Electronic Earthing System

hi

UPS should have dedicated earthing as it is not sync with the actual source and it generates power converted from batteries.

thanks

Is dedicated earth different from clean earth?.

Dedicated earth for electronic equipment is also 'clean earth', but it is isolated from the equipment earthing grid. Some equipment suppliers are of the opinion that interconnection between these two networks may cause damage to the electronic circuits due to voltage surges coming from equipment earthing grid during severe ground fault condition.

i have a query EE,

PLCC equipment earthing is done seperately and it is done through an isolated earth pit with an electrode. this electrode is not bonded to any earthing system whichever is present in the plant.

Why it is so?

I am confused since the standards quoted by you depicts a different picture altogether.

PLCC means ?????

Suppose your separate earth electrode has a resistance of 50 ohm to earth.

Site Supply 220V shorts through to case of small computer using that earth. Case (and everything connected to that earth) becomes 220V to anything connected to proper earth, could be lethal to a person.

Only about 5 amps flows - how can you guarantee a fuse blows etc to isolate the fault - safety standards require that fault is removed in ~5 seconds by fuse or breaker isolation.

From the point of view of instrumentation , using a separate earth for say, instrument common, is connecting it to an unknown potential/current acquired from whatever leakages may be about. Recently, several racehorses were killed by leakage from a buried high voltage cable in the paddock - jockeys felt shocks but fortunately not enough to injure them (horses do not wear boots). The HV cable stayed live until it was manually isolated.

<...Some equipment suppliers are of the opinion that interconnection between these two networks may cause damage to the electronic circuits due to voltage surges coming from equipment earthing grid during severe ground fault condition....>

Such an opinion does not correlate with experience here. There is a strong possibility that the suppliers include these sorts of statements to cover their corporate backsides and obviate warranty repair and replacement claims.

That is right! its just not opinion its fact! and moreover the purity of sine current delivered from the ups has its own mystery and never a friend of current generated from a alternator.

i dont agree with the elec expert to an extent because you can see that if we mix all in the same loop the the chances of complexity of faults are more and the place i am currently working with , its a sorry chance to do it.

I don't care whether anybody agrees with my posting. I have only quoted from the Standards 'verbatim'. If you want to violate the Standards, it is upto you.

Clean earth is intended to immunise electronic equipment against current surges in the protective earth for power systems. It is achieved by running a separate earth cable from the instrument earth bar in the panel to the earthing point. Thus, any fault current surge in the protective earth on the power system will have the least effect on the instrumentation.

Yes, I agree.

As noted by E.E.65, the wisdom of experience, put into Standards, is that far from a separate ground "electrode" being necessary or mandatory" having one causes trouble.

Electricity does not read "clean" or "separate". It goes its own way.

As PWSlack wrote, a separate instrument connection back to the earth you do have is better [and the isolation is essential for it to work]. But do not forget that there may be a big transient potential between instruments etc and the "power earthed" metal around them. The crunch point is that a short to earth in one equipment connects its live wire to its earth wire - its local earth gets half the supply potential.

Frank

I am not sure I understand you point, but if you are suggesting separate grounding electrodes for electronic equipment, that is incorrect. There can be only one GE. If you install separate electrodes, you are creating a dangerous situation. You can have voltage differences between the electrodes and thus potential for shocking employees. If you do install additional electrodes, they must be bonded together.

EE65 post makes this the rule per IEEE stds. Pw's post makes this same point.

If you have two or more ground electrodes interconnect so all are in parallel hence reducing earth potential.

As I understand from above discussions that we may have an electronic earthing grid as suggested by PWSlack to immunise electronic equipment from current surges and electrical noises of power system. But that should be interconnected with the protective earth grid to prevent any potential build up between the two networks. I think this arrangement also satisfies the requirements of the standards as mentioned by electrical expert.

The interconnection takes place once only, at the earthing point. The principle is to isolate the clean earth from the dirty earth as far as reasonably practicable, to minimise any effect on the instruments during the time it takes a protective device to operate and cut off the dirty earth fault current flow.

It doesn't matter what method of earthing is used at the earthing point so far as the clean earth connection is concerned.

Same thing was said in BS,to connect lightning,system, and telecom earths together

That's because earth is earth is earth.

PW Quote "The principle is to isolate the clean earth from the dirty earth"

Pw, how are you defining the clean and dirty earth? I am trying to compare your statement with the NEC. The NEC requires an equipment grounding conductor(EGC) to be installed from the power panel to the equipment frames. It is isolated from the system neutral down stream from the main disconnect. It may have more than one piece of equipment connected. For some installations, engineers will call for an isolated EGC that electrically is a direct run from the main switch to the piece of equipment with no other connections. Is that what you are defining as a clean earth.

The fact Earth is Earth is Earth is always right . Because Earth is a huge zero potential .

What i was saying is that we need to have different earth pits for the same i.e different grounding areas.

Difference between electrodes is never a picture as both of them are drawn with sufficient distance as per electrical practice.

The verticals related to all the IEEE standard could also be right as per books.

The current in an industry is always not clean it has its own distortions, harmonics of all grades.

I was working in a MNC bank till last October as a technical designation. MNC bank treating its power sources to be very critical had ups which was under critical category.

The standard that is been used in our country is to have different paths of these leakage current , fault current to travel such that it should not even cause a single Switch gear to trip as it would lead to business emergencies during unimaginable hours.

What if the Earth is Earth and Earth is Earth is always true. Why does it matter to have a single source ground.

Regards

How is your UPS frame is earthed?.-Is it connected to system earth?. Is there a separate earth for receptacles connected to the UPS output?.

Frankcorners,

Earth is so often referred to as the theoretical zero potential in texts that it is forgotten that nice moist, arable soil that we and plants like to live on or in has a resistance of around 100 Ω-m (dry sand is a near insulator). At that, I find it easiest to think that the resistance between opposite faces of an isolated cube with 1 metre sides is 100 ohms - it is high compared to a copper cable, is it not?

Recommended formulas in earthing standards suggest that one can expect a 3 metre long 50mm diameter rod hammered vertically into 100 Ω-m earth to have a resistance of 30 ohms to the theoretical zero potential.

You need 60 or so, in a grid with spacing of metres to get the ~0.5 ohm specified for electricity stations.

Compare the resistance of 50m of 2.5 sq.mm copper cable, about the smallest cross-section earth return which would be considered for high grade installations and no more than is required for bonding to a wash basin - it is about 0.4 ohm.

Suppose you have two 0.5 ohm earth grids, far enough apart to be "independent", the resistance between them will be 0.5 + 0.5 = 1 ohm. That is substantially more than the direct bonded path of 0.4 ohm to the source for the minimal bonding example I gave.

In practice, the "bonding together" resistance of most of a large installation would be much less than 0.4 ohm - if it were 0.04 ohm then only 0.04/1 or 1/25 of the earth potential difference between the two systems could transfer into one of them.

So just having a "separate" earth has isolated the cabling of the two systems, and consequently their interaction, mostly because low resistances to "earth" are difficult to get!

Practically, as PWSlack pointed out, it is better to insulate the sensitive earth return system with cables and insulators than to depend on the low, and variable, resistance between earths. Bonding back separately with one or two cables to the common source of an installation makes the possible potential difference a value which can be calculated and used for design.

If you want isolation between electronic systems and "noise", you begin with simple distance, as much as you can get! You do not put the microphone input of a sound amplifier at the power transformer end! Next you use shielded & twisted pair cable to connect to the pre-amp to reduce magnetic/electrostatic pick-up (better to have pre-amp in a metal box carrying the microphone socket). Test equipment with high immunity/emission standards has metal boxes within metal boxes with filters between for power and signal as needed.

No one calls for microphones to have their own earth rods "because this reduces noise", even though they may be transformer isolated. An earth would usually introduce noise, not reduce it.

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