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  1. DanDel

    Testing equipment for measuring powerline ground impedance

    The resistance to ground of a ground rod, ground grid, or any buried metal item is typically measured using a three-point, Fall-Of-Potential test, where two test rods are placed in a straight line out from the equipment under test. The typical distance is 100', which usually suffices for a...
  2. DanDel

    Arc flash calculations

    For the first question, 3-phase arcing faults are typically the worst-case scenario, and as already mentioned, L-G faults turn into 3-phase faults quickly. GF protection is not considered in AF calculations because the low trip setting would provide an erroneously low amount of AF energy. In...
  3. DanDel

    Getting 120V from a 240Y/139V XFMR

    Don't forget that if in fact the plant voltage is 580V phase-to-phase, you will actually have 253/146V on the secondary with the nameplate ratio nominal tap. Using the 578V tap, you will be close to the nameplate 240/139V.
  4. DanDel

    Pleading ignorance. Difference in induction motor vs generator?

    An induction generator is simply an induction motor which has been brought up to speed the typical way from a starter circuit and then driven slightly over synchronous speed by a prime mover attached to it's shaft.
  5. DanDel

    SEL-751A Modeled in ETAP

    This can be modeled in ETAP for any protective device as mentioned, but unless the manufacturer will guaranty this system will work on all instances of arc fault (which they won't; I've asked) I would not include it in an AF calculation.
  6. DanDel

    Power transformer energization

    Since so much time has passed since installation, a full set of field Acceptance tests should be performed per the NETA ATS spec, including Insulation Resistance/PI/DA, Doble PF (including all bushings), Turns ratio on all taps, and fluid DGA & screening. Also SFA (Sweep Frequency Analysis)...
  7. DanDel

    NEC code requirement

    Per the NEC 695.4(B)(22)(a) & (b), a Fire Pump motor circuit only needs short circuit protection. Not ground fault protection, not even normal overload protection: "(a) Individual Sources. The overcurrent protective device(s) shall be rated to carry indefinitely the sum of the locked-rotor...
  8. DanDel

    MV/HV Learning & Multiple Questions

    I would start with the IEEE series of books; 242 Buff Book(Protection & Coordination), 141 Red Book(Power Distribution), 241 Gray Book (Electrical Power Systems Commercial), 399 Brown Book(Industrial and Commercial Power Analysis), etc. The CT rating 'MR' does indeed mean Multi-Ratio. 'T'...
  9. DanDel

    Arc Flash Reduction Maintenance Switch

    I prefer to use the worst-case values on the attached label onsite, and keep the labels with the values from the maintenance-mode settings in a book kept secured by the facility maintenance manager. Since the worst-case labels are in effect for the vast majority of the time the equipment is in...
  10. DanDel

    Abnormal dga

    Outside of a possible thermal fault, with hydrogen that high, you probably have a partial discharge issue also.
  11. DanDel

    Polarization index

    500VDC is acceptable. The recommended voltage for Insulation Resistance testing for windings <600V is 1kVDC per NETA Table 5, so 500V will not cause a problem. Not sure what a PI test will tell you about hydrogen production. This is typically cause by partial discharge or hydrolysis.
  12. DanDel

    Run/Update short circuit current-ETAP

    I've modeled your system as close as I could, and I have gotten the same results. The only thing I can think of is that ETAP is using the total GF current instead of what is being seen through the individual CT as listed on the TCC kA page. Modeling the GF relay and (zero sequence) CT on the gen...
  13. DanDel

    Run/Update short circuit current-ETAP

    What is the value of the neutral ground resistor?
  14. DanDel

    unusual LTC DGA results

    'pete, I don't think you can perform an empirical chemical analysis based on the DGA values from an oil sample. The general opinion seems to be that an internal inspection is needed. I would imagine you would find a 'cloud' of carbon around one of the contacts or connections inside, which is...
  15. DanDel

    unusual LTC DGA results

    Hi 'pete. Looks like the tap change on 7/5 caused your increase in H2 and C2H2, which would be typical for internal arcing. It's tough to analyze DGA in a LTC since all the real bad gasses are caused by it's normal operation (switching contacts - arcing). Your other combustibles are coming...
  16. DanDel

    Run/Update short circuit current-ETAP

    Is it simply the sum of both 100A GF current from both gens? I'm not sure how you modeled the system, so I don't know. Not sure what you mean by 'TCC clipping current'; in ETAP, the TCC graph for GF will show you the GF current as a limitation for the end of a protective device's curve.
  17. DanDel

    sub sea cable tests

    In addition to typical insulation and conductor tests, I have performed Statistical Switching TOV (Transient Overvoltage)Analysis using ATP software. This is used to determine the adequacy of the insulation systems or the cable, along with the BIL ratings of attached equipment and MOV arrestors...
  18. DanDel

    600 V Bus voltage fluctuation is 12%

    Are the minimum and maximum voltages you mention instantaneous values? If not, loading (or some defect in the system) must be an issue. How is the voltage drop presented as a problem for the customer? Flicker/lights dimming, etc?
  19. DanDel

    Incident Energy with CL Fuse Equations

    In actuality the calculations, algorithms, and recommendations in use now for IEEE 1584 & NFPA 70E Arc Flash analysis are still in their infancy. You can count on continuous changes over the coming years based on more testing (hopefully).
  20. DanDel

    Recent Engineering Debacles

    Back to the Boston Big Dig Tunnel collapse - My understanding is that the manufacturer of the epoxy was sued and settled; even though they provided two different kinds of epoxy, and the wrong type was used for the ceiling lugs which collapsed. Either the engineers specified the wrong epoxy, or...

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