Resistance of cable 2

[hanksmith]

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

 

[ScottyUK]

Is it just one single conductor, or do you have another conductor of known properties on the same route?

 

[mgtrp]

I don't imagine the fact that it's embedded in concrete (if I'm understanding your post correctly) is going to make measurement of the resistance of a bare copper cable any easier regardless of what you do!

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.

 

[waross]

Well, if you ran an insulated 4/0 copper cable to the other end you could pass a current and then measure the voltage drop across the insulated cable and the bare cable. From the difference you will be able to calculate the contribution that the concrete makes to the conductivity. That long a run of bare copper encased in concrete, I would expect the resistance to ground to be only a few ohms. If the concrete is insulated from ground the effect of the concrete will be less.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hanksmith]

What about just using a multimeter with super long lead; basically use an insulated cable as an extention to the meter leads? Measure the resistance of the meter leads then measure the resistance of the ground cable and the difference would be the ground cable we are interested in?

 

[DTR2011]

A multi meter won't have the "oomph" (voltage to push the current), nor the accuracy. For low resistances, you need to use a 4 wire method of measuring and measure the voltage drop at the point you want to measure - this eliminates any lead resistance (and loading) effects on your measurements.

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.

 

[WoodrowJWeen]

Are you looking for the resistance from one end to the other or resistance to ground?

 

[rcw retired EE]

Since this is a bare cable, I assume it's purpose is to tie the two ground grids together to provide a low impedance path for fault return current.

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.