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  1. Mila2015

    Grounding CTs and PTs

    Thanks davidbeach. See attached file for actual PT configuration. This has dual secondary windings (Y and Delta) as you see. So, you're saying for the Y secondary, I connect A1X11, B1X11, and C1XII together, and ground middle wye point only correct? What about the delta...
  2. Mila2015

    Grounding CTs and PTs

    I have an installation where CTs and PTs are installed, but will not be used (not wired for metering nor protection). We will be connecting all phase CT terminals to ground to avoid an open CT situation, but the field contractor wants to also ground phase PT terminals. Is that typical? Those...
  3. Mila2015

    Equipment ground for 18KV syetem

    I agree each tap box needs to be bonded to earth at each box location, but do we have to pull equipment ground cable from one box to the other? As I explained, we're installing underground 18KV shielded conductors from one box to the other.
  4. Mila2015

    Equipment ground for 18KV syetem

    Sorry. Both enclosures are bonded to earth, but they're isolated from each other (iso-phase and tap-box) using "insulating bellows". The Iso-phase is grounded only at one location to avoid current circulation. Also, tab-boxes are bonded to earth (through grounding ring). I don't think bonding...
  5. Mila2015

    Equipment ground for 18KV syetem

    I'm installing 18KV underground cable from a new tap-box, to an existing one (see oneline attached). The iso-phases and tap-boxes are isolated (frames are not bonded) in all locations. Each Tap-box has a Power Transformer for metering. Through this set-up, both GSUs will be feeding separate AUX...
  6. Mila2015

    System grounding

    Thanks electricpete! Yes, I agree, I see NGRs more in the MV systems, rarely in 480v systems. Electrical model for that particular plant does show them on 480v systems though. I understand this would affect relay settings since ground faults are now limited to 50-100A depending on NGR size.
  7. Mila2015

    System grounding

    I'm feeding a 480v MCC system by (2) MV 4.16KV feeds, through (2) 500KVA Transformers. We're adding a 550KW emergency generator to one of the MCCs. The question came up, whether we should add low impedance resistors to EGEN and XFMRs to limit grounding current. I've seen multiple applications...
  8. Mila2015

    Standby generator sizing for loads containing VFDs

    I was told by VFD vendors that we need to double the size of generator supplied VFD loads. I also read many articles online saying the same thing, like the one at ABB site: https://library.e.abb.com/public/0007878791e17d0985257cca005bb048/LVD-EOTN64U-EN_REVA.pdf On the other hand, my generator...
  9. Mila2015

    Motor space heater requirement

    Thanks all! since it seems like we really can't predict what temperature will rise faster (motor or air), this issue exists in all cases, and affects every motor installed in non-temperature controlled conditions. In reality though, the majority of external installations I've seen (especially...
  10. Mila2015

    Motor space heater requirement

    Our specification indicates motors 20HP and above should be equipped with a space heater. I believe this has to do with a balance between price Vs condensation issues. My question is, how to determine when a motor space heater is required? I know we're trying to keep non-running motor...
  11. Mila2015

    Foundation re-bar grounding

    Sorry for lack of details. This is an outdoor pad with only concrete tank on it, and no electrical service. Foundation is enforced with concrete.
  12. Mila2015

    Foundation re-bar grounding

    I have a foundation with embedded re-bars. This will not have no electrical equipment on it. Question is, does the code require re-bar grounding?
  13. Mila2015

    2MW Generator Neutral resistor cable size

    It seems like I was referring to the wrong NEC section, as 250.36 maybe for systems <=1000v. For applications >1000v, "250.187 Impedance Grounded Neutral Systems" applies. However, sizing requirement for the Neutral conductor in this section is not clear. It reference sizing of "Grounded...
  14. Mila2015

    2MW Generator Neutral resistor cable size

    Thanks wross! This section from NEC I'm looking at looks like this: For 250 MCM (90 degree C),2001-5000v, single duct, the ampacity is 320A corrected to . I may have to use 2-1/C #250, since that's what I'm using for the phase conductors. With regards to shield, this is what it says: So...
  15. Mila2015

    2MW Generator Neutral resistor cable size

    You are correct! Based on NEC 250.36B, the grounded system conductor shall have ampacity >= resistor current rating, which is 350A. The question is, which table to use?! This would be single conductor, 5KV, insulated/shielded cable in underground PVC conduit. I think the closest table from NEC...
  16. Mila2015

    2MW Generator Neutral resistor cable size

    waross, thanks for the reply. Well, this cable is to feed a neutral resistor used for low resistance grounding. The resistor is rated for 350 A, 10 sec. The generator is 4.16KV. The cable will only see current during a ground fault. I believe the NEC tables you were referring were for continuous...
  17. Mila2015

    2MW Generator Neutral resistor cable size

    I have a 350A neutral resistor for a 2MW generator, and I'm trying to determine size of neutral resistor cable. I used equation: (1/A)^2 *t = 0.0297*log[(T2-234)/(T1-234)], and came up with A=23,643 (t=10s, T2=150, T1=90). This means I can use #2 AWG cable. The phase conducts are 6-1/C #250 (2...
  18. Mila2015

    Closing 480v Tie Circuit Breaker with no Sync check

    I have a situation where a client has a 480v, GE WPH Tie circuit breaker that needs to be manually closed. I didn't see sync checking in it's closing circuit, so I know there is no automatic voltage and Phase angle check, but the 500KV feeding network should be the same, and both units should...
  19. Mila2015

    Standby Generator Schematics

    Hi there. Does anyone has a typical standby generator Schematics showing interlocks, triping contacts, and indications? This is for a MV standby generator feeding a switchgear line-up through a feeder breaker. The switchgear has a main breaker, and is connected to another lineup through a tie...
  20. Mila2015

    Diesel stand-by Generator Circuit breaker

    cuky2000, thanks for the link. This addresses the IEEE standard I mentioned above, which specifically applies to Generators connected directly to step-up XFMR as I mentioned. Are there any examples of systems involving stand-by generators feeding MV switchgear? Why would that differ from other...

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