Doug,
Thanks for confirming that CSA 60950-1 has followed IEC 60950-1 and EN
60950-1 in specifying absolute temperatures rather than a delta above
ambient.
Are you REQUIRED to conduct heating tests at the highest rated ambient
temperature - in your example 30 degC? Well, that depends on whether your
equipment has a temperature controlled fan or similar.
If your equipment has no such fan or control mechanism then it is permitted
to run the test at lab ambient and extrapolate, see 1.4.12.3. For such
equipment therefore you will be able to run the tests just the way you seem
to have done in the past.
However, if your equipment does have such a such fan or control mechanism
then it could/would be unfair to run the test at (say) 20 degC and then add
10 degC (or 10 K if you prefer) to your results before comparing the
'corrected' results against the limit values: after all, in such equipment
the fan could possibly run twice as fast at 30 degC as at 20 degC. Hence
the requirement was added in part to make the test fairer. It was also
added because in the future it is expected that there will be a Part 22 that
will cover Equipment Installed Outdoors - and such equipment will obviously
work at temperatures in excess of 25 degC in the summer (well, in many parts
of the world). For manufacturers running their own tests there can also be
the. practical advantage of combining product safety tests with product
functionality tests (i.e. making sure the equipment doesn't fail to function
when run at its max/min rated temperatures).
Why change from limits based on a temperature RISE to ABSOLUTE temperature?
As indicated above, there are reasons why you may need to run tests at the
maximum temperature specified by the manufacturer rather than at lab ambient
and then convert. When non-safety engineers used IEC 60950 etc. it was
always previously necessary to explain to them that the figures given were
RISES above an ambient contained elsewhere in the standard rather than
absolute figures. In addition, if the equipment was not designed for a
maximum temperature of 25 degC but 32 degC or 40 deg C (to pick a couple of
common examples) then it was also necessary to adjust the RISE figures by
subtracting 7 K or 15 K (respectively for the examples given). What a
complicated way of doing things! In addition, why specify the limits for
Normal conditions as a RISE but those for Fault conditions (e.g. see Table
B.1) as an ABSOLUTE temperature?
Why were the temperatures changed from degrees K to degC? From memory,
these changes were made because many people (except, perhaps, the USA) think
in degrees C. Having made the decision to change from a temperature RISE to
ABSOLUTE temperature we could either have suck with the S.I. system of
Kelvins {note: not degrees K, just K} (giving the max temperature for a
Class A transformer to be 373.15 K, or their abouts) or we could say with
100 degC: we {the IEC Committee responsible for IEC 60950-1} decided on
degC.
All the best,
Richard Hughes [with a second 'h']
Safety Answers Ltd
From: [email protected] [mailto:[email protected]]
Sent: Tuesday, April 29, 2003 1:43 PM
To: [email protected]
Cc: [email protected]; [email protected]
Subject: Re: Surface Temperature Limits
Ladies and Gentlemen
Richard Huges commented that 60950-1 had changed to absolute temperatures
rather than a delta above ambient. I checked in CSA60950-1 and this is
indeed the case. Personally I preferred the delta measurement, as it was
easier to measure in the lab. The implications on testing are that the
heating test has to be done at the highest rated ambient temperature, e.g.
if the equipment is rated to operate at +5 to +30 Deg C, the heating test
has to be carried out at at an ambient of +30DegC. This now complicates the
thermals testing somewhat, in that it now has to be done in an oven,
instead of on the bench in the lab which is temperature controlled at
around 23deg C. Is this assumption correct?
A second question is does anyone know why the limits were changed from a
delta K to absolute degreesC?
Regards
Doug Beckwith
Rich Nute
<[email protected]> To: [email protected]
Sent by: cc:
[email protected]
owner-emc-pstc@majordom Subject: Re: Surface
Temperature Limits
o.ieee.org
04/25/03 12:47 PM
Please respond to Rich
Nute
Hi Brian:
> In EN61010-1 section 10 and EN60950 safety standards, there are surface
> temperature limits specification for Normal Condition based on an
Ambient
> Temperature of 40C. Our company makes analytical instrumentation and a
new
No, this is not true for IEC 60950.
The temperature limits in IEC 60950 are temperature RISE
limits, not absolute limits.
> product we are working on is functionally very susceptible to room
temperatures.
> So, the operational specification in our manual says it must be in a
room no
> warmer than 30C.
If you use IEC 60950, you would place the equipment in a
30 C ambient, and then measure the temperatures of the
parts in question, and then subtract the ambient to get
the temperature rise.
> Question is, is it proper to consider surface temperatures for safety
based on
> a 30C ambient which would give us 10 more range to work with?
Someone told
The operating ambient temperature range is a manufacturer's
spec. The safety standard does not tell you what ambient
temperature your equipment must operate in.
> me you could do this but it doesn't seem right to me. You are relying
on the
> building's air conditioning as a level of protection. Then I was told
that if
> the AC broke down it would be considered a Fault and then the 105C
limit would
> be applied.
No.
Ambient temperature outside the manufacturer's spec is
not a fault condition. The user's responsibility is to
operate the product within the manufacturer's temperature
spec just as it is the user's responsibility to connect
the equipment to the correct supply voltage.
If the AC broke down, what would be the ambient
temperature? The temperature is indeterminant, so no
meaningful measurements could be made, and, likewise, no
meaningful temperature limits could be set.
Best regards,
Rich
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