Most esteemed colleagues, we are looking for your professional opinion on a
UL 544 leakage limit (I think you will find this interesting):
1. This particular product uses a UL 544 evaluated direct plug in power
supply with outputs to the patient care equipment. In this case it is a
diagnostic unit that sends an audible tone to headphones (audiometer). The
plug in power supply Conditions of Acceptability indicate the outputs are
"not evaluated for patient leads" (i.e. applied parts").
2. Table 42.1 of UL 544 specifies leakage limits. "patient connection"
footnote "a" references testing of "patient leads (applied parts)"
connections. There is no written definition for "patient leads" or "applied
parts" in UL544.
As such NFPA 99 supplements UL 544 as it draws from the NEC and NFPA 99
(referenced in UL 544):
NFPA 99 defines the US definition of "Patient Lead" = "A deliberate
electrical connection that can carry current between an appliance and
patient. It is not intended to include adventitious or casual contacts such
as a push button, bed surface, lamp, hand held appliance, etc."
3. As the headphones of this audiometer are clearly not "deliberate
electrical connections" we conclude these are not "patient leads (applied
parts)" which would not fall under the limits for "patient connection" limits
per 544. The applicable limits would be as defined under "enclosure or
chassis" "grounded " or" double insulated".... Now be careful not to jump to
a conclusion yet..... You might say "enclosure or chassis?", but if you
examine this, you will find the footnotes reference UL 544's Enclosure
definition:
"Enclosure = That external portion of an appliance that serves to house or
support component parts, or both. Enclosure of patient care equipment likely
to be contacted by a patient include, for example, bedside monitors, bed
frames, dental chairs, and examination stands."
Our conclusion: Due to the US definition of "patient leads (applied parts)",
the earphones of an audiometer (patient care equipment) are subjected to the
leakage current limits for "enclosure or chassis", and not the limits of
"patient connection". For this particular application, we conclude that based
on the C of As, the output of the power supply has already been evaluated for
"enclosure or chassis" leakage limits.
Your Thoughts???
Drew
PS: If you care to look, CSA supports this position in that 50uA is related
to "cardiac tissue" limits only. See Appendix A of CSA 22.2 125 (500uA).
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