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  • Users: tommom
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  1. tommom

    Qatar, British Standard-RCBO, hazardous

    I live in U.S., doing design work in Qatar but am a little mystified by differences between Qatar (Kahramaa)/British Standard requirements and NEC/Nema/ansi/UL-land design. 1) Anyone have experience with RCBO requirements? Is there a reference book which explains this in more detail than the BS...
  2. tommom

    15 kV clearance issue

    I have a contractor who has located a junction box within 4'6" of the front of my switchgear - contrary to the drawings. According to NEC 110.34 (A), condition 2 for 7200 volts to ground, we need 5'0". What I would like to do is cover or coat the box with insulation to make it Condition 1...
  3. tommom

    Transformer impedances

    Is there a place I can get an IEC standard impedance for a say, 20 MVA, 110 KV-11 KV transformer, 50 Hz, for use in the Middle East, without laying down 3500 bucks for the IEC transformer collection? Thanks.
  4. tommom

    Transformer damage curve

    I am doing a coordination study, involving a 750 kva 12.47-480/277 volt transformer, with primary current-limiting fuse protection, in this case an EJO-1 fuse. I have never been clear on "how far up" the transformer damage curve I have to protect with the fuse. Is there something in the...
  5. tommom

    38 KV fault current

    A friend is designing a 300 kva unit substation, 34.5 KV primary. The available fault current is around 18,000 amps. However, he can't find a primary fuse which is rated higher than about 16,000 amps. What are his options for protection of the transformer?
  6. tommom

    Fire Alarm Strobes w/ indirect lighting

    Wondering if anyone has either designed a fire alarm system (strobes) to be visible when using a lit ceiling (ie indirect lighting) or been flagged for not having one? Any idea of the enforceability of such a design issue? Thanks.
  7. tommom

    Motor current imbalance

    A client of mine has a problem - his air-handling units are exhibiting a current imbalance of upwards of 15% on a 208 volt 3 phase system. He has only checked four of his six or seven units, and has not looked at his other equipment. I don't suspect severe load imbalance on the system -...
  8. tommom

    GF nuisance tripping-480v system-troubleshooting

    I have a 480 volt switchboard with a 1600 amp main circuit breaker with a GE type SS circuit breaker with ground fault trip, and feeder breakers of GE type SGH, SGL, SFH downstream of the main. These breakers feed lighting panels and rooftop units. Some of the feeder breakers and the main are...
  9. tommom

    Meggering 15kV cables

    A contractor recently told me he was going to hi-pot some cables some time before he energizes them. I suggested to him that he megger them at the same time, get a base line reading, then megger them again prior to re-energization and compare the readings to verify there was no damage or water...

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