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  1. healyx

    Switchboard Short Circuit Rating

    Hi Sibeen, The breakers in this board have to satisfy 2 different ratings. The breakers from individual generator feeds will not see the 88kA calculated total, they will only see their share of that (1/3) or 30kA rms. Breakers feeding other loads from this combined bus do need that 88kA+...
  2. healyx

    Generator/Transformer Grounding - When does distance become an issue?

    Hi All, I am considering a transformer with emergency generator combination. I have read some threads of pros and cons of using a 3 pole vs a 4 pole ATS. 3 pole seems to avoid some issues with potential floating neutrals but usually comes with the warning that having genset and transformer on...
  3. healyx

    NH DIN Fuses and Schneider Data

    Hi All, I am looking at some Schneider data sheets for fuse backup protection for breakers and switches. One table notes the fuse as "BS88 or gG" in one column and "HRC DIN type" in another. I took this to mean a BS Bolt-in or clip-in fuse would be the first column and a NH type fuse would be...
  4. healyx

    Group Derating Factor for Trefoil Formation

    Hi Alan, Can you tell us where you are so we know what standard applies? If you were in Australia (our standards are derived from IEC equivalents) I note the following. 3x400mm2 XLPE cables spaced at 170mm centres, buried direct (25 degrees c) would have a derating of 0.73, giving an...
  5. healyx

    Fuse Total I2t as per IEC 60269-2

    Thanks 7antoer4. Ch 7.7 in 60269-2 is repeated in each section isn't it? So is that note ("For the fuse-links covered by this section the maximum pre-arcing I^2*t values given in Table VI(6) of IEC 60269-1 apply for the maximum operating I^2*t values.") repeated for each section (fuse type)...
  6. healyx

    Fuse Total I2t as per IEC 60269-2

    Hi All, I don't have access to IEC 60269-2. Can anyone tell me if the standard provides total I^2t energy maximum values for the various fuse types. I have IEC 60269-1 but it only specifies minimum pre-arcing energies, I need the total energies. Cheers, Healyx
  7. healyx

    Fuse Modelling for Arc-Flash Analysis to IEEE 1584-2002

    Hi Eddy, Did you ever find a definitive source to confirm the the +10% and +15% tolerances?
  8. healyx

    I can't determine CABLE SIZE & CIRCUIT BREAKER?

    I agree with davidbeach, you need to ask the mechanical engineer for these loads. It is hard for us to even assist given every region will have completely different requirements. I Australia where our peak loads are for cooling (generally speaking), a rule of thumb for planning purposes would...
  9. healyx

    AS3851 Unable to calculate X/R ratio

    Reading between the lines - are you saying the peak (Ip) current is 3800A and rms current (Ik") is 3600A? If so, that peak current is way too low. I think it is probably out by a factor of sqrt(2). If so, the real peak current would be 5374A and X/R would be 4.11. This is close to what would...
  10. healyx

    X/R ratio for single phase to ground fault

    I'd say that would be correct. What are you using the result for? Is it to determine thermal equivalent current for the fault?
  11. healyx

    Circuit breaker practical trip time under short circuit

    I did a calc. Data Used: Tx to MSB. 240mm2, 4m, Cu XLPE, Single Core Trefoil. MSB to DB1. 70mm2, 25m Cu XLPE, Multicore DB1 to DB2. 25mm2, 15m, Cu XLPE Multicore DB2 to Control Panel. 16mm2, Cu PVC Multicore. With 630kVA, 4.5%, 415v Transformer, 500MVA upstream network, Cmax = 1.06. Max...
  12. healyx

    Circuit breaker practical trip time under short circuit

    What is the distance from 50A breaker board to your control panel? Is everything 3 phase?
  13. healyx

    Circuit breaker practical trip time under short circuit

    If you are going to use Schneider C60Ns throughout (including for the 50A breaker), you don't need the HRC fuse(s). Provided you checked the cascade table under the NSX100B, you are good to go. But yes, you can get fused HRC isolators. I'll do a calc for you later and post the results.
  14. healyx

    Circuit breaker practical trip time under short circuit

    Just done a search, the method where you calculate the limited rms current (Up-over-down) method has been disproved where you are trying to protect a current limiting breaker down stream. I couldn't find any IEC references for this but it is accepted to NOT use this method in North America...
  15. healyx

    Circuit breaker practical trip time under short circuit

    Looking at the current-limiting curve for a NSX100B, a 20kA rms fault is limited to 8kA rms. If you are saying the maximum fault level at the location this NSX100B is installed in 10kA, the NSX100B will limit this to 6kA (only just). As I said before, I don't really know if this is a valid...
  16. healyx

    MCCB single pole interrupting rating

    A line to ground fault close to the transformer could be of the same magnitude or greater to a 3 phase fault at the same voltage. So having a single pole rated at 87% seems like it would be under rated even for a solidly grounded installation. I couldn't find any mention of these single pole...
  17. healyx

    MCCB single pole interrupting rating

    Sorry, I wasn't sure about some of the terminology you were using, I'm in IEC land but I have looked at the bussman document now. I don,t think you are correct in saying Z0 in a solidly grounded system is zero, and not just because you need to include the phase cable impedances. The return...
  18. healyx

    MCCB single pole interrupting rating

    I might have this completely wrong but are you sure that Z0 is close to zero in a solidly grounded system? Zero sequence impedance is calculated by bridging the 3 phases at the source, then supplying with a single phase voltage source, so the path includes the 3 phase cables in parallel plus...
  19. healyx

    FAULT LEVEL

    You might want to include a voltage factor in wroggents answer depending on what part of the world you are in. IEC60909 would need 1.05 x 31618=33200A. If you are working out maximum fault level, you might want to neglect the network contribution in case it is upgraded later and depending on...
  20. healyx

    Circuit breaker practical trip time under short circuit

    I'm pretty sure that peak let-through current from the previous post is wrong. I think the Hager curve might be wrong, it doesn't seem to correspond with the let-through energy data and trip curve. I think the limited rms current for a 20kA fault would be closer to 15kA rms, not 2.5kA rms. So...

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