One can find a great deal of information on
a power supply by studying the manufacturer’s datasheet and other technical
articles, but sometimes more information is required for the actual
installation. Where is this
information? It is in the power supply’s
safety reports and certifications.
Failure to review and follow these can cause delays when system
certification is sought.
To keep this article simple, we will just
review a product that is certified to IEC 60950-1.
Usually there are three main documents; the
CB certificate, an IEC 60950-1 CB report and / or EN 60950-1 test certificate
and of course for North America, the UL or CSA 60950-1 test report. Due to confidential information like
schematics, full test reports are often restricted and may only be released
with a non-disclosure agreement (NDA). Fortunately
reproduction is allowed by the test houses for the relevant pages of the
report.
Usually the CB test certificate, which
should always accompany the CB test report, is just two or three pages. This is often public information and details
the part numbers that have been certified, their input and output ratings along
with the safety standard (including revisions and amendments). Its function is to give a quick snapshot of
the product and to show if all the certifications are current. A product that has out of date certifications
may only be suitable where the safety bodies have allowed the use of
“grandfathering” for older systems, and will not suitable for new designs or
major system upgrades.
To reduce cost, many power supply
manufacturers are using the CE Mark to indicate compliance with EN 60950-1
rather than pay for and maintain a separate EN 60950-1 test report and
certificate. In this case the CB test
certificate (and CB test report) will indicate that the product was
“additionally evaluated to EN 60950-1”.
This is perfectly acceptable.
Even an abridged CB or UL 60950-1 test
report (the full report may extend to over 300 pages) has useful
information. The section “Engineering
Conditions of Acceptability” has the all-important details for how the product
should be used.
For example:
Are the outputs
SELV? Those outputs that are not should
be insulated or have their access restricted to ensure that an operator or
service technician cannot receive an electric shock.
Do any outputs
have hazardous energy levels? 240VA is
considered potentially dangerous if a screwdriver or metallic item accidentally
shorts them, and a cover should be installed to protect them. Metal watch straps have caused serious burns
to car mechanics when they have shorted the positive battery terminal to the
automobile body.
Is “field wiring”
allowed? If not, any cabling has to be
attached by trained personnel. Products
like DIN rail power supplies do allow field wiring and do not have crimped wire
terminations.
The maximum
investigated branch circuit rating is given.
This reflects the size of the circuit breaker that was used during the
safety testing, particularly when abnormal tests were performed.
The investigated Pollution
Degree rating is stated. A rating of 2
is normal for office or laboratory equipment.
That product should not be used where a pollution degree of 4 is
required for an outside application where it may be subject to rainfall.
Proper bonding to
the end-product main protective earthing termination is listed as required or
not required. Failure to correctly earth
the product can result in electric shock.
The temperature
class of any magnetic component components is given. Usually this is Class A (105oC)
and system testing should check to make sure that is not exceeded under worse
case conditions.
“The following
end-product enclosures are required:” Here the types of enclosures are
indicated for mounting the power supply in.
If an open frame power supply is being used, the report will state that
it has to be housed in an enclosure.
Other notes may be
listed, like product orientation.
Many power supply companies are now posting this
information on their website, along with the CE D of C (Declaration of
Conformity); even some distributors are doing this too. The recent surge of amendments to the
standards though is keeping many webmasters busy!
