Point to point ground impedance test

[Paykon]

Do you recommend AC equipment or DC equipment for testing and why?

 

[trosepe]

550 to 600 cps is typical and the most accurate. DC is influenced by galvanic curents in the earth. 60 cps is influenced by stray power system currents. The ground measurement is very sensitive and the accuracy of the result is compromised by using either DC or 60 cps.

 

[Paykon]

I am looking for procedure of conducting the ground-grid integrity measurment test(Point to point ground impedance test)using DC equipment?

 

[trosepe]

Go to this site:

http://www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

Should answer your questions.

 

[Paykon]

The link explains about ground grid resistance measurment. I am looking information about the ground-grid integrity measurment test using DC equipmentor high current injection test?

 

[trosepe]

Need more information on your system and what you trying to do. The ground 'grid' has a earth resistance which can be measured using the procedures outlined in the link. If you are trying to determine the inegrity of individual ground grid connections: you can do that by injecting AC or DC current thru the connection and measuring the milivolt drop across that connection. A better and more accurate way is to buy a 100 amp DLRO (Digital Low Reistance Ohmmeter). This unit injects 100 amps DC thru the connection or contact under test, measures the voltage drop across the connection, then clculates and displays the resistance of the connection in micro or milli ohms.

 

[Paykon]

Troepe,

You are right. We can inject 100A DC current and measure voltage drop and use a shunt resistance to measure current to calculte the resistance of the connection points for point to point connectivity test of the grid.

 

[Paykon]

One more question. How do we select the selected points for point to point connectivity test in a station to see what points are connected to the grid. If there is no drawing for the grounding system.

 

[trosepe]

You would only use the injection method to test visible bolted, clamped or cadweld connections.

If you are trying to determine if some or all of your substation equipent has a good connection to the ground grid or mat, the 100 amp injection test will not work due to possible multiple current paths.

The best method is to measure the absolute earth resistance of the ground mat using the three point fall of potential method and then measure the resistance of each piece of equipment to the ground mat using the direct method. Both tests are outlined in the 'Getting Down to Earth' paper and both are performed using a four terminal earth resistance test set which operates at 550 to 600 cyc. In this way you will be able to determine the absolute earth resistance of each piece of equipment.