Hello from San Diego:
Juan Pena wonders why we have multiple standards (i.e. IEC 65,
IEC 950, and IEC 1010) when the products are very similar in
construction.
I would go further and include IEC 601 in the list.
Let's look at the big picture, the construction of products
for safety.
1. Electric shock protection from primary circuits (isolation
of primary circuits). All products must have isolation of
the primary circuits from accessible conductive parts AND
from secondary circuits. This is a common requirement among
ALL of the standards. Indeed, when one actually constructs
the isolation, it meets the requirements of all four IEC
standards. (There are some differences in spacings
requirements as each standard interprets IEC 664 in a
different way!)
2. Electric shock protection from secondary circuits. Today,
most secondary circuits are SELV. So, no specific isolation
is required. Some products involve high voltage for crt's
or other devices, in which case isolation must be provided.
Again, the actual construction is the same for all products,
whether a computer display, a TV display, an oscilloscope
display, or a cardiac monitor display.
3. Electric shock protection from external connections. Here,
the products have some differentiation. For ITE, we have
low-voltage SELV interconnections which don't require any
special considerations except in the case where power is or
could be involved, in which case the ampacity of the
interconnection must be taken into account. Another
interconnection is the isolated circuit such as telephone
connections (ITE), measurement connections (T&M), and patient
connections (M&D). In all these cases, the isolated circuit
is isolated from accessible conductive parts and from secondary
and primary circuits. Only the degree of isolation differs
among the circuits.
4. Protection against electrically-caused fire. Fire, of course,
only occurs under fault conditions. All the standards include
fault tests. Some standards include enclosure requirements
on the premise that the enclosure can contain a fire (just as
a stove contains a fire). There are several different approaches
to fire, all of which are independent of the kind of equipment.
5. Grounding requirements. These are very consistent across the
different products. Indeed, CSA has their No. 0.4 standard
which applies to all products.
6. Double insulation requirements. These, too, are very
consistent across the different products. Here, UL has a
generic standard for double insulation.
The point of this discussion is that safety requirements are NOT
product-dependent, but rather hazard-dependent. Protective schemes
SHOULD be prescribed on the basis of hazards, not on the basis of
the type of equipment. In reality, this is what the safety standards
do. But the authors of safety standards get caught up in product-
specific details. IEC 950 is an example of a standard with many,
many product-specific details which are technically meaningless.
For example, spacings in low-voltage semiconductor secondary
circuits are dependent on the primary voltage. This is ridiculous.
It presumes that impulses from the primary will be transmitted
to the secondary. If this occured, then the semiconductors would
be destroyed. And we know that semiconductors in secondary
circuits do not fail when the primary is subjected to an impulse
overvoltage.
ECMA TC-12 is currently writing a hazard-based safety standard.
This should demonstrate my contention that one standard CAN
apply to multiple product classes.
Best regards,
Rich
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|Richard Nute |Quality Department |
|Hewlett-Packard Company |Product Regulations Group |
|San Diego Division (SDD) |Tel : 619 655 3329 |
|16399 West Bernardo Drive |FAX : 619 655 4979 |
|San Diego, California 92127 |e-mail: [email protected] |
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