Resistance of cable
Resistance of cable
(OP)
Does anyone know of a method to measure the resistance of a long ground cable that has been installed?
The cable in question is a bare 4/0 copper cable installed in the concrete of a conduit bank. The conduit bank is about 2km long (1.2 miles) and I have to ensure the resistance is low enough to meet my design needs and to ensure it was installed properly.
Thanks
Hank
The cable in question is a bare 4/0 copper cable installed in the concrete of a conduit bank. The conduit bank is about 2km long (1.2 miles) and I have to ensure the resistance is low enough to meet my design needs and to ensure it was installed properly.
Thanks
Hank






RE: Resistance of cable
RE: Resistance of cable
If the most important thing is just that the ground grids are the two sites are tied together effectively, rather than the cable resistance, you might be able to pass current through an installed insulated conductor between the sites and return it through ground, measuring the voltage drop. Note that this would include the resistance of the insulated conductor. 2km seems like a long way to try measuring the voltage at the remote end using an additional spare insulated conductor.
RE: Resistance of cable
Bill
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RE: Resistance of cable
RE: Resistance of cable
How badly do you need this measurement? There is test equipment available made to measure such things, but it is likely beyond your budget. Perhaps talk to a service company. You could cobble something together with a few car batteries and a suitably sized resistor that would probably work as a current source.
I haven't tried this myself, but was thinking that a transformer winding resistance test set may do the trick and most service companies will have one. The test set will likely give up to 40A DC (with the oomph) and can measure down to micro ohms. If you have other cables in the conduit (de energized, of course) you can use them for send/return of current and voltage measurements.
RE: Resistance of cable
RE: Resistance of cable
If that is the case, you don't need to know the resistance, just the effective impedance between the two grids. If you have an insulated cable in the duct bank, you can do an injection test at both ends to measure thae impedance. (See IEEE 81.2 "Guide for Measurement of Impedance and Safety Characteristics of Large, Extended, or Interconnected Grounding Systems" paragraph 8)
Tie the insulated cable to the grid and 4/0 ground at one end and connect a generator between the cable and ground at the other end. Raise voltage on the generator until measurable current is flowing, say 10-20 amps, or higher but within the rating of the insulated cable. The generator voltage will be the local GPR plus the voltage drop in the test lead. Measure the step and touch potentials at both ends using a voltmeter. Measure currents in the duct bank ground wire and other ground paths such as an overhead line ground wire. Swap ends and repeat test at the other end.
To get fault GPR, ratio the measured voltages up by the estimated fault current to generator output current ratio.
Comparing current measurements on the 4/0 ground wire at both ends during the same test will give indication of its continuity.
For accuracy, run the generator at 55 or 65 hZ and use a signal analyzer to filter out only the test voltages and currents. The standard also describes how to calculate the errors caused by induced voltages in the test leads.
There are several safety issues involved with this test. I recommend hiring a testing company.