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Switch Contact Rating Question

Discussion in 'Electronic Design' started by AndyS, Apr 7, 2007.

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  1. AndyS

    AndyS Guest

    Andy asks:

    I was looking at a switch that specified it's contacts rated
    at 100A in a 12 volt system, and at 50A in a 24 volt system.

    So, can anyone tell me why the current ratings are different
    for the voltage applications ?

  2. Guest

    If you look at the power handling ability (W) it is the same! In
    effect the contacts are rated for a certain maximum wattage.
    It is difficult to think of it in those terms but that is the way it

  3. Phil Allison

    Phil Allison Guest

    ** Groper Alert !!!!!

    ** Cos it is a **switching ** current rating !!

    Means the ability of the switch to BREAK a current flow of so many amps in
    a circuit with so many volts available after that break opens.

    Its all about how DC current arcs behave - the more the volts, the harder
    it is break the circuit successfully.

    Too many DC amps at too many volts with too small an opening gap = a *
    continuous DC arc * that heats and melts the metal surfaces to bits.

    ....... Phil
  4. Phil Allison

    Phil Allison Guest


    ** BOLLOCKS !!

    ** No it is not - you know nothing, bullshitting ass.

    ........ Phil
  5. The ratings are also different, in general, for AC vs. DC and for
    different types of load (resistive, tungsten, motor).

    Some types of current and/or load simply cause more contact wear due
    to arcing (or the switch may even have difficulty in breaking the arc
    at all), or the type of load may have a large surge current on 'make'
    that tends to weld the contacts.

    Best regards,
    Spehro Pefhany
  6. Switch ratings have more than one limiting property.

    When the switch is closed, the contacts are heated by the
    RMS current passing through them, and the current limit for
    this operation depends on the contact material, whether the
    contact point just meets or wipes at closure, the contact
    pressure, the thermal conductivity to the surroundings that
    heat sinks the contact point, etc.

    As the contacts open, an arc is drawn that cumulatively
    degrades the contact surfaces. So the load current and
    voltage limits depend on both the off state voltage, the
    load current, whether the current is AC or DC (AC has two
    moments per cycle when the current goes through zero,
    letting the arc go out, easier), whether the load is
    resistive or inductive, how fast the contacts separate, how
    far the contacts separate, the expected cycle life of the
    contacts, etc.

    As the contacts close, they approach, bounce, possibly wipe,
    and increase their contact pressure, just as the load
    current through them is rising. The load current limit here
    depends on whether the load is inductive or resistive
    (resistive load currents rise almost instantaneously) and if
    resistive, if they are incandescent (low cold resistance
    produces large inrush current), how many bounce cycles,
    whether or not the contacts wipe (to smear the arc oxide
    damage out and freshen the metallic surface), etc.

    Different cycle conditions (closing, carrying, opening)
    produce different kinds and quantity of damage, so either
    different life expectancy for a given rating (voltage,
    current, AC or DC and load character), or different ratings
    for the same life expectancy.

    Banging two pieces of metal while they carry current or
    block voltage, with predictable results, is an art form.
  7. more stuff here

  8. That's a good question. Doubling the voltage shouldn't halve the current
    rating. Why not ask the makers?
  9. Phil Allison

    Phil Allison Guest

    "Homer J Simpleton"

    ** See the Derating Curves on page 2.

    10 amp ( switching capacity) rated relay contacts drop to 1.5 at 50 volts DC
    and nothing at 100 volts DC - even with purely resistive loads.

    ........ Phil
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