In message <[email protected]>, dated Tue, 21 Jul
2015, Pete Perkins <[email protected]> writes:
Product committees need to understand these issues on both the
product safety and EMC side and include appropriate requirements In the
standards to mitigate the interference noted.
The EMC committees are aware of the issue, do understand it and are
working hard towards resolution. But it is proving very difficult to get
consensus among the very many interests concerned. This is not all to do
with reluctance to change. The introduction of filters brings
second-order issues to the fore. Shunt capacitors (often of tens or even
hundreds of microfarads) on the supply deeply attenuate PLC signals and
can drive the power network towards, or even into, resonance.
One can't respond to Pete without mentioning touch current. (;-) It is
indeed a significant issue, but there is only so much that can be does
by setting standards. For example, just a modest 100 PCs might be used
in one building. if each is limited to 1 mA touch current (not easy,
considering the EMC filtering required), that's 100 mA in the protective
conductor, which is too much. (In USA etc. where 120-0-120 rules, there
can be substantial current cancellation in the protective conductor, but
that is an installation-level advantage and is difficult to allow for in
standards.)
Making everything safety Class II looks attractive but with networking
you have an extended system of conductors with no connection to ground.
After a nearby (not direct) lightning strike, you probably no longer
have a network. After a power cross, you may no longer have any
employees.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
When I turn my back on the sun, it's to look for a rainbow
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-
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