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Circuit Breakers

Discussion in 'Electrical Engineering' started by Steven Stern, Jul 25, 2003.

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  1. Steven Stern

    Steven Stern Guest

    My home is 20 years old. Do circuit breakers ever need to be replaced with
    age? Steven
     
  2. sometimes!
     
  3. happyhobit

    happyhobit Guest

    Steven,

    If you get false trips or cannot reset the breaker, then replace it. A
    breaker is a thermal device; current causes a bimetallic strip to bend. Time
    won't affect it.

    A short circuit may destroy it but a normal trip should not affect it. A
    hundred trips might but not one or two.

    Jay
     
  4. Eric Palmer

    Eric Palmer Guest

    This depends on the prospective short circuit current. And ratting of
    breaker. In a little square there is a number like 6000 or 10,000 this is
    the prospective short circuit current maximum. With larger breaker sometimes
    two figures one causes damage the other not. When the electrical system is
    checked every 10 years or with change of use or ownership then this is
    measured together with the earth loop impedance. The breakers are then
    checked for burning and ratings together with the size of cables etc. If
    they do not comply with the regulations of the year then a note should
    detail any points not conforming. Since the regs change year to year there
    are normally some no conformity although this does not mean there is any
    danger. Needs £1000 worth of test equipment so not really within the scope
    of a DIY man.

    All best Eric
     
  5. Rowbotth

    Rowbotth Guest

    OK, but seldom does a home breaker see more than on short circuit. They
    will see the odd overcurrent, and of course the GFCI breakers see ground
    faults more frequently, but the garden variety of breaker won't see a a
    short circuit. (Industry is a whole different breed of cat, but the
    original post was about household breakers.)

    In this country, homes have had circuit breakers in general usage for
    over 40 years. I've heard nothing about replacement of home breakers.
    (Getting a new breaker for an old panel is a challenge, but that would
    be about the only reason I've heaard of to replace them.)

    HR.
     
  6. Dave M.

    Dave M. Guest

    A short circuit may destroy it

    Except for the inconvenience, fuses are vastly superior protection because
    of their near transient response.

    Dave M.
     
  7. Brian

    Brian Guest

    Use once and throw them away.

    --
    If it is not broken, I cannot fix it.
    If I screw with it, it will kill me.
    Payday is Friday.

    It's Volts that jolts, but Mills that kills!

    My home is 20 years old. Do circuit breakers ever need to be replaced with
    age? Steven
     
  8. Rowbotth

    Rowbotth Guest

    Except for the inconvenience, fuses are vastly superior protection because
    of their near transient response.

    Dave M.
    [/QUOTE]

    OK, but the downside is that users (like residential folks who don''t
    really understand this) will replace a 20 Amp fuse if a 15 Amp fuse
    opens to clear a fault. People have a much greater reluctance to
    replace a breaker. Also, they don't have to spend money every time the
    fool thing operates and causes the whoozit to stop working...

    Balance the certainty of control of a breaker against the incremental
    superiority of a fuse - you decide what T.C. Mits should be using.

    What?

    HR.
     
  9. John Wilson

    John Wilson Guest

    Well, that's an oversimplification, though it expresses the right idea.
    IIRC, the NEMA (US) standard for molded-case circuit breakers includes a
    short-circuit test that goes something like:

    1. Close the short circuit. The breaker under test trips.

    2. 10 seconds (IIRC) after step 1, reset and close the breaker under test.

    3. The breaker under test trips again.

    The test only requires that the breaker interrupt the fault twice, with
    some conditions about emitting fire, molten metal, etc. The standard
    gives a list of conditions like broken arc chutes and other signs of
    destruction that are _not_ considered failures, if the breaker performed
    the two interruptions successfully and wouldn't have started a fire or
    something in the process. Needless to say, this is a design test, and a
    breaker that has been tested this way is _not_ then packaged up and sold
    to the public.

    And yes, the standard does recommend replacing any breaker that has
    interrupted a fault at a current close to its short-circuit rating. That
    said, it generally requires a solid connection physically close to the
    breaker to allow anything like the calculated short-circuit current to
    flow. At low voltage (<=600V), a little impedance goes a long way. The
    NEMA standard test includes some impedance between the breaker and the
    fault. This impedance is provided by a specified length of specified
    wire, and for small breaker ratings, the test impedance considerably
    reduces the short-circuit current that the breaker is actually subjected to.

    I just plain don't have a feel for the damage done to a molded-case
    breaker clearing a fault at, say 10% of its short-circuit rating.

    73,
    JohnW
     
  10. In Europe, breakers have two tripping components -- a thermal trip
    for overload protection (slow acting) , and a magnetic trip for fault
    current (short circuit) protection (very fast acting).

    Breakers are marked something like C32 M6000.

    The 32 is the steady current the breaker will pass without tripping.
    As you start to exceed that, the thermal trip will operate -- it
    depends how far over you go just how long it takes to trip, but at
    twice the current, a typical operating time would be ~100 seconds
    for a EN60898 European breaker, and 10 seconds at 4 times the rated
    current.

    The 'C' is the rating of the magnetic trip, which can be B, C,
    or D. ('A' is not used to avoid confusion with A for Amps.)
    The magnetic trip provides the fast breaking for a fault current,
    and it operates in 10ms (half a mains cycle). This must be rated
    higher than the inrush current of the appliances on the circuit or
    the breaker will trip when applianced switched on. Type B will trip
    between 3 and 5 times the steading current rating, type C will trip
    between 5 and 10 times the steading current rating, type D will trip
    between 10 and 50 times the steading current rating.
    Type B is the type most commonly used in domestic situations, and
    sometimes type C on lighting circuits to reduce the chance of a
    filament lamp which arcs over from tripping it. Type C would be
    used for appliances on dedicated circuits with large motors such
    as aircon units. Type D is only used in commercial/industrial
    premises and requires special consideration for the cable safely
    handling fault currents. The older breakers to UK standards used
    Type 1, 2, 3, or 4 for the the rating of the magnetic trip, with
    2, 3, and 4 being very roughly equivalent to B, C, and D. Type 1
    was in practice too sensitive to find many uses, and does not have
    an equivalent in the European replacement standards.

    The M6000 is the fault current the breaker can safely interrupt
    without damaging itself, 6000A in this case. M10000 is another
    common rating. If you exceed this, the breaker might be damaged
    interrupting the fault current. If you grossly exceed this, the
    breaker may be unable to interrupt the fault current, relying
    on the upstream protective device and likely causing considerable
    damage to itself and the surrounding area and the downstream
    cable carrying the fault current. Some manufacturers allow an M6000
    breaker to be uprated to M10000 if the upstream protective device
    is a BS88 100A cartridge fuse, which is what would be used in the
    UK in cases where a very high prospective short circuit current is
    available at the domestic premises supply.
     
  11. Tom Horne

    Tom Horne Guest

    Bussman is in the fuse manufacturing business. There main competition
    is the manufacturers of breakers. It is in there best interest to make
    the cost of fuses appear to be lower than the cost of breakers. We can
    know the maximum fault that a breaker has been subjected to by doing the
    withstand calculation for that point in the system and assuming a bolted
    fault.
     
  12. daestrom

    daestrom Guest

    Well, if we're complying with code, we know the calculated short-circuit
    current is at least equal or less than the short-circuit rating of the
    breaker ;-)

    But IIRC, there is a method described in IEEE somewhere for calculating the
    expected short-circuit current based on the kva of the line transformer, the
    length/size of feeder to breaker and from breaker to fault.
    I think they are if you're going to replace the breaker each time it clears
    a fault. But many faults aren't direct shorts. I do know that in
    industrial settings, we require a breaker to be racked out and inspected if
    it opens under fault. But don't know how you can do this with molded case
    units.

    And a downside of fuses is I wouldn't try to remove them under load. This
    means a separate disconnect adding to the total cost.

    daestrom
     
  13. daestrom

    daestrom Guest

    Ah, but in distribution schemes, breaker trip settings can be coordinated to
    clear faults and minimize affected area.

    daestrom
     
  14. daestrom

    daestrom Guest

    'use'??? You must have a deep pocket to replace breakers every time you
    open one ;-)

    daestrom
     
  15. Ben Miller

    Ben Miller Guest

    In addition to what you described, the utility needs to advise the available
    short-circuit amps at the transformer primary. The calculations are routine
    for anyone who designs electrical systems, and very straightforward for the
    typical residence. However, the average homeowner doesn't know how to do
    this, or even that it should be done.

    Also, when was the last time an electrical utility advised a homeowner that
    new distribution transformers or feeders had been installed that might
    increase the available short-circuit amps? I suspect there is more than one
    home where the panel is under-rated, although it might have been fine when
    originally installed.

    Ben Miller
     
  16. Rowbotth

    Rowbotth Guest

    Breakers are also not intended for multiple operations - like switching.
    Contactors are rated for multiple switching operations, but not
    breakers. (However, as long as you limit your switching to a few dozen
    in their lifetimes, breakers will do the trick.)

    A couple of other things which could be added are that it is a natural
    reaction amongst the untrained to slap in a larger fuse when one blows -
    or one with a different characteristic. This does not happen with
    breakers. And generally, fuses are rated for short circuit operations -
    unless you get the right rating - and subjecting them to a sustained
    overload can lead to an interesting and frightening failure mode.

    It is also a little bit too easy to stick a penny inside the screw-in
    type of fuses that used to be so common for residential applications.

    HR.
     
  17. Gym Bob

    Gym Bob Guest

    Breakers are rated for one fault type trip and then need to be replaced.
    Read the fine print.
     
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