Hi Lauren:
There are different definitions for "safety-critical
component" based on the different needs for identification
of such components. I'd like to list a few, and then
offer yet a different definition of a safety-critical
component.
1. A safety-critical component is a component which
appears in a safety report such as those produced
by third-party safety certification houses. (Doug
McKean)
2a. A safety-critical component is any component the
failure of which would lead to a hazardous condition
of either the product or the system. (Richard Woods,
John Allen)
2b. A safety-critical component is either component in
a two-component scheme intended to provide protection
(against a hazardous condition) in the event of a
fault in one of the components. (George Alspaugh)
3. A safety-critical component may be a specific
construction, e.g., smooth, rounded edges, or color
of wire insulation, or a specific rating, or a
warning, etc., rather than a component. (Gregg
Kervill, George Alspaugh)
If you can't identify the hazard against which the safety-
critical component provides protection AND how it provides
the protection, then either the component is not safety-
critical, or the safety function it provides is not known.
There is one word that rarely appears in the discussion
of safety: safeguard.
A safeguard is a device or a scheme of construction which
renders a product or system as safe. A safeguard has
certain parameters which are critical to its effectiveness
as a safeguard.
Rather than identifying safety-critical components, we
should be identifying the safeguards and their parameters
applicable to the specific scheme or product.
For example, Basic Insulation is a safeguard against
electric shock. Its safety-critical parameters include
voltage rating, dielectric withstand rating, and
temperature rating.
An enclosure may or may not be a safeguard, depending on
the construction of the product. If the enclosure is a
safeguard against anything, then one of its parameters
is robustness sufficient to withstand the impact test.
Depending on the hazard against which the enclosure is
providing protection, there may be other applicable
parameters.
Another safeguard is the two, independent fixings of
wire terminations where the wire carries a hazardous
voltage (or is adjacent to hazardous voltage).
So, which is a better description?
The wire terminal is a critical component.
Or:
The wire terminal is provided with two, independent
fixings.
A warning is a safeguard in that it imposes a specified
action on the part of the user. Its parameters include
color, font, and size. (Gregg Kervill)
A list of safety-critical components is largely useless
because the safety function of the component and its
safety-critical parameters are rarely identified.
On the other hand, a list of safeguards must necessarily
identify the "hazard" which the safeguard protects
against. (And, "safeguard" avoids the problem of "safety-
critical feature" and "compliance-critical component"
identified by John Allen and Oscar Overton.)
Best regards,
Rich
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