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The PC enclosure is the most important
component as the first line of defence.
The enclosure should contain any emissions from the collection of cards
from the various manufacturers. This, then, limits external connections
as sources of emissions and immunity. Non-compliance with external
connections can be resolved with screened cables but care should be
taken as this only moves the problem “down the line” and cables must be
specified in the user instructions.
PC enclosures can be purchased for less than £20, but, like
everything else, you get what you pay for. It is false economy to
purchase enclosures without supporting evidence of their compliance and
that evidence must be impeccable.
I-Bus/Phoenix manufacture a range of industrial PC chassis all of which
are tested at BSI labs with a selection of CPUs, PSUs and PC cards. BSI
have a semi-anechoic chamber certified to EN55022 Class B and the
hundreds of tests that we have performed have proven
repeatability over several years.
Although the limits for industrial IT equipment are EN55022 Class A,
I-Bus endeavour to meet the more severe Class B which gives integrators
confidence with a 10dBuV/m margin.
Recent experience with an IT system at BSI Labs resulted in complete
retesting with an alternative UPS as the original failed to comply with
the IT Directive during surge immunity testing to EN61000-4-5. The UPS
shut down and remained in the idle condition.
User intervention was required to bring the Mission Critical IT system
back on line.
It was extremely fortunate that this was discovered before systems had
been installed.
EN50091-1 (safety) and EN50091-2 (EMC) are “the standards for UPS used
in operator controlled areas”. EN50091-2 cross refers to IEC 801 with
IEC 801-5 (surge immunity) for “future use” and therefore not
applicable at this time.
Whilst this approach may be legal, it can be argued that the UPS does
not comply with EMC Directive when used as intended, unless UPS
manufacturer declares that the UPS must not be used with IT equipment.
Unlikely!
IT equipment uses EN60950 (safety) and EN55022 (EMC emissions) and
EN55024 (EMC immunity). Other standards may be applicable depending on
the application.
UPS standards do not require testing for surge immunity, whereas the IT
standards do.
Compliance to EN61000-3-4 harmonics is also typically required.
Experience has shown that PSUs manufactured prior to 2001 may not be
compliant. It is possible that some current stock may only just be
compliant. Harmonics is cumulative; therefore, several different PSUs
in one system may result in system non-compliance.
It is the SYSTEM INTEGRATOR who takes ULTIMATE responsibility for the
CE Mark and Declaration-of-Conformity.
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