ground' - this is the 'usual' way of looking at a DC circuit. If you ground the positive side, +V is '0V' and -V is 'below ground'. Connecting both to ground is not a good idea.

Jein ( A nice German word meaning Yes and No!).

True you would make that point RELATIVE to ground 0 volts, but no extra current would flow just because of that and you need higher current to blow a fuse.

Basically the circuit remains unaffected....

Bingo.

I cant imagine how an ungrounded DC system would even work. If you can't complete the circuit you have no electrons flowing anywhere to do any kind of work. Or am I missing something here?

Let me think; If positive is hot then negative has to be ground and if negative is hot then positive must go to ground and the electrons just go round and round, round and round, round and round. Unless of course you have a break in the loop as in no ground.

It is common in oil and gas applications to run ungrounded dc systems due to critical loads. The battery chargers do however have - & + dc ground alarms.

The only problem is trying to find the ground when you receive the alarm..

YOU ARE RIGHT BUT DO YOU NOW WHEN WILL THE FUSES BLOW?

Hi AAWZ,

Please do not SHOUT in your posts as this makes my eyes bleed.

The fuses will blow when the current through them gets high enough. In an ungrounded DC system this will be when the +ve conductor is connected to the -ve conductor with a sufficiently low resistance. The fuse will not blow if either +ve or -ve is shorted to earth, but will, of course, if both are.

You are bringing some sense and good order into the Blog, I gave you a GA.

I never realized that so few other CR4 users neither fully or even partly understand simple electrical theory!!!!

Your comment of:-

Of course, some folks will want to insist that one or the other conductor must be ground, this is of course the case in vehicle DC systems, but this is more convention than necessity.

Is intrinsically wrong.

The chassis of most cars is used as the negative conductor (many years ago the chassis was connected to the positive side of the battery in all cars!!)and is called "ground" as a convention or name, but it is not connected to ground per se!!

The same goes for aircraft and ships......although when a ship is immersed in sea water, it has a natural ground path to Mother Earth as well!!! But not in fresh water, or should I say, not one you could rely on!!!

Whether or not a car chassis, ship's Hull etc is connected in any way shape or form to earth or not, does not affect its normal operation at all......

You do not appear to have any basic electrical knowledge at all, that is not a critic, just a statement of fact.

Most DC circuits are never ever connected to ground, your portable radio, your mobile, any aircraft, your car, your electronic watch etc etc etc, but they still work.

A connection to ground is not a requirement for almost any circuit (let alone DC circuits) to work!!!!

Connecting to ground certain parts is usually needed when high AC volts are used to help protect the user from fault currents.....

Sometimes ground is used to shield sensitive signals from interference from other signals.....

Andy,

I'm sure you do not mean to be rude. I have expressed before that I am not an engineer. I have been an auto mechanic for over 30 years, however, and at least on autos in America the cable that goes to chassis and engine block are ALWAYS referred to as ground. If I went into an auto parts store and asked for an earthing strap I would either be given a quizzical look or laughed to scorn. If you read my post again you will see in sentence #3 a question that indicates I may have a limited knowledge.

Yes, in my second paragraph I made a snotty comment (my bad) based on my experience in automotive electrical. I will admit my knowledge is incomplete and I have learned new things right here.

Please try to avoid rudeness. I will do likewise.

I wasn't being rude, I was stating facts. You would know exactly if I was being rude....!!

One fact being that the chassis on most vehicles is used as the negative line and has got the colloquial name of ground....everyone knows that if you connect anything that needs 12 volts (or thereabouts) DC to a plus from the battery and to the chassis you get a supply. But really it is the negative connection or "cable" in a different form! It saves weight but actual negative cables would be otherwise far better and then the chassis could be used as a safety (real) ground if needed....

A true ground is a safety feature and does not have any large current running through it, at best nothing at all! It is only there to protect people (and other equipment, but primarily people) from high voltages and a current on the ground should cause fuses and/or other safety devices to swiftly act and cut off the main power....

A cars chassis has current running through it and is part of the circuit. Therefore it is NOT a safety ground ever. Ground in this case is really just a common name that has got attached to the chassis.....I suppose calling it "negative ground" would be a little better than just calling it ground......semantics again!!

Reminds me of a not overly pleasant experience once had hooking up batty in a positive-ground vehicle: a farm tractor. No need to say more.

Andy, (notice I address you by name)

I think we have a problem of definitions or semantics as it were. It could be the cultural divide I suppose. For instance, what you call 'fact' looks like 'opinion' to me. For instance, what I call a chassis 'ground' (and is generally accepted as such - not just by word of mouth but by every technical manual I am aware of) you call what? It is unclear just what you call the negative side of an auto electrical circuit.

If I drive a ground rod into the earth next to my car and attach a cable between it and the chassis will it then be properly grounded? If the negative side of the system has juice running through it, Why don't I get electrocuted when I touch the negative battery post or any thing attached to it. It is only when I connect the two sides that any current flows to make a fan turn, a light glow, a starter motor spin and/or any other electrical work that I require. It is now obvious to me that this is a different kind of ground than was refered to in the original question here, but it is still ground. Perhaps we should get an opinion from an automotive electrical engineer on the subject, or, and I doubt you could lower yourself to it, you could agree that I have some 'ground' to stand on with my understanding of 'ground'.

Sorry, but a full and proper explanation of the subject should not be on such a Blog, it should be part of the training of a technical person, such as I had largely in the RN and later in the OU.....

It still sounds like I am talking down to you, that is not my intention, I am not seeking to offend anyone.

Take my word on it, the name "ground" for a negative earth chassis is a well known "habit" of the whole population when referring to cars and the like. That does NOT make it technically correct......A lot of people in the UK call all Vacuum cleaners "Hoovers", which is a brand name.....it is not technically correct either.....its a habit!

The same word "ground" was also used when we had positive earth vehicles (up to the middle 60's or thereabouts). So those cars had a "positive ground"......confusing ain't it?

I will say it again, a "safety ground" cannot have any current running though it to supply motors, radios, ignition systems, computers etc in a modern car......as does the chassis.

A rod stuck in the ground next to your car does not make it any "safer" than it was without. In fact it makes the car slightly MORE attractive for a lightning strike and all the resultant possible damage to the sensitive electronics that modern cars have. You will be ok inside as the chassis forms a "Faraday's Cage". Also, you will need a very long cable if you want to drive somewhere!!! So impractical and not even the slightest bit safer!!!

What you seem not to want to understand is that the main point is that a "ground" on a vehicle does not imply safety as many seemed to think if you read back in this Blog, you also. That was the point I was trying to make clear. Keep clear of any thoughts of this as they are simply not true!!

The third wire in a normal mains cable is also called ground, it is often green or yellow of a mixture of both (but depends on the laws where you live as to its actual color!), this is a REAL "safety ground" and one should not confuse the two things as being in any way similar. Which is all that I was trying to get across, nothing more, nothing less.....

Please continue to call the "ground" on a car "ground" or even "chassis ground", whatever you wish, but do try to avoid the thought that it is in any way "safe".....

Also, the likelihood of getting a shock/electrocuted from pure normal 12 volts is very, very unlikely......if you ever feel anything, I can assure you that it will be considerably higher than 12 volts for some reason!!

The greatest dangers with 12 volts in a car is not its voltage, but the possible current that a lead acid battery can deliver into a bracelet or a metal watch strap and severely injure the wearer as it glows suddenly red hot and no fuse happens to be in circuit at that time!! A friend of mine bridged between positive and a screw in the chassis, that was over 30 years ago, he still has the scars to show for it.....

The other dangers of such batteries are the gas and say a spark or lighted cigarette....many people have been blinded in this way......ignorance is not always bliss!!!

My intention was to clarify for all, as this blog has demonstrated that many here do not possess some basic knowledge in this area.......and avoid "Bliss"!!

Think torch (flashlight) they work fine!

If you have faulty wiring or some other fault that happens to ground the system, it ceases to be an ungrounded DC system and becomes a grounded DC system that will work just fine. The usual reason to make a DC system ungrounded is this reason.... if for some reason something goes wrong and a conductor grounds... no problem, you just have a grounded system and everything keeps running. This is why a lot of heavy industry that uses DC systems will use an ungrounded system.

I know what you wanted to say, but you did not achieve it!!

Your comment will have confused the uninitiated even more , it was not your intention I am sure, but you should read it through slowly and maybe you will understand what I am getting at!!!!

Just meant in a friendly way to help, not to upset you!! You obviously know what you meant to say!!!

Have a great day anyway......

You are probably right that the uninitiated would probably not get what I said. I happened to be half asleep when I wrote that. I still think what I said was valid.

Perhaps this will come out more clearly....not for Andy's edification since he obviously understands the concept.

The reason a lot of the larger power systems we use are grounded is due to safety. What happens is that the ground becomes a return path that allows your current to flow if you have a damaged wire somewhere that is touching ground. This will enable your fuse/breaker/other overcurrent device to trip thereby protecting anyone that could come in contact with the chassis of the equipment from being shocked or electrocuted.

The reason some people use ungrounded systems (AC or DC) is that it makes the equipment more fault tolerant. If you have the same fault in an ungrounded system you will not trip any breakers/fuses/other things, this is because the current can not get back to the power source by taking that route. Therefore, the system doesn't go down.

Not sure if that is any more clear, but I hope it is.

The end result is that in a true grounded system you should have little if any current running through the ground. If you do that means you have more than one ground. In an ungrounded system if you get grounded, you should likewise have little or no current running through that ground, hence no tripping of fuses.

I gave you a GA for a clear and concise explanation that I personally feel everyone has a really good chance of understanding. Many thanks from me at least!

I could see from the original that you really knew what you were talking about.....it was just not quite clear enough!!

Dear Friend

Fundamental question, are you talking of fuses in power line?, I think so because fuses are not provided in earth path. Fuses in power power line will blow ( NOT TRIP ) ONLY if current flowing through it increases beyond its rating. Thus when there is earth leakage or fault, then current in power line may not increase beyond the fuse rating. Hence if it is not increasing, then there is no reason for fuse to blow.

These are two nearly independent issues but may be related, depending on the nature or extent of fault

We are an ISO 9001-2000 ( TUV certified ) manufacturer of battery chargers with distribution boards for installation in power plants, substations, petroleum refineries, industrial & commercial buildings, off shore oil platforms etc., for more than 30 years. Apart from mainland India, we have installations in Bhutan, Myanmaar, Vietnam, Tanzania, Malaysia, Indonesia, Zambia, Kenya, Srilanka, Bangla Desh & Nepal.

Thanks and regards

Ashok Toshniwal, Bangalore, India, 919448491293

Why don't fuses in ungrounded dc system like battery charger trip if a small ground fault in the load side?

—aaawz

Are you asking why battery charger fuses did not blow when you momentarily touched (shorted) the POS cable clamp to NEG (GND)? Or reversed DC connections to a battery?

Guys and gals, It's great to talk semantics, but the implied question is that a small overload current has happened in the conductor and the OP wants to know why the fuse didn't blow.

From my memory of the SAE handbook on fuses, they will cary a 10% overload for more than 10 minutes, a 25% overload for as much as 3 minutes and a 35% overload for as much as 30 seconds but will "fuse within 0.5 seconds on a 300% overload". (From memory, but go look up the standards on fuses.)

Basically a fuse provides protection against severe overload to protect the wiring and such. They are not regulator devices and so provide poor protection for minor persistant overloads.

I suspect that the OP has a smaller than critical fault and the fuse has not blown.

GA.

Well put, but I am afraid many will not understand! Sadly!

Mr Guest, Good point.

I have worked with 24VDC controls for ride systems. Most manufacturers ground one side of the 24V bus, it is a requirement in most of Europe. The Disney manufactured attractions I worked on floated the DC and used a ground fault detection to alarm of a problem which would allow the attraction to be shut down in an orderly manner so that a repair could be made. Ground fault detection is accomplished by tying both the + & - to ground through large resistors. If there are no ground faults, the current though the resistors will be equal with 12V being be dropped across each. The resistors are sized large enough so that they cannot provide current enough to power an actuator (motor, solenoid, etc.)

With a grounded system, a fault to ground in an actuator circuit is not detected until the control system attempts to activate the actuator. Of course a fuse blows, but unless each actuator is fused separately, it is going to be tough to find the source of the ground fault.

Guys and gals,

Don't take this personally, but I want to have a little rant. If I'm off the mark, then score it as off topic, but please read it first and then re-visit what has been posted in the thread.

I'm really frustrated at the piffle that is floating about in this thread. I have resisted the opportunity to go back and distribute "off topic" scores all over the place. I'm even more surprised at the "good answer" scores flying around to responses that don't provide an answer to the question asked.

Instead of arguing amongst each other about competancy to answer the question or picking out issues with each other or bragging about what you've done in the past has ANYONE answered the poor guy's question?

Question:- "Why don't fuses .... trip in a small .... fault?"

Answer:- Fuses don't blow on a small fault, they are there to protect circuits from catastrophic failure.

Only DickL has even hinted towards this answer. It seems that we are "the voice in the wilderness"

Why don't fuses in ungrounded dc system like battery charger trip in small ground fault in the load side?.

I think this is the reason people are talking so much about grounded vs. ungrounded. Now, it is possible that the questioner did not explain the problem in a way that would enable a good response, or he could have explained it right on the mark. There can be two interpretations of the question.

First:

I have an ungrounded DC system that got grounded but the fuse didn't blow, why?

To which the answer would be that the fuse shouldn't blow in this instance if you have only one ground in your circuit.

Second:

I have an ungrounded DC system that I had short the positive and negative sides for a short period of time/wasn't a direct short.

This would be as Just an Engineer has explained so well before would be because the fuse is not rated to blow quickly on a small fault current, but will blow quicker as the fault current rises. If this is your problem and your equipment is extremely current sensative you can get quick blow fuses that are designed for electronics that will blow much quicker in an overcurrent condition. However, if you are running a motor or something that has a large amount of current coming in at the start, you will constantly blow fast blow fuses. Of course if you are going this route, I would consult either an electrical engineer or, if it is a very small device I would ask an electrician who could help you out.

Unless you know what you are doing, NEVER just change the design of an electrical system without consulting someone who knows what they are doing.

We almost always go off on slight tangents, that is half the fun of CR4.....for many of us at least!!!

I did have the opinion (maybe wrongly) that he had been answered several times, are you implying that each and every one of us may only answer the question and nothing else?

I saw that there had been some answers and some of them were incorrect, so we could all go off and do our own thing again, correcting those wrong thoughts and impressions.....your post implies this too that answers were already given.....so whats the Beef?

Dear Friend

Pls check my reply, as well

Thanks and regards

Ashok Toshniwal, Bangalore, India

Hi,

A noraml battery charger body will be earthed on either side whci will be connected to the main grid of the system. During earth fault, leakage current will lead to tripping of Battery Charger fuse.

To avoid this provide a separte earth rod for the Battery Charger and do not mix up with the main grid.

N.Suresh

What about tracked vehicles, not rubber-tyred like tanks, but things like bulldozers? Do these need to use full circuit chassis wiring, not chassis ground, to have isolation from earth ground?

Hi,

Irrespective of vehicle with insulated tyres or uninsluated steel in case of bull dozers/ earth moving equipemnt the battery must be kept over an insulated bush in the cabin.

N.Suresh

I can only ask either why? or what do you mean? as I personally could not make any sense from your post, sorry.....

It should be me saying sorry, but appreciate your gesture. I was just passing by and not being an expert by any stretch, I was just trying to find how, other than pneumatic-rubber tyres, and how important or not, to keep a vehicle's chassis ground, or conductor if you prefer, isolated from Earth's ground, or conductor if you prefer. So I wanted to know if vehicles with tracks not driven by tyres, or with metal wheels, use some other method, such as not using chassis conduction, to return current to battery or power generator. I hope this makes more sense this way.

Not really.....sorry!!

Moving vehicles as a rule are completely independent of "real" ground. They may have a safety ground of their own, a car is NOT one of them, as it has no safety ground, nor is "Ground" really "Ground" either.....it is just the -ve conductor of a small DC system!!!

Years ago it was the +ve conductor, but it was changed as the old method caused extra corrosion on the chassis.....

So then, why is earth a "real" ground, other that in the sense of real estate? What makes it any different from an other DC or AC connection point?

Well put, could not agree more.

I have just read through most of this blog again, particularly the original question and a lot of the original answers.

1) Due to the fact that the questioner does not understand relatively simple electrics (not his fault, and we always try and help still!), he has actually phrased his question badly, which results in a proliferation of answers, like a shotgun blast so to say, as anyone would know and understand who DOES correctly and fully understand the subject inquestion.

2) Due to the inherent error in his knowledge and question, this has caused many correct bits of information to be supplied by many people, and a similarly large number of either irrelevant or wrong information. Some of this wrong info I an several others have tried to educate those persons a little as to why the info is wrong or misleading. But as the old saying goes, "You can lead a horse to water, but he has to drink all on his own!"....

3) This amount of mixed quality or information here that probably gives rise to the original blogger reading ALL the information and not having a clue as to what is correct and what is not!!

4) Then there is the terminology, which as I have also tried to explain, gives rise to misconceptions as to the way parts of the circuit actually work, which probably was the original cause of the confusion for the original poster!! Ground, that must mean safe, or not? for example.....

As many know, actually having a safety ground can in certain circumstances actually make the circuit more dangerous for the worker working on it!! Not always safer.....as can NOT having a ground be also dangerous, it depends upon the application and how it was built completely.....

5) Then, as a typical CR4 Blog (this is the part I really like), we all go off on our merry way exploring all the various facets of the original question that were not addressed by the original questioner!!! I personally find this to be the "fun" part after the meat and 2 veg part!!!! But obviously you don't find this "fun".....

I really do not know or want to try and think your way personally, so you yourself will have to think up a way for you to deal with it on CR4 or not in the way you see best, but as I said before, its your problem, its your "beef", so you have to "like it or lump it" because thats the way CR4 often is and the greater number of people here online seem to accept it for what it is in the way it is.....

Your post name, does it reflect the real truth? as all the "Real" Engineers that I know and have worked with, were to a man, all interested in the associated areas and problems as well and they were NOT fixed on certain terminology and conceptions as you appear to be, they were all really flexible....even outside of their personal areas of knowledge, in fact even more so in those areas.......so each meeting we would all gain some extra knowledge from another and be HAPPY to have gained something......

You do not strike me as being in the slightest bit happy about all this either, which is really really sad. I hope that changes for you really soon.

Have a great day.

Why don't fuses ¹ in ungrounded dc system like battery charger trip in small ground fault² in the load side?.

Above is the original question.

1. Which fuses specifically and at what rating?
2. Define small in terms of current rating of item 1?
3. Explain/define the 'ground fault' which would cause current to flow to ground or any change in current.

If an 'ungrounded sytem' has some single part of it connected (through) a fault to ground (or earth or any other plane) there is no reason to suppose any current will flow.
A connection to ground from two points at different voltages will cause a current to flow.

The above statement may be at odds with some particular practice/definition or situation*. Someone has already mentioned semantics, and as this forum is based mainly on the written word we have little else to go on.

Maybe there are separate ground wires with current monitoring?...whatever?..in the absence of concise technical details we are all floundering...maybe we should be seeking clarity rather than sniping from our high horses....

Now, if you don't all behave it's off to bed with NO supper

Del