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Charles-
I
would suggest that you find a consultant familiar with EN60950/IEC60950. He will
be able to assist you far better than this reflector.
Again,
let me stress that the actual clearances required will be a function of your
particular application, device characteristics, and operating
environment. These may be determined from an exhaustive reading of EN60950.
It isn't fun or easy, but it's the only way. Let me give you some
examples:
a)
your PD is an IP phone - its environment is a standard office (no salt water,
conductive dust, etc.) and there is no source of mains voltage in your PD. Your
device is SELV and will have the most relaxed requirements for
spacings/clearance/creepage.
b)
your PD is a motor controller - the control logic runs from 802.3af power, but
the motors operate on 440 VAC. The controller is in a steel mill. In this case,
your environment is completely different from the benign office cubicle. EN60950
would not even be the appropriate safety standard; in the US UL508 for
industrial controls would be the correct one, and in Europe the industrial
section of the CE Low Voltage Directive. Spacings, creepage and clearances would
be much more severe due to the environment and the high mains
voltages.
The
term "adequate separation" is undefined precisely because the requirements may
vary per the above examples.
Finally, Clause 14.3.1.1 defines several methods of measuring isolation
that use testing methodology defined in EN60950:1991. It does not
indicate conformance with EN60950, but simply uses the test methods of
EN60950. Please note that three different methods are available for doing the
isolation testing and the implementer may chose whichever one they
want.
Prior to P802.3af, isolation in 802.3 was intended to prevent earth
loops and to eliminate common-mode voltage problems with the data communications
interface. The 802.3 standard is not a safety standard. The amendment of
P802.3af is also not a safety standard, but provides pointers to EN60950:2001 to
formally recognize the concept of the Limited Power Source and to align
isolation testing with the 2001 edition. Again, these pointers are used to
define testing techniques for determining isolation for the data communications
interface. P802.3af will open a maintenance request to determine of the
references to EN60950:1991 may be updated in all cases to point to
EN60950:2001.
Regards,
Steven B.
Carlson
President Co-Chair, ESTA Control Protocols Working Group Chair, ESTA ACN Task Group http://www.esta.org Chair, IEEE 802.3af DTE Power via MDI Task Force http://www.ieee802.org/3/af/index.html Secretary, IEEE 802.3 CSMA/CD Working Group http://www.ieee802.org/3/ High Speed Design, Inc. 11929 NW Old Quarry Road Portland, OR 97229 503.626.4206 FAX 503.626.4206 scarlson@xxxxxxxxxxxxx
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