Hello from San Diego:
Several messages have suggested some values of current
that cause ventricular fibrillation. The values are all
over the map, and are much lower than those reported in
the research literature. Here are some "better" facts
together with the source of those facts.
According to Biegelmeier and Lee, "New considerations
on the threshold of ventricular fibrillation for ac
shocks at 50-60 Hz," IEE Proceedings, Part A, Vol. 127,
No. 2, March 1980, pp 103-110...
Research shows that the heart is not susceptible to
fibrillation except during the T wave.
The threshold of fibrillation for a heart exposed to
an electrical current during a single T wave is
relatively high, but for each consecutive T wave during
which the current is present, the threshold decreases
until it is about one-tenth of its initial value.
For currents imposed hand-to-hand, the thresholds are
believed to be above 500 mA if present during a single
T wave, and above 50 mA if present during three or more
consecutive T waves (heart-beats).
Furthermore, the shock current must exist for the full
duration of the T wave.
The T wave duration is about 15% of the heartbeat cycle.
If we assume 60 beats per minute, then 15% of 1 beat
(1 second) is 150 milliseconds.
150 milliseconds is 7.5 cycles at 50 Hz, or 9 cycles
at 60 Hz.
So, for fibrillation, the current would need to be at
least 500 mA for at least 7.5 cycles at 50 Hz or 9 cycles
at 60 Hz.
Or, the current would need to be at least 50 mA for at
least 3 seconds.
GFCIs/RCCBs are designed to operate in 1 cycle/20 milli-
seconds or less at the rated current (GFCI = 5 mA,
RCCB = 30 mA). During that one cycle, the current can be
up to 166 mA (assuming 250 volts and body impedance of
1500 ohms).
So, GFCIs/RCCBs provide protection against fibrillation
by limiting the DURATION of the current applied to the
body, given that the body impedance will cause a current
up to, say, 166 mA (250 V source) until the GFCI/RCCB trips.
(At lower voltages, the current will be proportionately
lower.)
Here are some ballpark numbers for 50-60 Hz currents:
0.5 mAOnset of sensation
5 mAOnset of tetanization ("can't let go")
50 mAOnset of fibrillation
500 mAOnset of cardiac, respiratory arrest
5000 mABurns
Tetanization (freezing) only occurs with ac, and only then
in the frequency range of 20-100 Hertz. Tetanization does
not occur with dc or with higher frequency ac.
There is a famous experiment about the turn of the century
by D'Arsonval where he passed a current of 1 ampere through
several humans, hand-to-hand, at 10 kHz, to light a light
bulb!
Best regards,
Rich
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: ri...@sdd.hp.com
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