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From: emc-pstc@ieee.org on behalf of Robert Johnson
[johnson@ITEsafety.com]
Sent: Friday, June 01, 2007 9:18 PM
To: ieee
Subject: IEC 60950 fire enclosure openings
I have come up with a question and will try to strip it to its
essentials.
Consider a part which requires a fire enclosure, as a hypothetical
example a primary rectifier bridge with reinforced insulation. It is
safe to touch, with no live parts, so top openings or side access is not
restricted. A bottom enclosure is provided out beyond the 5º projection.
The question is what is the fire enclosure? Clause 4.6.1 places no
limits on side or top openings since access is not an issue. In fact it
appears the opening can extend from the bottom up the sides and over the
top and down the other side to the bottom.
Is it necessary for a fire enclosure to envelope the part?
The next difficult question is what is a part placed in this opening? If
parts are placed in the opening (anywhere beside or above the part
requiring the fire enclosure), they must be V-1 (etc. see 4.7.3.2), but
if outside the opening they may be HB (etc. see 4.7.3.3). No distance is
specified, so in my case it appears a plastic window 12 inches away
appears to be occupying an opening in an enclosure.
I can apply 4.7.1 method 2, but do the enclosure requirements of method
1 fail to be usable in this situation? Any opinions?
Robert Johnson
ITE Safety
-----
From: emc-pstc@ieee.org on behalf of rnute@san.rr.com
Sent: Friday, June 01, 2007 9:51 PM
To: 'Robert Johnson'; 'ieee'
Subject: RE: IEC 60950 fire enclosure openings
Hi Bob:
Been there, done that.
Regardless whether the requirements for side and top
openings APPEAR from the text to be for prevention
of electric shock, they also apply to fire enclosures.
The way the standard is written, the text appears to
not require a fire enclosure for the sides and top.
Clearly, this is an oversight in the writing of the
standard.
If you submit the construction for certification,
you will find that the cert house will impose the
side and top opening requirements.
Or... you can perform the fault-all-components test
and qualify the construction as not needing a fire
enclosure. This was my solution.
Best regards,
Rich
-----
From: emc-pstc@ieee.org on behalf of Robert Johnson [johnson@ITEsafety.com]
Sent: Saturday, June 02, 2007 11:19 PM
To: richn@ieee.org
Cc: 'ieee'
Subject: Re: IEC 60950 fire enclosure openings
My attempt to put aside the access rules got in the way of the question.
Another way of putting it is, if you have a solid top which overhangs
the bottom by the 5 degrees, there is no limit to the size of the side
opening. It appears the opening can be the size of the side. In other
words, is there some undeclared maximum to dimension L in Figure 4D?
Bob
-----
From: emc-pstc@ieee.org on behalf of rnute@san.rr.com
Sent: Sunday, June 03, 2007 2:58 PM
To: 'Robert Johnson'
Cc: 'ieee'
Subject: RE: IEC 60950 fire enclosure openings
Hi Bob:
As I said, been there, done that.
Look at it this way. Openings in the sides are
permitted IF they are related to the ingress of
foreign objects with respect to the circuits
below the openings. If there are no openings or
circuits below the openings, then you still need
a side. I know, this is poor rationale. But,
you need a side, and it needs to be in proximity
to the circuits.
Here's my story: Consider a printer with an
opening in front for paper input and paper output.
The main PWB is vertically-oriented on one side of
the opening. The components are on the PWB side
facing the opening. The PWB was supplied with
SELV, but not LPS. So, a fire enclosure was
required. The design included a fire enclosure
bottom, a fire enclosure top, a fire enclosure
side (non-component side of the PWB), but no
enclosure for the component side of the PWB except
for the opposite side of the printer. With a
great deal of contortion, some of the components
were accessible through the front opening.
Clearly, nothing could fall onto the circuits
through the front opening of the printer.
My favorite certification house would not accept
the opposite side of the printer as an enclosure
for the component side of the PWB. The opposite
side was probably 12 inches away from the PWB.
Nor would they accept the front opening as
complying with the fire enclosure side opening
requirements.
I'm not arguing with your interpretation of the
side opening requirements. I am advising that
you cannot win with a certification house that
no side is required.
Best regards,
Rich
-----
From: emc-pstc@ieee.org on behalf of Robert Johnson [johnson@ITEsafety.com]
Sent: Sunday, June 03, 2007 10:34 PM
To: richn@ieee.org
Cc: 'ieee'
Subject: Re: IEC 60950 fire enclosure openings
Rich,
I sort of figured that this condition might be unacceptable to some
agencies and that is why I brought up the question.
However I am uncomfortable with a situation where they don't like the
standard and will make up their own rules after someone comes to them
with a product. If they have a maximum value for L, then they should
state it or explain the limits, not just pick some arbitrary value
later. It also leaves it up to whether the inspector notices or cares.
Obviously in your case they confronted the situation but did not resolve
it for anyone else.
Bob Johnson
ITE Safety <http://www.itesafety.com>
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