Neutral to Earth is Aprox 4 Volt

IT is due to 3 phase imbalance loading, causing current flow in Neutral line. Some imbalance is unavoidable. You must try to balance th3 3 phase loading as much as possible.

Dear Mr. Bravo

First of all thank you

I would like ask you few more questions

It means my earthing pit resistance is ok now which is around 3 to 4 ohm after opening or disconnected with earthing strips, there is no need to more improve my earthing pit?

What is the right method to check my earth pit resistance -

1. Without opening the earthing strips or after opening the earthing strips?

2. What should be earth pit resistance?

3. What should be neutral to earth voltage?

Kindly guide

Thanks & Regards

SAWAN

The neutral voltage is simply the expression of the neutral current and the earth loop impedance. To reduce it, either reduce the earth loop impedance or the neutral current or both.

The neutral current is simply the vector sum of all the phase currents and an expression of phase loading imbalance.

http://en.wikipedia.org/wiki/Earth_loop_impedance

The resistance of the earth pit has nothing to do with your readings. The earth pit is a zero reference point. When you have current flow in the neutral you have a voltage drop. This makes the point where you measured the voltage at a higher level than the earth pit. The only way to reduce the voltage difference is the reduce the neutral current flow. Or you could increase the neutral conductor size. 3.2 volts is not that high. Unless the current in the neutral is zero, you will always have a voltage difference.

Are you concerned about the voltage on the neutral relative to earth zero reference, or are you measuring 7 volts on the building steel and grounding system relative to the earthing pit? These are two different problems.

What is the voltage between the building steel and the earthing pit? If you are reading 7 volts there, then you may have great neutrals and a rapidly decaying building due to galvanic corrosion.

If 7 volts on the neutral is too high for your equipment... You will have to achieve better load balancing, or use larger or better bonded neutrals. Your spec of 1 volt may be ambitious. On the other hand, 7 volts on the grounding system relative to earth would be cause for all kinds of concerns.

1) Is the total series resistance under 1 ohm from panel LT to transformer neutral to bonding point to earth pit, or just the series from TN-TB-UPS-DG?

1.1) If the resistance did not change then you did not measure correctly, or the point where you measured is not connected to the grounding system is not connected, or the entire earthing pit is lined with an insulator which is preventing contact with earth relative to the power source. It must - must - change if you change the ion concentration up or down by adding water or salt. Are you certain the earthing system is absolutely bonded to the earthing pit, or have you somehow measured the system above ground, and the "other system" below ground, but not across the two tied together? You suggest you removed the connection to the earthing strips...

1.3) And then you saw a change to 7 ohms but on what side - the earthing system or the building earth bonded to the transformer neutral?

I suspect that what you really have is a connection between neutral and earth beyond the panel due to a skinned neutral or whisker or nicked jacket, perhaps even in an underground pipe which may have a lot of moisture or water in it. We just pulled underground line out of a building that had the same problems you describe. One tiny nick from sloppy electrical work had us chasing 7 volts one day, 20 the next, 2 or 3, then 30, all depending on how much water was in the pipe or humidity in the air.

Have your neutrals meggered again. Check all your insulation integrity. I bet you will find that your pit is fine, and your earthing system is fine up to the transformer and up to the panel.

1. no all are separate not in series

i want to describe you in detail we have one 400KVA D/Y transformer, 11KV/415V

for this transformer we have 2 body GI plate earthing and 2 neutral copper earthing.

From the pics you can understand and same for DG set and for LT & UPS we have only 2 body earthing for each.

You must bond all earthpits including those for lightening protection together. This will achieve a better earth resistance.Secondly check the harmonic content of the system. If it is high, it should be corrected by using appropriate measures. If even after that the problem persists then increase the neutral to ground conductor size.

Dear Mr.Sawan

Could you throw some light on the installation details like the vector group of the transformer,the primary earthings done at your installations ,details of your distribution boards,whether 3/4 pole Breakers/MCCB's etc used.What about your neutral earthing?What is the system of earthing adopted?You may have to have the Parallel neutral earthings done at Transformer ends.Is your neutral -Body earths mixed?What is the capacity of the UPS and what about the neutral of the UPS?

Kindly have me some details.

radhakrishnan k a

System of neutral earthing is Copper plate earthing (600x600x5) CU with charcoal and salt in 12 feet of dig.

Yes we have Parallel neutral earthings done at Transformer ends.

no mixing between body and earth

Capacity of UPS is very small 20 KVA Make - DELTA, 3Phase In and 1Phase out and output neutral is directly loop with the raw power.

Elect Single Line Distribution

Regards

SAWAN

Dear Sawan,

The circuits are not clearly visible. But in your original post you have mentioned that there are 36 nos. SPM machines( electronics based) as loads. Please note that neutral to earth voltage is simply multification of current thro' neutral and earth loop impedance. You have already reduced the earth impedance also balanced the three phase loads. Therefore, it is apprehended that, due presence of high quantum of third harmonic component(due to electronic SPM machines) which are passes thro' neutral to ground, this voltage drop may occur. So please arrange to check third harmonic component in the power supply.

Thanks,

Manindra.

hello sawan , charcoal and salt mix water is good for grounding system, but you need to dig 12meters deep to solve that problem, and the verygood reading of grounding system is 5 to zero ohms that is the standard grounding, and we incounter that kind of problem before by follwing this spec, and also pls retigh all electrical terminal so you will preventing high amp / voltage reading

keep up the good job guys !!!!

Sawan,

Please carry out harmonic analysis of your loads. The zero sequence currents due to third harmonics are additve i.e all three phase currents add up in the neutral and generally these are the cause of high neutral current and consequent voltage drop. Eliminating third harmonics with proper supression or filter will take care of your problem.