Bill, et al,
To help bring about a better understanding of the basic issues involved,
this is a summary of some specific work.
There is a large ongoing work within the IEC regarding leakage current
issues.
IEC 60479 deals with the effects of current on the human body. This
committee is a mixture of medical researchers and engineering/scientific folks
who are formulating the basic information on these effects. As you can
understand, most of the results are based upon historical data since it is
difficult to collect data in these times. The committee is expanding the
description of the effects and expanding to coverage of the results; their
standards are developed in sections that are updated on a revolving basis.
IEC 60990 deals with the methods of measurement to touch current (as body
leakage current is now being called). These measurement methods have been
implemented in a number of IEC standards including IEC 60950 (safety of ITE &
Telecom equipment) and IEC 61010 (safety of measuring and testing equipment)
along with some other standards. This standard is currently being reissued as
updated. I am currently the convenor of the committee responsible for this
standard.
IEC 61201 deals with the development of touch voltage data corresponding
to the current levels described in IEC 60479. A substantial update of these
requirements is in progress.
There are a large number of folks involved in the work described above.
Some serve particular committees, others serve on several or all the
committees. By & large these folks have a good understanding of the basic data
and it's application in the real world. You can find the names of the
committee members on the IEC website.
There are also National Experts who support the work of the IEC
committees.
Many of these same members also serve on IEC product committees developing
particular requirements. The Hintz paper - described earlier in this thread -
reflects the work that is going on within the IEC TC108 committee who is
preparing a new standard melding the safety requirements for both commercial
and consumer electronic equipment (now separately described in IEC 60065 and
IEC 60950).
Finally, much of the large scale work described above applies generally to
all equipment with the caveat that medical applications have special
requirements - because you can't depend upon the resistance of the skin to
provide the first line of defense. The medical folks have described specific
requirements that they deem acceptable in IEC 60601.
It is true that hospitals have quite a bit of trouble separating the
patient environment from the general environment and applying appropriate
requirements to each area - but that is largely the responsibility of the
hospital engineer.
A couple of good references are: Bruner & Leonard, 'Electricity, Safety &
the Patient', 1989, Yearbook Medical Publishers, Inc. - ISBN 0-8151-1291-2 and
Reilly (ed) along with Antoni, Chilbert, Skuggevig & Sweeney, 'Electrical
Stimulation and Electropathology', 1992, Univ of Cambridge Press, 1992 - ISBN
0-521-41791-0.
My own experience is similar to John Allen's - when demonstrating touch
current levels most folks won't stay on the 3.5 mA long enough to get a
reliable current reading as it's too painful. This level is just below the
let-go limit described in 479. Let-go is being more commonly used as the
limit under fault conditions in equipment evaluation.
I hope that all of this is helpful.
:) br, Pete
Peter E Perkins, PE
Principal Product Safety Consultant
Tigard, ORe 97281-3427
503/452-1201 fone/fax
<mailto:[email protected]> [email protected]
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