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  1. nchrista

    Does Grounding Rebar (Ufer) Weaken Concrete?

    The Green book has a section that actually provides formulae and a brief but succinct description on ufer grounding and when it may weaken the concrete. It appears that concrete is impervious to AC current but DC presents a problem when it breaches 60A. The concrete has a rectifyng effect on...
  2. nchrista

    Does Grounding Rebar (Ufer) Weaken Concrete?

    jghrist, I know about the NEC requirement but I'm being asked to verify that grounding the rebar does not weaken the concrete. Neither IEEE-80 nor 142 shed any light on the issue that I've been able to find. I think common sense should dictate this one. Not only would not grounding the rebar...
  3. nchrista

    Does Grounding Rebar (Ufer) Weaken Concrete?

    Hello All, Does anyone know where I can find information on this topic. Particularly helpful would be a study that provides ground fault current vs structural concrete strength and/or life expectancy.
  4. nchrista

    Ground Potential Rise Vs. Touch Potential

    jghrist, Can you also post the GPR contour map. As I've stated, it makes sense to me that the touch would be safer in the direction of decreasing voltages. As you've stated, the GPR contour is the inverse of the touch, and it shows safer in the direction of increasing voltages. I believe this...
  5. nchrista

    Ground Potential Rise Vs. Touch Potential

    I understand the equations and the relationship between GPR and Touch Potential. I believe what GroundMat has shown me is correct only had to adjust my thinking to interpret the results correctly. GroundMat shows a safer arrow going in the direction of increasing potential. What the good...
  6. nchrista

    Ground Potential Rise Vs. Touch Potential

    Ok, I think I see what's going on. Most of my confusion was coming from the 3-D model of the GPR. The safer arrow points in the direction of higher voltages, which intuitively didn't make sense. Following the definition of touch potential: The touch potential is the potential difference...
  7. nchrista

    Ground Potential Rise Vs. Touch Potential

    Though the entire grid was modeled, the grid area under concern here was confined to the area of the largest fault currrent, 60kA at one of the utility feeds. The 50V touch potential is graphically produced by the SKM GroundMat software. The site has been modeled, both in excel with...
  8. nchrista

    Grounding Requirements

    The nature of a delta is such that there is no neutral to be grounded, I believe it's referred to as a floating delta (though somewhat reduntantly). The only way to ground a delta system is either to break open the delta (thus an open delta) and ground one of the legs or to leave the delta...
  9. nchrista

    Cable Sizing

    You have to be careful to use the low voltage tables for low voltage and medium voltage tables for medium voltage. Not all cables are created equal. Also keep in mind that the XLPE cable is not an NEC type. The NEC just provides a guideline on cable properties. It is left up to the engineer...
  10. nchrista

    Ground Potential Rise Vs. Touch Potential

    Hello All, I created a ground grid for a multi-peaker unit site with soil resistivity in the neibhborhood of 10.7 ohm*m. There is approximately 7000 meters of 4/0 copper with some 500 kcmil in the heavy fault (60kA) area. The grid area is in the neighborhood of 300km^2. My calculated GPR is...
  11. nchrista

    A cable question?

    In our case we have a substation with both AC and DC loads. Though we will likely use separate cables within one conduit, the loads will involve short distance connections to some microprocessor based relays and other connections to motors, heaters, and lights. Thanks to all for your input...
  12. nchrista

    A cable question?

    No, I literally mean in the same cable. Meaning, I have a multi-conductor cable with several conductors going to supply an AC load and 2 to supply a DC load. Is there some reason (NEC or otherwise) that this can't be done? Since you mentioned it, what about in a conduit? Yours, Nick
  13. nchrista

    A cable question?

    Can DC conductors be used in the same cable with AC conductors?
  14. nchrista

    Power of Katrina

    Yeah, and the energy required to make two electrons touch would be enough to lift all the water out of the all the worlds oceans to a height of 4 feet and hold it there for 1 second. It doesn't have to be a hurricane to be big. Yours, Nick
  15. nchrista

    Where can I find a table of Rt or Kt battery sizing values?

    Hello All, Does anybody know where I can find standard Rt and Kt values for sizing an industrial Lead-Calcium battery? I've made the entire iterative spreadsheet recommended by IEEE-485, but it does me no good without those values. Also, does anyone know the design factor used for sizing the...
  16. nchrista

    Duplex Motor Control for Crane Bridge Drive

    How much time should I alot for supplier to train plant crane operators?
  17. nchrista

    Duplex Motor Control for Crane Bridge Drive

    There's a structural engineer taking care of that part, my job was to take care of the electrical portion. From what I read in his portion of the spec. the bridge structure is nearly adequate to handle the uprate, but will need upgraded wheels, bottom plate reinforcement, and web stiffeners. A...
  18. nchrista

    Duplex Motor Control for Crane Bridge Drive

    I have a question concerning the motor system for a crane bridge drive. I'm writing a spec. for a 15 ton upgraded to 25 ton (by mechanical means with reconditioned motors) overhead electric crane. Presently the bridge has a duplex drive (for 2 DC mill type 800 series motors), series wound...

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