Re: Tek warning


regman10
 

According to "Tektronix Biophysical Measurements" it only takes 100mA-2A to
cause cause ventricular fibrillation and probable death (60 Hz, arm to arm).
("cannot let go" is between 10mA and 100mA). Voltage is not as important as
current which is dependent on other variables such as arm to arm or arm to
leg, barefoot, standing in water, humidity, skin resistance, etc.

-----Original Message-----
From: Peter M. Munro [mailto:pmmunro@hotmail.com]
Sent: Monday, January 17, 2005 2:31 PM
To: regman10@comcast.net; TekScopes@yahoogroups.com
Subject: RE: [TekScopes] Tek warning


I don't know if Gary means that 220V plus literally "fries" the heart - it
is true that the heart can be seriously disrupted and in the very
unpredictable extreme fibrillation can occur.

If anyone is interested enough there is a good analysis and commentary on
the physiological effects of electricity on the human body in "Electrical
Safety Engineering" by W. Fordham Cooper, isbn: 0-408-00289-1. Mr.
Fordham
Cooper advocates Schaffer resuscitation, (manually applied to the rib cage
from the front), despite conventional medical opinion favouring
exclusively
the mouth-to-mouth method, with airway recommended for safety from AIDS.
This is because breathing assistance and heart massage are applied at the
same time. He states that this can be effective up to one hour after the
accident although no cases of success after this period are known. The
definitive source however is IEC 479 which has graphs on all aspects of
current through the human body. It does point out that effects on live
subjects are speculative as comprehensive test are not possible.

I know that it is possible to survive 240 V across the arms, the most
dangerous path, as I once, 43 years ago, had this experience when trying
to
connect a live chassis record player (phonograph) to a correctly grounded
amplifier. The record player had been modified by a local repair shop and
should have had an audio isolating transformer fitted to allow an external
output to be connected safely. Even at 15 years old I knew this but should
not have assumed that anyone charging money for this kind of work would
know
or care.

I do remember trying to shout for help and being very interested to note
that my voice appeared, to me at any rate, to be modulated with mains hum.
Eventually I was able to let go by shear will power but I felt weak for
several weeks afterwards and still have the burn marks on one finger and
the
opposite thumb.

Despite the risks, my experience is that the non-technical British public
does not really understand the dangers of electricity and often takes
unwarranted risks.

Most colleges and universities are unwilling to expose their students to
more than 24V and it can be argued that this is counter-productive as they
come to regard all bare conductors as safe. Forty years ago electrical
labs
had switchboards with open 'knife' pattern switches carrying 440V (AC) and
students were expected to follow the safety training. (Some have even
suggested that it was better to eliminate those who didn't as soon as
possible)! Despite this I have never heard of an accident in a college lab
although there may have been some.

The point about greater current being necessary at 110V is well made. Not
all manufacturers were/are as responsible as Tektronix in dual marking the
current rating of back-panel fuses.

There has been considerable discussion about good practice in first
powering
up an old 'scope but I don't ever remeber anyone advocating what we know
here as a "Portable Appliance Test" (PAT). Could that be because all of
you
know that careful visual inspection is equally important - maybe even more
so? Unfortunately many members of the public, and some who should know
better, put far too much faith in elaborate test instruments which can't
see
lethal defects.




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