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How much current safe for 30m extension?

Discussion in 'Electrical Engineering' started by Sammo, Feb 12, 2005.

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  1. I just got thru working with some equipment that we inherited that had
    the fuse in the plug. In this case, the luser that had replaced the
    fuse used 32V auto fuses instead of the proper 125V or 250V rating. I'm
    glad I caught that problem. But fuses in plugs are rare, usually the
    only thing between the breaker panel and the equipment is the 15A
    breaker built into the power strip.

     
  2. I don't know about the UK, but in the U.S. most decent plugs come with
    three large headed screws that have a square washer under them, with one
    edge of the washer hanging over the edge of the brass pin, so it retains
    the copper wire. So if you can unscrew the screw 3/16" (5mm) and get
    the copper strands in there and screw it down, then it would hold almost
    that big a conductor. Of course getting it all thru the hole in the
    cord grip is another matter..
     
  3. Bob Eager

    Bob Eager Guest

    Some earlier plugs were like that. These days, there is a hole in the
    end of the pin, and a grubscrew into a threaded hole at right angles
    that clamps the wire. Remember the wire is thinner over here.

    And of course, nearly all of our plugs have fuses in them. A 15A breaker
    is not too good at protecting a 5A flexible cable to an appliance.
     
  4. Spehro Pefhany wrote:
    SNIP
    Yep!
     
  5. John Rumm wrote:

    SNIP
    _FUSE?_ 1/2 of their plugs don't even have an earth!!
     
  6. Stefek Zaba

    Stefek Zaba Guest

    Aye - and their stores [sic] will happily sell you an adaptor to let you
    plug those weird-ass 3pin plugs with that pinko-liberal ground pin into
    an honest red-blooded real man's 2-pin outlet...
     
  7. Yeah, but everything's gone cordless nowadays, even the weed wackers.
    So who cares? ;-)

    I think the appliances that have a plug with only two prongs have to be
    double insulated, and meet stringent leakage specs.

    I got one for you. I checked the fuse panel of a really old bldg (well
    for the U.S., anyway..) - almost 100 yrs - and it had the ground
    fused(!) Now _that's_weird.
     
  8. Was it 3-wire DC originally??

    Peter
     
  9. I have to concede that my knowlege of colonial wiring pracice is
    somewhat out of date. My family was part of the UK "Brain Drain", a
    period of history where engineers (my father, not me) from here got _1st
    class_ berths on liners taking them to the new world!
     
  10. John Rumm

    John Rumm Guest

    Depends on if you are in serise or parallel with it!



    --
    Cheers,

    John.

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  11. Guest

    A ~ 2 volt drop at 4 amps means ~ 1/2 ohms. You had 800 feet of wire - 400
    out and 400 back. That means your extension cord wire had to be rated at
    (1000/800) * 1/2 = .625 ohms per 1000 feet. Your extension cords had to
    be made from #6 or #8, given those numbers. That is highly unlikely.

    A typical heavy duty extension cord 100 feet long would most likely use
    #14 wire. #14 wire is rated at 3.1 ohms per 1000 feet. At 800 feet, if
    the 4 amp load was running, there would be a drop of about 9.9 volts.
    If #12 wire (rated at 2 ohms per 1000 feet) were used in the cords,
    the drop would be about 6.4 volts. The ohms per 1000 feet numbers
    come from table 9 in the National Electrical Code.

    There has to be an error in this, somewhere.

    Ed
     
  12. Darcy B

    Darcy B Guest


    What is earth fault loop impedance?

    (Yup, I have tried Google!)
     
  13. Sammo

    Sammo Guest

    I am the original poster with two 30m extension cables. You refer to
    RCD protection.

    I have got a plug-in RCD device but I know nothing about earth fault
    loops. It is similar to the one in this illustration.
    http://www.argos.co.uk/wcsstore/argos/images/9828331A59IFN104773M.JPG

    Where should my RCD device be plugged in to best avoid the problem of
    earth fault loops:

    (1) At the mains supply end where the first extension goes into the
    mains socket.
    (2) In the middle of the two 30m extensions.
    (3) At the far end of the two extensions where the appliciance I am
    using is plugged in?
     
  14. Sammo

    Sammo Guest

    The cable reel has two ratings. I took the unwound rating to get
    10A/2400W.

    Ha! You have spotted my lack of knowledge about these things - like
    current carrying capacity. I wasn't too sure how these things are
    determined.

    However, if the cable has resistence then presumably current capacity
    (ignoring voltage) gets reduced. I am thinking of W = I^2 * R.

    I posted just now and asked about RCDs (see copy below). If you have
    some info on using an RCD then I would be pleased to hear.

    Thanks, Sammo.

    ==========

    MESSAGE ID = <
    I am the original poster with two 30m extension cables. You refer to
    RCD protection.

    I have got a plug-in RCD device but I know nothing about earth fault
    loops. It is similar to the one in this illustration.
    http://www.argos.co.uk/wcsstore/argos/images/9828331A59IFN104773M.JPG

    Where should my RCD device be plugged in to best avoid the problem of
    earth fault loops:

    (1) At the mains supply end where the first extension goes into the
    mains socket.

    (2) In the middle of the two 30m extensions.

    (3) At the far end of the two extensions where the appliciance I am
    using is plugged in?
     
  15. Sammo

    Sammo Guest

  16. Sammo

    Sammo Guest


    CU?
     
  17. = Consumer unit. The RCD should protect the whole of the "outdoor" section.

    Earth Loop Fault Current = the current that would flow in the event of a
    live to earth fault. This current is calculated from the earth loop
    impedance (measured or calculated) including suppliers cable. This
    value is then compared with standard graphs for various protective
    devices (fuse, circuit breakers) to ascertain the disconnect time. If
    this does not meet the requirements, additional methods of protection
    are required. This could be changing to a faster breaker, and/or adding
    earth leakage protection.
     
  18. John Rumm

    John Rumm Guest

    It is the sum total of the resistances af all the wiring in the path to
    earth or ground. It is significant because should a major fault occur
    (like a wire falls inside an appliance and shorts to the casework, or
    you cut through an extension lead), the earth fault loop impedance will
    place a limit on the maximum current that can flow to earth.

    Ideally this fault current wants to be large, so that it causes the
    protective device (fuse / breaker) to open quickly. We have regulations
    in the UK that require a circuit with socket outlets to disconnect in
    under 0.4 secs in these situations.

    The fault loop impedance is also dictated by the impedance of the earth
    provided at the supply where it comes into the house. In the UK there
    are three common ways the power company can provide the supply - with
    two of them (known as TN-C and TN-C-S (aka PME)) the supplier provides a
    good earth (i.e. typically well under 1 ohm) which when used with
    suitable fusing and circuit breakers should result in good disconnection
    times in most cases. However if you add long circuits with undersized
    earth conductors (i.e. big extension lead for example!), then the
    impedance creeps up and lowers the fault current that could lead to much
    greater electrocution risk. The third type of supply common over here
    does not supply an earth at all (this is typical for power fed via
    overhead wires - typically into rural locations), and relies on a local
    earth rod that is staked into the ground. These tend to give much higher
    resistance earths and hence you can no longer rely on them to allow
    enough fault current to flow, so the whole installation must be
    protected by additional Residual Current Device circuit breakers (RCDs)
    to detect any leakage from the circuit and cut off the power that way.
    (RCDs are also mandatory here for all socket circuits that may
    potentialy feed portable equipment that could be used outside).

    There is some more info here:

    http://www.tlc-direct.co.uk/Book/5.1.1.htm



    --
    Cheers,

    John.

    /=================================================================\
    | Internode Ltd - http://www.internode.co.uk |
    |-----------------------------------------------------------------|
    | John Rumm - john(at)internode(dot)co(dot)uk |
    \=================================================================/
     
  19. John Rumm

    John Rumm Guest

    See my reply elsewhere in this thread for a fuller description of earth
    fault loop impedance. Also see here for more background:

    http://www.tlc-direct.co.uk/Book/5.1.1.htm
    No - but better than 3
    no - but better than not at all!


    Having the RCD at the start of the cable will mean the whole cable is
    protected by it. So should you accidentally do the "hedge trimmer
    through the extension lead" exercise, the RCD hopefully will cut off
    power to the lead, rather than leaving live exposed conductors flopping
    about in the wet grass under your ladder!



    --
    Cheers,

    John.

    /=================================================================\
    | Internode Ltd - http://www.internode.co.uk |
    |-----------------------------------------------------------------|
    | John Rumm - john(at)internode(dot)co(dot)uk |
    \=================================================================/
     
  20. cable is not that expensive, it's copper after all.I can see in an invoice
    that I paid 28 cents a meter for a 3G1.5 mm^2 A05VV-U cable.A 2.5 mm^2
    conductor, single core, which we generally use to connect washing machines,
    costs 9 cents a meter .Or 6 mm^2, for ranges, 22 cents a meter.Why don't you
    ask an electrician to construct you an extra heavy duty extension cable,
    with industrial-grade plugs?I did in college, when doing my Practicum, for a
    high-temperature oven, three-phase, 25A, which plug tended to overheat.Cost
    was negligent.

    --
    Tzortzakakis Dimitri?s
    major in electrical engineering, freelance electrician
    FH von Iraklion-Kreta, freiberuflicher Elektriker
    dimtzort AT otenet DOT gr
     
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