Field Testing Underground Conduit For Grounding Suitability 1

[OhioAviator]

I have an existing facility that was built 30+ years ago where the 480V power is supplied by transformers with ungrounded delta secondaries. As such, no equipment grounding conductors were pulled through the conduits between the ungrounded delta transformer secondaries and the motor control centers, or between the motor control centers and the individual motors out in the plant. Instead, the rigid steel conduits are used as the equipment grounding conductors, which the NEC does allow. The vast majority of the conduit runs between the motor control centers and the motors are buried in underground duct banks. I am concerned about corrosion over time that may have occurred with the buried rigid steel conduits and do not have high confidence that the existing buried conduits are actually capable of carrying the likely fault currents that could develop in the event of a double phase to ground fault. In my mind, it isn't adequate to simply conduct a ground resistance test between each motor and the motor control centers. Even though that test does measure the resistance of the ground return path, it doesn't measure how much current the 'equipment grounding conductor' (the steel conduit in this case) can actually carry during a fault without failure of a corroded conduit joint or the conduit itself. Does anyone know of a test that can be performed on buried rigid steel conduit to determine its fault current carrying ability, and hence it's suitability to continue to be used as an equipment grounding conductor? Am I worried over nothing?
Thanks in advance for your help.


Galatians 2:20

 

[Skogsgurra]

This is really a coincidence!
We are faced with a similar problem. It is about a meshed ground grid in an Ex process plant. This plant is 40+ years old and there are signs indicating that there might be several local problems with the ground grid. We actually do not mind if the earthing resistance is OK or not - it is the continuity of the ground grid we want to verify.

There are devices that can send heavy currents through the grid (normally used for primary test of protective relays), but those devices are rather heavy and also need a 63 A 400 V supply and are therefore not easily employed to test the continuity on hundreds of down conductors across a 1000x1000 m process area.

Also, we need to find out the inductive part of the grid's impedance, we therefore need to test at low frequencies and we need to test at higher frequencies and calculate R and L from the measurements taken at around 50 Hz (R) and between 5 and 10 kHz (SQRT(R[sup]2[/sup]+wL[sup]2[/sup])). We are now doing the final lab tests with our portable device that couples between 10 and 20 A inductively coupled current to the down conductor, measures driving EMF and current and then synch detects the current, smoothes, divides and finally shows the result in milliohms. We get stable readings down to 1 milliohm with a fluctuating 0.1 milliohm figure.

It is a lot similar to the ground grid clamp testers that you can buy from C&A, Fluke and other manufacturers. Only with better resolution and able to use realistic frequencies. The commercial ground grid testers are not, we tested, usable on ground grids with 300 feet AWG 2/0 and 4/0 meshes because the L/R ratio is rather high, we see 10 - 100 millisecond, and therefore the typical 1500 - 3000 Hz frequency used by the clamps doesn't work very well. It is better suited for earth loops in cables, where L/R is kept at 100 microseconds, or so.

We are, as I said finalising the tests during the next weeks. I will be back with results when we have them.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[stevenal]

I doubt that a corroded joint would suddenly fail to carry current during a fault, since fault currents are pretty difficult to interrupt. The current would simply arc across the failing joint until interrupted by protection. Normal ground testing at 300 or 600 A should pick up resistive returns. The scenario you describe would require simultaneous ground faults on different phases at different locations. The chance of such scenario can be greatly reduced by using a ground detection system to detect and address the first fault before a second one occurs.

 

[OhioAviator]

Thank you for your reply, Skogsgurra. I'd be very interested in learning the results of your testing.

Stevenal... the problem is a little more complicated than this, I'm afraid. We already do employ ground detectors and you're right about their use reducing the chances of a double phase to ground fault occurring. What I didn't mention was a mandate from executive management of our company to replace all ungrounded delta transformers with grounded wye transformers as soon as possible. I know... there are all sorts of technical discussions that can occur (and have occurred) relevant to the technical desirability of using an ungrounded delta system. However, when the people signing your paycheck say, "Thou shall...", I don't argue unless there's a darn good reason. Therefore, I don't dare install a solidly grounded wye system without full confidence that the existing grounding system will adequately carry rated fault current long enough to trip upstream protective devices. I am also considering employing a resistance grounded system that will limit the fault current to a known value of around 25 to 50 amps. But even with that, I want to know that the existing grounding system is of a low enough resistance value and able to carry the 50 amps of limited fault current continuously. If the ground resistance is too high initially, or becomes too high during a fault, then the touch potential during a ground fault could increase to a value that might be unsafe.
Make sense?

Galatians 2:20

 

[Skogsgurra]

In "our" case, it is about a 400 V TN system, solidly grounded. The PSC is somewhere round 30 - 40 kA, depending on contribution from running motors. There has been an incident with suite of damaged bearings and that followed after a ground fault in the plant.

Having a bad connection in such a system is very much like electric welding with the ground clamp far away from the welding spot - currents will be all over the place. And that is what we suspect. And that is why we do this test.

We have already tested "Bearing Inception Voltage" and found motors where we are down to a few volts. You do not need much resistance to produce two volts at a 40 000*sqrt(2) A peak fault current. Bearings are damaged in microseconds. At least, a crater that develops into a failure over time, is created in microseconds.

Even if we find a low enough resistance, the current "front" (di/dt) is sometimes so fast that even a minisule inductance will cause enough voltage to create a crater that represents a potential problem for the bearing and can reduce its life.

We feel a little like pioneers here. Anyone out there that done similar work?



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[dpc]

The scenario you describe would require simultaneous ground faults on different phases at different locations.

Unfortunately, that occurs all too frequently with ungrounded 480 V systems in industrial facilities. The first ground is ignored - sometimes for weeks, months, years, then a second phase goes to ground somewhere else.

Based on experience, and review of available test data, I have little confidence in the use of steel conduit as a grounding conductor, even when new. After being in place for 30 years, I would have even less confidence.

The fact that conduit is still allowed to be used as a grounding conductor in the US is mainly a testament to the influence of the steel conduit manufacturers on the Code Making Panels that deal with this issue.

We require a ground wire be run in every power circuit, regardless of system voltage or conduit material and nearly all consultants that I have talked with do the same thing.

That said, it would be difficult to test the conduit system for impedance at fault current magnitudes (and frequency).

 

[OhioAviator]

Thanks dpc. I have the same lack of confidence in continuing use of conduit as the equipment grounding conductor. All systems that we have installed over the last 15 years or so have ground wires pulled with the phase conductors in each and every conduit, regardless of the conduit material.

I'm not sure about code compliance, but what about this....
It might be possible to dig a shallow (18 to 24 inches) trench between the transformer pads and the MCC building, and a number of similar trenches between the MCC buildings and the buildings/steel structures where the motors are scattered. If we can do that, and install parallel runs of 4/0 AWG stranded bare copper grounding cable in those trenches, it would provide a parallel low resistance equipment grounding path back to the MCCs and ultimately back to the source transformers, would it not? Is there a code issue or technical issue that would prohibit this approach?

Galatians 2:20

 

[dpc]

Here's a link to the classic paper by Richard Kauffman/GE on this subject:

http://www.powercet.com/uploads/files/GE-More_Specific_Eqpt_Gnd.pdf

As you will read in this paper, running the ground wire external to the phase conductors is not nearly as effective as running them inside the conduit, due to the increased reactance - especially with steel conduit. It could offer improvement over the existing situation and should not be a Code issue, since you still have the conduit system in place.

Where the conduit is accessible, checking all grounding bushings, etc is certainly worth doing.

 

[OhioAviator]

Excellent paper, dpc. Thank you for the link.
I believe that if we end up installing a neutral grounding resistor to limit the ground fault current to 50 amps or less, I would feel a lot more confident that we can continue to use the existing conduits if we supplement them with added parallel runs of 4/0 stranded bare copper grounding cable as described above.

Still, I would appreciate any ideas as to how to test the existing conduit already in place to see how much voltage drop would occur across the conduit if about 100 amps were driven down the conduit from end to end. Perhaps a MultiAmp type large current source could be employed using long heavy cables stretched out to reach both ends of each conduit?

Galatians 2:20

 

[DTR2011]

you may want to check Omicron CPC 100 + CU-1 accessory. You can perform inject currents at off frequency (40 & 80 Hz) and average the two. Either find a service company with it or rent one.

 

[OhioAviator]

Thank you, DTR2011. That instrument looks pretty nifty, especially since it can do tan delta testing of cables.
Thanks!

Galatians 2:20

 

[Skogsgurra]

Skogsgurra. I'd be very interested in learning the results of your testing

OK, here is a sample screen. Text in Swedish, comments in English.

We have been able to locate quite a lot of bad connections, both above and below ground. We have done tests in a biogas installation and a 130 kV installation so far. The biogas is a rather new installation. We have not yet tested in the refinery. That installation is 40+ years and close to the sea - we are looking forward to find what the salty mists have done to the installation.

http://gke.org/pub/files/Ground Grid Verification.png

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.