Darrell, Rich
What if the overlay is a membrane key pad or has embedded LEDs?
Then it is not a decoration.
This is my situation
How does this change the evaluation?
Regards
Chris Wells
Sr Des. Eng. Cutler-Hammer
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Original Text
From: C=US/A=INTERNET/DDA=ID/richn(a)sdd.hp.com, on 4/30/97 6:29 PM:
Hi Darrell:
1. Do overlays outside the 94HB enclosure material need a
flame rating?
The answer depends on which certification house you are
submitting your product to.
According to the standard, the overlay would be considered
a "decorative part." A decorative part is defined as a
part, outside the enclosure, which has no safety function.
(IEC 950, Sub-clause 1.2.6.5.)
According to the standard, decorative parts are required
to be 94 HB or better. Except: "Small" decorative parts
are exempt from being required to be 94 HB. "Small" is
defined by example, nameplates, mounting feet, key caps,
knobs and the like. (IEC 950, Sub-clause 4.4.4, third
paragraph.)
Some certification houses will tend to ignore ANY decorative
parts, while other certification houses will negotiate what
size or particular decorative part needs to be 94 HB.
In practice, MOST plastics are UL-certified and flame-rated.
I would be surprised if your overlay was not made of UL-
certified and flame-rated material. Furthermore, overlays
are usually not critical as to choice of material. So, the
easy way out is simply use a UL-certified plastic for your
overlay.
2. Double-sided tape, not required for safety or function.
(If the tape is not required for function, then why use it?
My guess is that the tape is required to hold an insulating
sheet in place, and the insulating sheet is required for
function; therefore, the tape is required for function.)
If the insulating sheet is required for safety, then the tape
would perform a safety function of holding the required
insulating sheet in place. If this is the case, then the
tape would need to be a UL-certified tape rated for the
temperature incurred in the product.
If the tape is truly functional, then the tape would be
treated as a "small part." Specifically, flammability
requirements do not apply to thin insulating materials such
as adhesive tape used directly on any surface, PROVIDED that
the combination of the thin insulating material and the
surface comprise 94 V-2. (IEC 950, Sub-clause 4.4.3.3, third
dashed paragraph.)
3. Adhesives not relied on for safety.
Adhesives are addressed in IEC 950, Sub-clause 4.3.22. This
only applies to securing specific safety-related parts. The
requirement specifies tests for aging properties. There are
no requirements for adhesives for functional purposes.
However, UL 1950/CSA 950 address adhesives in Annex P.2,
under Sub-clause 4.3.22. Adhesives must comply with the
adhesive requirements of UL 746C (the standard which you like!).
Be aware that some certification houses will want to evaluate
the consequences of failure of the adhesive. That is, if the
adhesive should fail, and the part should move around within
the product, could it bridge a safety insulation, or short a
circuit so as to cause a fire?
4. IEC 950 not helpful on plastics.
I invite comments on the "user-friendliness" of IEC 950 and its
clones. And for other standards. Which safety standard is
"good" in regard of "user-friendliness"? What are those "good"
characteristics? What are the characteristics of a "user-friendly"
safety standard?
Best regards,
Rich
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