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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

    What is red book, white book?

    Depends upon the context. IEEE publishes are series of "standards" for various topics of facilities engineering - the series is known as the "Color Books." http://standards.ieee.org/colorbooks/sampler/shop.html UL also publishes a couple of books which tend to be known by their colors, e.g...
  3. 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...
  4. tommom

    NEC Table 9 Design Criteria Question

    Some people correct the resistance values to 25 degrees C when doing short circuit calcs - that's conservative and doesn't change the answer much.
  5. tommom

    CT TESTING

    Just take a voltage across each input of the bus differential relay (if it's for example a GE SVD bus relay). Any voltage substantially higher than zero (if your bus has tested out) will tell you if you have a failing CT - just maybe not which one.
  6. tommom

    MCC Low Megger Reading

    Just a question: are all the fuses pulled from the PT's?
  7. tommom

    changing the PF on my installation

    Doesn't the voltage regulator have a power factor or VAR control? I assume you're paralleling this generator with the grid?
  8. tommom

    VFD feeder -dv/dt

    I thought there was a problem with cable length, shielded or no. The length apparently depends upon the voltage first, then slightly by manufacturer.
  9. 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.
  10. tommom

    Fusible vs non-fusible disconnects

    On 208 or 240 volt systems when the transformer is under 150 KVA, non-fused should be fine. On nearly every 480 volt system, the interrupting current is over 10kA, so fused should be used unless it's protected by an upstream dedicated fuse sized to protect that switch/circuit.
  11. tommom

    Delta-delta versus Delta-Wye

    Grounding the system will reduce the possibility of a line-line fault going through their ground system. While a fault will trip a breaker off line, and thus shut down a piece of equipment or the System! Maintenance is much easier with a grounded system. Trying to find the fault when the...
  12. tommom

    Switchboard vs Distribution Panel

    Another way to look at it is a switchboard is floor-mounted, while a panelboard is wall-mounted (although very large ones might reach the floor). One other criteria that we use is up to 1200 amps - panelboard - 1600 amp and above is a switchboard.
  13. tommom

    Audible Fire Alarm Coverage

    Look at the SFPE handbook - there is a calculation procedure in there that should give you just what you need for audible alarm coverage. The calculation is fairly straightforward - adding & subtracting dB based on the construction.
  14. tommom

    Motor Starter used as a circuit breaker?

    It's not listed for the application - I wouldn't do it. That starter is sized for a motor circuit, which is sized under different code and UL rules. I don't think it should be done.
  15. tommom

    Typical Unit Price Items

    Means is low on overhead work, in my experience.
  16. tommom

    Over Currrent Protection to Generator/Fire Pump ends

    NFPA 20 states the locked-rotor currents for 480 volt fire pumps - double check your number with that.
  17. tommom

    Arc flash on secondary of 480V Transformer

    Consider using a 750 kva transformer. 500 kva transformer often has a very low impedance (300's do also), allowing a very high arc-flash value. 750 kva transformer standard impedance is 5.75 +- 7.5%, giving a far more reasonable (but still PRETTY high) value. Wye-wye connection may also be a...
  18. tommom

    2 Phase 3 Wire to 3 Phase 4 Wire 208/120V- Scott Connected Xformers

    You might have to get something specially made for this. These probably aren't readily available, but they should be "makeable." The three-phase you get out of a Scott transformer is really three-phase. The mfr has to tap one of the coils at 87.7 % (I think that's the number) and connect that...
  19. 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...
  20. tommom

    38 KV fault current

    Thanks, stevenal. Will see if we can use this in the app. All the distribution is underground, so it's possible this might not do the trick.

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