Isolation Transformers

Discussion in 'General Electronics Chat' started by twosacrowd, Jul 8, 2009.

  1. twosacrowd

    Thread Starter New Member

    Jul 3, 2009
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    Hi guys,

    Can anyone give me the low down on PAT testing equipment powered from an Isolation Transformer. Bit confused on this since the powered equipment is derived from an isolated supply and not the building supply, though it has a common ground.
    A general description on why we'd use an isolation transformer on patient connected equipment as opposed to grounded equipment would be helpful, I must be missing something here lol
    Thanks.
     
  2. twosacrowd

    Thread Starter New Member

    Jul 3, 2009
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    Cheers Alberto, link gives some useful info. Looking for some perspective on PAT testing in that regard too. Time for more Googling I think :D
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    What difference do you expect the isolation transformer to make to the PAT test?

    Bearing in mind that most powered electronic equipment contains an integral isolation transformer anyway.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
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    Haven't a clue what a PAT test is (posters, please don't assume people know what your acronym means). But you say "patient", so do you mean this is in a hospital environment? If I recall correctly, medical equipment is required to have AC leakage currents a few orders of magnitude less than typical consumer electronics (microamps vs. a milliamp or so).
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    At the risk of the wrath of Ratch

    (a bit like the wrath of Khan, only more so)

    I will explain.

    PAT = Portable Appliance Test

    So yes Ratch, PAT test contains a redundant word.

    It is a requirement of the UK electricity regulations, introduced more than 10 years ago, for all portable appliances to be subject to this test, including any cables which plug them into a permanent mains supply. A portable appliance, in this context, is one which can be unplugged, regardless of how big or heavy it is or its electric load.

    In theory the test is required annually.

    Calibrated testers apply set high voltage to the conductors and measure the leakage between points on the casing (including plastic) and the conductors and earth.

    Different classes of appliance have differing max leakage, and appliances such as pcs can be tested at lower voltage.

    Hope this helps.
     
    Last edited: Jul 9, 2009
  6. twosacrowd

    Thread Starter New Member

    Jul 3, 2009
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    Hi Studio,
    Since the PAT is usually applied to testing of an instrument from it's own mains lead from the building supply e.g. earth resistance, insulation, leakage etc we now introduce the isolation transformer which supplies from it's secondary an isolated ac source to power instruments such as PC's and in our case a patient connected eeg measuring system.
    With this additional cabling,(mains cable to transformer and then IEC lead from the secondary to PC or eeg instrument) the earth resistance increases to about an ohm, which is 10 times the 0.1 ohm passable level under 60601 standard. I can't see how I can achieve a pass value. Perhaps it needs a specific risk assessment? Thoughts?
     
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