Hi Gregg:
> Consider the number of PRODUCT standards written - these reflect the
> accepted degree of protection (Operator or User) for each product under
> specified operating conditions and accepted uses.
I invite consideration of INJURY, and the means by
which an injury can occur from a product. It is
NOT the product that causes the injury, but an
ENERGY SOURCE that is the cause of the injury.
For an injury to occur, energy must be transferred
to a body part.
Prevention of injury is by means of prevention of
transfer of energy to a body part.
Consider a thermal (burn) injury. A burn injury
will occur if sufficient energy is transferred from
a hot part to a body part. For example, consider
aluminum foil and an aluminum bar, both at the same
high temperature. The aluminum foil will not cause
a burn, while an aluminum bar will cause a burn.
The difference between the two is the amount of
energy stored in the aluminum and available to be
transferred to a body part.
> Simplistically USE will change safety. (Indoor - Outdoor and Underwater
> lights). Using the SAME STANDARD approach there is a VERY REAL risk of over
> design being forced upon manufacturers and product costs soaring.
I would suggest that the CONDITIONS under which the
transfer of energy occurs is the salient point, not
the product or its use. These conditions may enhance
energy transfer or they may impede energy transfer.
Clearly, electric shock energy transfer is enhanced
by water or moisture contributing to the energy
transfer mechanism. The mechanism is that water is
not an insulator, is a fluid, can displace air
insulation, and can provide a conductive coating to
a solid insulator. So, a generic standard would
specify that where moisture or water is present, then
safeguards must be employed to prevent the water from
compromising the insulation.
If the generic standard is written correctly, it will
not force overdesign. Requirements can be written in
the from of "If moisture or water is present, then..."
Such conditional statements prevent overdesign.
> More subtle situations exist. We have recently obtained a UL Listing for a
> UK product (The First Pocket CDR) - the test lab suggested '065 but I
> insisted upon '950 because (amongst other things, the Creepage and clearance
> distances between the two standards is not compatible and as a result '950
> provides a higher level of protection for the user). There is no issue when
> two '950 products are interconnected, but the user safety of the '950
> product MAY BE REDUCED if a non'950 product is connected.
The presumption is that the larger the creepage and
clearance distances, the safer the product.
A clearance is really the use of air as the insulator.
As with all insulators, the state of being an insulator
is a function of the applied voltage. At low voltages,
the material is an insulator. At some high voltage,
the material breaks down and becomes a conductor. The
breakdown voltage is well-known.
For the purposes of safety, the question is, how much
margin should be between the maximum applied voltage
and the breakdown voltage? At some point, the margin
is excessive, and overdesign is forced on the
manufacturer. For the purposes of safety, there is no
good rationale for having different margins in different
safety standards, especially '065 and '950.
I contend that we must understand the physics of
safeguards in order to use them effectively. A generic
safety standard can address such attributes.
Best regards,
Rich
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