Connect with us

1000 MCM single conductors

Discussion in 'Electrical Engineering' started by Dave, Nov 22, 2004.

Scroll to continue with content
  1. Dave

    Dave Guest

    Hello, I have to replace some old wooden vertical raceways (out the top of a
    PDC xfmr room) that have 1000 mcm single conductors, with some steel tray.

    Does anyone know of any health hazards to do this while their is large
    currents flowing?

    Thanks, Dave
     
  2. Dave

    Dave Guest

    Dave
     
  3. SQLit

    SQLit Guest

    Health hazards? None that I know of. I doubt that the feilds produced would
    be high enough to offset your cell phone usage or the 2 way radio you might
    use daily.
     
  4. Old, you said? 1000 Kcmil with current in them and you are going to replace
    the wooden support with tray while they are energized? Yes, I would say
    there is a health hazard - your life. De-energize the conductors or use
    proper protective equipment including gloves, face shield, insulated gloves,
    and have a worker qualified in cpr and who is also equiped with the proper
    ppe standing by.
    If you are in the USA I would ask for a voluntary OSHA compliance inspection
    and a safey plan.
     
  5. Dave

    Dave Guest

    Thanks, They are 480 V

    Not high voltage.

    Dave
     
  6. Bob Peterson

    Bob Peterson Guest

    I think the point is that conductors of that size might well have
    substantial SC capacity, which if you managed to nick a wire could result in
    a very serious arc flash. you should really try to do this with the power
    OFF.
     
  7. 480 volt is deadly when it comes to arc flash. After working for six months
    at a 24 year old industrial facility that had a lot of distribution at 480
    volts, I would not assume any thing as safe anymore. Once I turned on a
    480 volt three phase 70 ampere main breaker and got the shock of a life
    time. The plastic breaker handle cover had gotten broken and only the metal
    part was left. There was 277 volts to ground on the metal. I was wet and
    had wet gloves on. At this same facility cables had been stretched very
    tight in 1982 when a 120 pound per square foot snow load broke down the
    cable tray support structures leaving the cables taut. The cable were never
    replaced. You never know what took place in the past. Do not assume that
    anything is safe in the electrical world!!! While working next to a
    journeyman with over 30 years experience I witnessed his screw driver
    exploding while he was in the bucket. The 400 ampere 480 volt breaker that
    he had turned off was still hot on the load side!!! The breaker was
    defective. A journeyman I know was working some cables in the back of a
    distribution center in that was hot. One of the existing cables came out of
    its terminal and cross phased. It blew him several feet and burned him
    badly. This stuff is dangerous. I know a person who had a bad experience
    with arc flash who now never ever stands in front of 480 volt equipment when
    he turns it on. He turns it on with a stick while he stands to one side.
    Talk about arc flash shy!
     
  8. Dave

    Dave Guest

    Thanks for all the safety advice. The job is complete. I replaced the old
    wooden raceway with modern electrical tray. I was not really worried about
    arcs. I was worried about the extended time being in close proximity to any
    EMF surrounding the cables. I made sure that no forces were put upon the
    teck connectors. This PDC room was designed 40 + years ago with the
    secondaries coming straight out the roof of the room. Most PDC rooms these
    days have the secondaries coming out the side of the building or out the
    bottom, which makes it much easier to seal around the cables / tray etc. for
    moisture.

    Speaking of sealing, does anyone know of any potting compound one could use
    in a framework around the tray and cables to seal where they exit the room?
    We already have foam, but the Engineer wants a potting compound pored in a
    framework around the cables and tray.

    Thanks, Dave
     
  9. Bob Peterson

    Bob Peterson Guest

    if you are worried about EMF, you are seriously worried about the wrong
    thing. There is no serious science that suggests short exposure to such
    levels of EMF you experienced has any negative effects. There is a LOT of
    experience that says that the work you performed is quite hazardous. the
    problem is that while the chance of something going wrong on any individual
    exposure to this type of risk is fairly low, the consequences are very
    grave, and often fatal. Arc flash is something you should take very
    seriously, as it has killed and/or seriously injured a number of people over
    the years. You ought to take some time to read up on this, so you will at
    least understand how dangerous what you are doing actually is.
     
  10. Bob Peterson

    Bob Peterson Guest

    The short circuit current is somewhat independant of voltage, but tends to
    be higher for higher voltages. Think about it this way. The only thing
    that limits the current in a phase to phase fault is really the wiring, and
    1000 MCM wire has very little resistance.
    The load is irrelevant. What creates the dangerous arc fault is the
    available short circuit current.

    <snip>
     
  11. The impedance of the transformer has a big effect on the short circuit.
    A 2000 kVA transformer 208 V 3 phase transformer with 5% impedance will
    have a short-circuit current of more than 110,000 A at its secondary
    terminals (neglecting primary system impedance). 2000 kVA, at 480 V and
    5% gives only 48,000 A on a short. For the same power rating, of course
    the higher voltage uses smaller wire, with a resulting higher branch
    circuit impedance.

    So, at the same transformer power level, I would expect generally that
    higher voltage distribution systems have a *lower* short-circuit
    current.

    The only place I've ever heard of that couldn't rely on HRC fuses (200
    kA interrupting rating) is network power systems with multiple network
    transformers connected - these have 300+kA fault levels...at 208 V 3
    Phase.

    Bill

    (Bussman publishes a guide to a simplified fault level calculation
    procedure. )
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-