Finding Faulty DC Circuit

How does the <...battery earth fault...> become apparent/notified?

We get annunciation in battery charger with earth fault current in range of +/- 30mA

I would start with looking at your documentation (database) of cable failures to look for trends in the cause of the failures.

Exactly. Prevention of recurrence has to be a priority. GA

Ever thought to try a GFCI?

A quick google search shows some DC GFCI producers.

I have only worked with AC GFCIs but I'll be glad to hear about your DC GFCI experience.

Greets

I believe they already have a similar circuit protection device for they detect a fault. The OP is asking for another approach in troubleshooting the root cause of a detected fault.

In substations I have seen insulation monitoring systems. These just detect ground faults, without tripping the circuit. I understood it has something like this integrated in the battery charging unit.

Some of these insulation monitors also have selectivity using a transducer on every cable so it can not only tell if there is a fault but also on what cable. Pretty neat. The OP probably does not have this sort of system installed. Maybe can share some details with us.

I'll share the details if I am able to find it.

I knew about earth leakage monitoring unit of Bender IRDH where a central unit inject small current of diff frequency and we have small ct in each outgoing circuit and they all connect to earth leakage monitoring unit which all communicate central unit. When insulation of the cable is fail, ct in that circuit will detect the earth leakage and also main unit will find out which circuit is faulty.

But we are not able to install this in our refinery due to lot of substations and swgrs.

From the responses, I think that many do not realize that your system is normally ungrounded, so the first fault gives you warning to go fix, before a second ground fault occurs, with subsequent loss of protection. With your 30mA sensitivity, about 3600 ohms to ground, ground current measurement using clamp meters or other locating instruments appears to be unpractical, especially considering the complexity of the distribution system.

It seems unusual to have motors & controls included in switchgear protection power, normally used for relay / breaker tripping & closing power.

The method used in most installations I have experience with is to limit the scope of the dc distribution, where a large site might be required to maintain up to 10 station battery installations, with very limited exposure to external influence, easy to troubleshoot & protect.

i think you are doing the best you can with what you have to work with, but I would develop a plan to transition your equipment towards a design that is easier (& perhaps cheaper) to maintain. Rather than attempting a reset after each circuit is brought on line, you should be able to monitor the ground fault with a pair of indicating lamps to ground, the lights will change intensity, one bright, one dim, when the ground fault shows up. No need to attempt a reset.

Could you tell the shorted cable with an amp tester? If so you might just check each feed from the central feeder location...You might need to establish a historic record of amp draw for each feeder...

You might try AC coupling a Time Domain Reflectometer to the DC power cable. You will have to be careful with selecting the DC blocking, RF passing capacitor selection and how one connects and disconnects to the TDR. This might tell you the cable run distance to an RF short circuit but with cabling not a standard RF impedance with a known propagation velocity the distance calculation will not be accurate if it can be seen at all. The far end of the cable run will most likely look like an RF short, too.

There is a method of using an "O" scope as a TDR,using the "Z" input,but it has been many years since I have done it,and it did require knowing the characteristics of the cable.

https://hizantennas.com/A%20Quick%20TDR%20Setup%20Using%20a%20Tek%20475%20scope-1%207N%20edits.pdf

Get a DC clamp-on ammeter. If you clamp around dc supply and return wires, there should be no net current. If there is a dc fault to ground, you will see a net current.

That will work unless the ground fault is to the shield,or included within the cable itself.

Also,he stated a "earth fault",which is not necessarily a ground fault,so that is another kettle of fish.

The best way is as you showed,with the megger.

It means isolating both ends of the cable,but it will give definitive results;no more guessing games.

I give you a GA for your answer.

<...a "earth fault",which is not necessarily a ground fault...>

OK. What's the difference, then?

A Ground Fault can be a short between the + and Minus conductors in a DC circuit,or between a phase and neutral in an AC circuit.This can happen without the outside insulator being damaged.

Many DC supplies have an isolated ground(common return).

An Earth Fault is a connection from a conductor to an Earth ground,like to a metal conduit or the leakage of the outer insulation in a buried conductor into the soil or media surrounding it.

Ground faults have the potential to carry more current than Earth Faults.

Not all dc systems have an intentionally grounded power leg, so a plus & minus fault is just a fault. I never did understand that there was any difference in meaning between earth & ground, U.S. tend to use ground, Europe tend to use earth, often modified as Protective Earth, PE. I assume meaning an intentionally grounded equipment component, to serve as a conductor of abnormal current?

In the USA,a Grounded conductor is used to carry unbalanced load between two current carrying conductors,and this is bonded to a ground rod or grounding grid.It is usually white or gray in color.

The Grounding conductor is used to carry a fault current from the non-current carrying- equipment metal parts to a ground.

It is usually green,or green-yellow stripe.

There is much confusion about the two terms,and are easily confused.

The way I remember is this: If the neutral is broken,it can carry the full voltage of the circuit,and you are DEAD.You will be buried in the Earth(Ground-dead)ground rod.

Groun-ding,if broken,you will get a "Ding" when touching metal parts,but this can sometimes be fatal.

The green wire(grounding) is reserved for protection only,and carries no other current,even though it goes to the same place as the neutral eventually.

Kinda like the emergency lane,only used by emergency vehicles.

Bonding and grounding rules can be pretty confusing at first and the NEC has a whole section devoted to it: See Sec. 250.

Here is a link if you wish to purchase the 2020 version:

https://www.contractorresource.com/national-electrical-code-handbook-2020/

A ‘grounded conductor’ carrying current is called ‘neutral’, to not be confusing.

The neutral if opened usually will not be much different than ground in a 3 phase system, unless there is a ground fault on one of the phase conductors. Capacitive coupling tends to keep it there. On a single phase system, the voltage to earth or ground is typically half of the system line voltage, with neutral interrupted, not necessarily deadly on a 120V system, at 60V. It could startle you into falling off your ladder, tho.

However, if you place yourself in the interrupted return or supply path of any load above 60-70 volts, you will get full voltage there until current starts flowing, then plenty of voltage & current to kill. Doesn’t matter if neutral is grounded or not, it’s only grounded in one place, at the source. That’s the situation you are referring to.

Tell that to the families of the people that have been electrocuted when cutting a copper pipe that was part of the bonding circuit under a house.

The fatality of the shock depends on the path through the body and how effectively the person is grounded.

It only takes a few milliamps through the heart to make it fibrillate.

The NEC requires GFCI receptacle that are accessible from ground level.

They are not required if access is above ground level,such as a wooden deck because the connection to ground is high resistance,even though I recommend it anyway.

A wet wooden deck with treated wood can be an effective ground.
The wood is treated with mineral salts and metals which can make it conductive.

Do you have any sources for people being electrocuted by cutting the grounding connection, e.g. copper pipe?

This is probably exceedingly rare.

The only way to get any appreciable current back through an earth ground is to have a line-to-earth fault and then cut the grounding conductor (pipe) and insert yourself into the circuit assuming the circuit breaker didn't trip on an over-current due to the fault in the first place.

There is another possibility where the grounded conductor (neutral) back to the transformer has broken and due to load imbalances on the secondary, you can develop some currents on the grounding conductors as a result of the unconnected center-tap causing the neutral current to flow through the earth grounds of the pole pig and the residence's panel. You will usually feel tingles at a hose bib or faucet because the currents in the grounding conductor raise the potential of that 'ground' be it a pipe or faucet.

There should not be any currents on any grounding conductor except for a fault condition. (The one exception is the older three-wire range and dryer connections where there might be some 120V loads in the range or dryer.)

I have only personal empirical evidence,a local plumber was killed in this manner.

Here is a link on safety procedures when working on copper pipe.

https://www.worksafe.qld.gov.au/injury-prevention-safety/electricity/electrical/electrical-safety-for-plumbers-metal-water-pipes-can-shock-you

And here is another reference link:https://www.safetysolutions.net.au/content/electrical/article/safety-alert-on-water-pipes-1070193013

And another:

https://www.safetysolutions.net.au/content/electrical/article/safety-alert-on-water-pipes-1070193013

I do not know of any installation in a home where the neutral carries zero current all of the time;There is always an imbalance current carried by the neutral.

A refrigerator or freezer turns on a hair dryer is turned on;these all can cause an imbalance current to flow in the neutral.

A bad water heater element,if the water pipes are not bonded to the ground rod,can put a voltage on the faucets and metal plumbing fixtures.

Most plumbing codes require that you provide an insulated jumper of appropriate size,around a copper pipe to be cut.

Copper plumbing is required to be bonded to the ground rod with at least a #6ga copper conductor.

Cut this pipe,and you are now in parallel with the ground rod and some current will flow through you to ground, from either side.

If you are lying on your back,it can go through your heart.Not good.

The neutral in your home or other single phase circuits carries ALL of the load current of your 120V circuits, so is only zero when there is no load. 240V circuits don’t use the neutral. 3 phase circuits can balance to almost no neutral current when there are no line to neutral loads, or if each of the line to neutral loads are closely balanced, equal. The voltage above ground potential on any neutral is low unless there is a line to ground fault, open circuit in the return AND you measure the load side of the interrupted return AND the load is connected, or you have a short circuit active in the line.

Wrong.If both L1 and L2 have the same amount of current,then the neutral carries no current.

If L1 is pulling 10 amps,and L2 is pulling 10 amps,the neutral current is zero,same as a 240 volt load.

As I stated,this Rarely occurs,so there is always Some current on the neutral.

I do not feel the need to teach basic electricity 101 today,but I will summarize:

When 2 resistors are in parallel,there will Always be current flow through both resistors,the amount depending on the resistance of each.

If you touch a neutral,and your body resistance to ground is low enough,you will carry a hazardous amount of current through your body.

Read the links I posted above for authorities that recognize this fact.

Neutral current can be the same where they combine in the panel, which is less than 2% of the circuit exposure. Otherwise neutral current is same as load current, as explained earlier.

Neutral in the narrow case you describe is at ground potential, so is at essentially 0 volts, so no hazardous voltage to ground.

No, it is right. The problem is whether we are talking about the current on the grounded conductor from the panel to the transformer or the current on the grounded conductor from the panel to a particular load that is serviced by 120 V. Sheesh.

Don't be so condescending. I have a master's degree in electrical engineering and over 36 years experience in defense and aerospace. I happen to know that RW has over 50 years experience as an engineer in pulp & paper mills, lime kilns, and other industrial plants working with numerous electrical installations. Trust me, we know what we're doing.

I apologize for sounding condescending,and did not realize it until I read my post again after it aged a bit.My bad.

I tend to get a bit crass sometimes when I have had too much Pinch.

Back in the 1960's the telco's had problems with 120vac appearing on their phone lines.They discovered that some houses were not electrically bonded to the water pipes,and a bad water heater element was putting voltage on the system.From then on,they required that all new phone installations would also bond the ground rod to the water pipes using a #6 AWG ground wire.

A water system with metal plumbing that has a loose or non existent bonding ground can be very hazardous to any person working on it, under certain conditions,especially if cutting the pipe.It is recommended to put a jumper around the place of cut to prevent the potential differential.

Insofar as qualifications,I have been a Unlimited Electrical Contractor since 1987,which is reciprocal in 7 states.I am now retired due to some health issues.

I have supervised and directed installations in many industries,including turn key process control,instrumentation,programming,as well a total plant power distribution,from 19.2Kv,7.2Kv,4.160kv,1000+Hp motors all the way to duplex receptacles.

Some jobs were pre-engineered,some were not, and I was required to calculate total load,all branch circuits,etc.,as well as submit a winning bid,which as you know requires knowledge of and cost of all components used in a job,as well as man hours.Small errors can cost big bucks.

I realize I have a lot to learn from the theoretical aspect,but I base my knowledge on empirical evidence from my experience,however I stand to be corrected when I make an error,without taking offense,but with gratitude that I have gained knowledge.

You and HTRN are both right. This is because both of you are using ambiguous shorthand jargon in this discussion and expect others to understand what you mean.

The neutral conductor in a cable from any 120 VAC load to the distribution panel must always carry current regardless of any other loads on the distribution transformer. The neutral conductor in the cable from the distribution panel to distribution transformer will not carry any current when a purely 240 VAC load is applied and when the current loads of each split phase 120 VAC load draws identical currents in magnitude and phase angle. Usually the split phases never exactly match so some smaller current still exists on the neutral conductor between the panel and transformer but it can happen.

But really you should start with Article 200, Use and Identification of Grounded Conductors.

The handbook is a little cheaper if you order directly from the NFPA.

Hi Vijay,

I'm trusting memories from way back here, but we use 110V DC batteries for substation protection systems too, and as another poster has mentioned, these are ungrounded (floating) systems. I recall the panel having an analogue meter (not digital!!) with a switch, that would by default measure your regular 110V DC output from the batteries. The other positions would measure from +110V to ground and from -110V (actually 0V, just the way it is marked) to ground. You could then at least see if you have a positive or negative cable that is developing a ground fault, and that reduces fault finding time. But we often had to switch off or disconnect circuits to find the specific cable.

I started looking into equipment that can speed up the fault finding process, but left the department before making headway. Items such as the Kongter DC Earth Fault Detector and similar models by others (such as Megger, but pricey!) seems what you are looking for, this digitrace system also seems interesting.

Have you considered using a TDR time domain reflectometer which sends an high frequency AC signal down the cable under test and the signal shown on the display will show a short, open or discontinuity in the cable under test?

An open circuit will display a vertical uptick on the screen while a short will show a down tick on the screen and a discontinuity will show as an offset.

The transmission can be inductively coupled or capacitively coupled to the cable.

It does take some interpretation of the results and the dialectric constant of the cable under test needs to be entered into the TDR. If you don't know the dialectric constant but know the cable length then dial up a constant which gives a display corresponding to the cable length. The dialectric constant of the cable changes the frequency of the signal making the cable appear shorter than it is. RF radio theory will assist in understanding this phenomena. The display will actually show you the injection connection and the constant and length of the connecting leads needs to be taken into account.

I have used then on long underground cable faults though the cable was deenergised at the time, as there is too much OHS when testing 66KV or 11KV cables but control cables have been checked in the powered up state. The TDR will even show an antenna feedline fault on RF systems, it takes some time and skill to learn to use one but on long cables an accuracy to 0.3m is easily achieved and then it becomes easy to check in the area for damage, and often multiple faults can be detected.

While all the current advice is valid and good, you need to be prepared that there are in fact multiple faults in the system, potentially in more than one conductor set.

If the cabling is of an age and manufacture that one is failing, then potentially the others are also close to failure.

I'm not saying to rewire the whole plant, just suggesting that when you fix the failure that you eventually identify, do not be surprised if the same failure mode happens, to be eventually traced to another section of the same cable run, or even the supply to another machine/process.

If the -ve on the <...110V DC...output...> is anchored to earth somewhere by design, then a fault between the +ve and earth would trip a circuit protective device to disconnect with a BANG! and an accompanying shower of sparks from the area of the fault. Such faults are usually accompanied by discolouration of the metal components in the area of the fault and maybe some local deterioration of insulation. The -ve would not do this.

Therefore "undetected" earth faults are on the black wires and not the red ones. That narrows it down a bit.

you can deduce within a couple of inches WHERE a short circuit, or open circuit fault, is along a line by using Time Domain Reflectometry tricks. Basically you sent a rising edge of a pulse down the line....it hits the deflect and bounces back to you.

The time it takes to return is read off the meter, and you can calculate the distance away that the fault is. If the fault returns in-phase, it was an open circuit. If the return is 180 degrees flipped in phase, the fault was a short circuit.

this works VERY well in things like shielded twisted pair, or coaxial RF lines, where the impedance is pretty well controlled. You do not have that, so there will be other non-fault discontinuities along your line, AND you will not exactly know the speed of transmission (i.e. the speed of light loaded by the cable insulation's dielectric constant), because you are not using controlled RF cables. But still, it would be one way to find the fault pretty precisely.

Find a "cable fault tester" or "time domain reflectometer for cables" and away you go