Static electricity

  1. Hi

    I have seen electric technicians while racking out high tension circuit breakers, use a wire that has one end grounded and they use the other end to touch the bus bars on the breaker in case of a static charger, they carry out this procedure before they do any work on the CB in case they get shocked by a static charge, is it necessary to carry out such a procedure even tho the breaker is completely isolated ?

    How can static charge be built in a case like that, where there is no friction ?
     
  2. jcsd
  3. Simon Bridge

    Simon Bridge 14,852
    Science Advisor
    Homework Helper
    Gold Member

    That's like asking if it is necessary to always assume that a gun is loaded.

    Charge can also be induced.

    The main point is that the workers have no idea what has been happening to the component.
     
  4. vk6kro

    vk6kro 4,059
    Science Advisor

    Large power transformers can have a lot of capacitance between the windings and the core and the metal case.

    This capacitance gets charged and discharged from the voltages on the windings in normal operation.
    If the supply is removed rapidly some residual charge could be left on the capacitance from the winding to its metal surroundings.

    This capacitance can be deadly when transformers are being tested. Large DC voltages are used to test transformer breakdown capability and if this is not carefully discharged, it can persist for days.

    Someone was killed about 20 years ago by a transformer which was still holding a charge two days after high voltage testing.
     
  5. A few points : In general I would not refer to this as static, but residual charge - because the insulation on Medium (600-72KV) and High Voltage ( >72KV) - although I do not know of any rack out HV breakers...... ok back on track, because of the very high insulation, it is typical / common to have a residual charge left on the breakers conductors.
    As a matter of practice - safety policy etc.... EVERY conductor you touch MUST be grounded..discharged first. You do not touch ANYTHING unless you KNOW ( not just think) it is safe.
    -- A note back on the HV systems, it is also common for just the wind to leave a static charge on UNGROUNDED conductors - large transformers have pumped oil cooling systems, also a problem.
     
  6. vk6kro

    vk6kro 4,059
    Science Advisor

    I think I would add "don't touch it anyway".

    I have seen these charges build up again even after being discharged.

    You can always use insulated pliers etc to keep your fingers away from potential danger.
     
  7. "Do not touch" does not really work if you have to work ON the breaker - the disconnect points (stabs/receivers) are cleaned and this is where many of the test connections are made. Once dawn out of the cubicle, and discharged - there is little risk, make on these is 36KV Class, up to about 3000A - I have not seen draw out breakers higher voltage than that. - Although we used to wrap a wire or have light alligator jumpers on all points not under test - these can get induced V from hi-pot testing.
     
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