I will make only one comment on this issue and this comment is not a
statement from UL but rather a fellow IEEE member.
Rich Nute stated "Injury doesn't occur until the onset of fibrillation,
about
50 mA hand-to-hand."
Sorry Rich, this is simply not true. There are a terribly large number of
injuries every year resulting directly and indirectly from much lower
currents. Injuries as a consequence of "startle effect" can be very
serious. In addition, small children by the hundreds are shocked and
suffer nuerological damage or worse as a consequence of currents
significantly less than 50 mA.
As engineers working to develop effective, safe products that improve our
world, we have to keep in mind that there are all types of people out there
that use or abuse electrical products including the children and the
physically frail and that in many cases the injuries suffered are an
indirect result of shock.
Sorry if this seems preachy, no offense intended.
Mike Windler
Underwriters Laboratories Inc.
International EMC Services
E-mail: windl...@ul.com
Fax:847-272-8864
Phone: 847-272-8800 (ext. 43409)
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Original Text
From: "Rich Nute" , on 9/9/97 4:24 PM:
Hello from San Diego:
Mike Conn suggests that 1 mA through the heart muscle can
induce fibrillation, and that anything greater than 5 mA
through the hands is a dangerous current.
I think that 50 uA is a better value of current applied to
the heart muscle that is likely to induce fibrillation.
This is why leakage current from patient-connected equipment
is limited to 10 uA.
Note that physical blows to the chest can induce fibrillation.
During open-chest surgery, simply touching the heart muscle
can induce fibrillation. This is because the heart muscle
works much the same way "the wave" works in a ballpark when the
fans rise to their feet and raise their arms. They do so as
the folks adjacent to them rise. So, too, for the heart
muscle. It gets an initial trigger at the sinoatrial node.
The initial muscle contraction at the sinoatrial node spreads
along the atrium to the atrioventricular (central wall) node.
At this point, the contractions divide to either side of the
ventricals, passing around the outside of the heart cavities
back to the starting point.
Any trigger to any relaxed heart muscle can cause it to
contract, and thereby spread out of sync to the other parts
of the muscle. This is fibrillation.
The 5 mA number came from the work of Karl Geiges of UL in
his study of leakage current from radio receivers in the late
'40s. Geiges and Charles Dalziel, University of California
Berkeley, studied "let-go" currents. Geiges tested UL
personnel. Dalziel tested graduate students. Independently,
they concluded that 5 mA was the maximum allowable current
that persons of all ages could let go.
(Geiges work is published in a UL monograph. Dalziel's work
is published in the IRE and AIEE journals of the time.)
Since Dalziel was the inventor of the GFCI, it follows that
the trip current for the GFCI is 5 mA.
The issue is, "What is dangerous?"
Many years ago, Claude Haggard, Medford, Oregon, taught and
demonstrated the dangers of electricity to various groups,
from school children to granges. He demonstrated arm-to-hand
"can't let go" using a conductive arm-band and a "defective"
electric drill in his hand. He would "freeze" at from 7 to
12 mA.
But, he was not injured. Following the demonstration, he was
as good as new! I enjoyed lunch with him following one of his
demonstrations.
But, Haggard was very careful by adjusting the current up to
the point where he was just "frozen," and no more. And, he
was very careful that the current path was down the arm, and
not across the thorax.
He also demonstrated the effectiveness of the GFCI -- without
any current limit. He would hold the "defective" drill and
touch a grounded panel with the upper part of the arm. Almost
no sensation. (I haven't had the nerve to try this!)
So, more than 5 mA is not "dangerous" in that it does not
cause an injury. But, being "frozen" is scary.
Injury doesn't occur until the onset of fibrillation, about
50 mA hand-to-hand.
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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