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)

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.

www.physics.ohio-state.edu/~p616/safety/fatal_current.html

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

Defibrillator design:apply 400 J of electrical energy across torso in 200 msec

jdd/SecOrd123.mdlwith alph = omeg = 16

Ground Fault Interrupter

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 ___________

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.

Isolation: blocking DC

• Stimulation electrodes with a capacitor in series

• Micro-switch: 100 usec and 100 uA

• Isolation Amp

• Optical isolation

• telemetry

danger from Electromagnetic waves

• Hard UV

• X-rays

• Microwaves

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.

example of a ground mecca

www.wikihow.com

clamp for cold water pipe

Another POV: Coupling

• Conductive coupling (ground chains)

• Capacitive coupling (wires running near each other)

• Inductive coupling (loops that enforce Faraday's Law)