electrical safety, grounding, shielding, isolation and noise effects of 60 hz electrical current...
TRANSCRIPT
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Electrical Safety, Grounding, Shielding, Isolation and Noise
• Effects of 60 Hz electrical current • Defibrillator design• Ground fault interrupter (GFI) design • Faraday’s Law and ground loop noise
• Dangers of EM radiation (xrays, hard UV)
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Human grips 2 copper handles connected to 60Hz power source
• Threshold of sensation• Threshold of pain• Let-go current• Respiratory paralysis• Ventricular fibrillation• Burns
See Fig 14.1 in Walter H. Olson chapter, "Electrical Safety” pp 751ff in Webster (ed) Medical Instrumentation (1992): Work of former UC Berkeley advisor Charles Dalziel.
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www.physics.ohio-state.edu/~p616/safety/fatal_current.html
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Electro-shock therapy for suicidal depression
DC current to promote bone healing
AC current to mask sensation of pain
Transcranial direct current stimulationThe case of the Russian frontal cortex stimulator
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Defibrillator design:apply 400 J of electrical energy across torso in 200 msec
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jdd/SecOrd123.mdlwith alph = omeg = 16
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Ground Fault Interrupter
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How a GFI works• A ground fault is ___ current greater than ___ milliamps
flowing from ____ to _____ through an unwanted _________.
• A ___________ can detect a ground fault.• The primary of the _______ is ___ and ___ flowing in
______ directions. The primary ____ is non-_____ if there is a _____ current to ground.
• The small secondary _____ (due to _____’s Law) must be ______ed and _____ed.
• The ___ and __ current goes to an electric _______whose pistonmechanically _____s the ____ wire.
• The switched-out hot must have a ____ to hold it in place because the _______ to the GFI is _____.
• Once the ___ is ____ed then a ____ ____ is pushed to release the _________ and allow the _______ hot wire to ___________
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How a better GFI works• A ground fault is __ _______ greater than ___ milliamps
flowing from ____ to _____ through an unwanted ______. • A ____ _______ can detect a ground fault.• The primary of is ___ and ___ flowing in and out of
the__________core. • The induced secondary ______ is sent to an analog
______ ;when it exceeds a ___________the digital output of the __________turns on a 1-shot pulse that turns on a power __________ connected to a charged _______.
• DC from_____ ______ pathway flows to a ________whose internal ________ open s a snap-action _______ in the ___ path.
• Once a human ___ the ____ then a ____ ____ can be pushed, which flips back the ______ switch to _____ the hot pathway.
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Isolation: blocking DC
• Stimulation electrodes with a capacitor in series
• Micro-switch: 100 usec and 100 uA
• Isolation Amp
• Optical isolation
• telemetry
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danger from Electromagnetic waves
• Hard UV
• X-rays
• Microwaves
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Ground Mecca:
• Arrange that all wires grounding various points in a circuit are connected to one plate/site, so as not to create any ground loops, which could generate 60 Hz noise by way of Faraday’s Law.
• Faraday cage (grounded copper screen) to reduce EM signals (inc 60 Hz hum) being picked up as noise by lengths of wire acting as antennas.
• Twisted pair to minimize induced noise in circuit loops. • Hose clamp on cold water pipe for better ground. • Shielded, grounded coax cable as another means of
stopping lengths of wire from acting as inadvertent antennas.
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clamp for cold water pipe
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Another POV: Coupling
• Conductive coupling (ground chains)
• Capacitive coupling (wires running near each other)
• Inductive coupling (loops that enforce Faraday's Law)