Grid faults- Neutral-Ground at 70V!

lazza

HI forum.

Wasnt quite sure under what topic to put this as it isnt directly related to solar, but I know there are some seriously good electrical experts and I hope you can help me.

My house is in Spain, where there is a 230V  T-T system, the neutral being grounded at the transformer substation a few hundred meters away (in theory). However, the last few days my RCD has been tripping.

When I measured the voltages I was in for a surprise. L-N is normal at 230V. But, N-Ground has been oscillating between 60V and 70V! When this drops below 50V the RCD no longer trips. Voltage between L-Ground is around 280V .

My first thoughts were a problem with my ground rod. However, i've been to my neighbours houses in the street and they all have exactly the same anomaly.

I have called the distribution company, who i have little faith in. So I'd like to know what on earth could be causing this anomaly in voltages?

Thanks for any assistance
Larry

Aguarancher

I have seen that caused by a bad transformer ground at the pole. The power company in my area, has made a point the last couple of years, of going around and checking all the transformer ground wires at the poles.

BB.

The other possibility is that somewhere in your neighborhood, there is a home/business with the hot/neutral swapped (they grounded/shorted the hot wire to earth somehow--either direct connection, bad insulation, etc.).

I have hunted down bad wiring (and small scale circuit board shorts) by connecting to a common point (earth ground in this case)--And go to each home and see which has ground to Hot with near zero (or lowest voltage). The point where Ground to Hot is lowest is probably near the point of shorting.

I have not tried this with electrical, but you could try "direction finding" similar to RF signal hunting. Take two metal rods and place them X meters apart and measure the voltage between them.

Then take one of the rods and rotate 45 degrees and drive it into the ground and measure.

The point at which the measured voltage is the greatest, a line through the two rods will point towards (or 180 degrees away) from the source of the voltage. (and conversely, the point at which you measure zero voltage will point 90/270 degrees from the voltage source).

This is exactly how you rotate a directional antenna to find a radio source.

Then move XX meters away and try the same experiment. In theory, you would find the second line--And where the original and the new line cross should be the source of the ground voltage.

Be careful when driving rods/measuring voltages--I have measured >60 VAC and received a good electric shock from two ground electrodes 50 feet apart when there were grounding problems around electric pumps.

Electrical flow may be quite varied due to ground water/moisture levels and free ions (pollutants, fertilizer, irrigation, water pipes in ground, etc.)... So it may not be accurate enough to work (vs free air signal propagation for radio waves).

The gas company measures ground potential to find problems with gas line galvanic protection systems (gas lines are charged and insulated from earth to prevent corrosion).

-Bill

lazza

The subcontractor for the distribution company has found the root cause. A house down the street is responsible for the deviation. When they quit their live wire, the neutral-ground problem disappears, dropping back down to 2V. They said it is probably faulty equipment, a bore-hole pump or a deep freezer that normally cause this anomaly. What I dont understand is the physics of it (maybe inetdog can explain?).. how can faulty equipment in one house draw the Neutral-Ground voltage of the whole street from 0V to 70V?

Aguarancher

Very interesting. I would say BB was close to the problem. Would have thought the problem house in question must not have a very good ground rod. Glad your mystery was solved.

lazza

The guy from the company just came round and said it was actually some computer equipment- that puzzles me even more, how can faulty equipment cause the neutral to ground to change so dramatically?

vtmaps

lazza said:

The guy from the company just came round and said it was actually some computer equipment- that puzzles me even more, how can faulty equipment cause the neutral to ground to change so dramatically?

Anytime you inject current into the ground (that could be lightning or faulty equipment), the ground where you inject the current can be at a different potential than the ground somewhere else.   So your ground rod was at a different potential than the ground rod the power company was using. 

That's not a problem until you have a common point of reference.  There is at least one copper wire that connects from your house to the site with the different ground potential.  That wire provides the common point of reference... it has the same potential at each end, but each end of the wire is referenced to a different ground rod.

--vtMaps

lazza

Mmm vt Maps, not sure it works like that here. Our ground rods are separate in each house- the neutral is grounded at the transformer. The house causing the problem is at least 200metres away.... still struggling with the physics of all this 

inetdog

lazza said:

Mmm vt Maps, not sure it works like that here. Our ground rods are separate in each house- the neutral is grounded at the transformer. The house causing the problem is at least 200metres away.... still struggling with the physics of all this 

Am I correct that your neutral is not connected to the ground rod at each house, as it is here?

lazza

sorry for the delayed reply - yes correct- neutral is only connected at the transformer station