Hi Peter:
> I agree with you with your statement "rather than address products, safety
> standards need to address hazards". However, the "hazard" standards must be
> made taking into consideration the environment in which the product is used
> "light industry, heavy industry, medical, office, outdoor, home, etc.". For
> example, a product's leakage current should be stricter if product is used
> in outdoor non-protected envieonments or as medical equipment. Similarly, a
> product's mechanical hazards should be guarded more if used by ordinary
> persons and less if product is used by professionally trained persons.
I agree that the limit values for various safety
parameters depend on the situation.
And that is my point. The situation defines the
limit, not necessarily the product. (However,
often, specific products are likely to be used in
specific environments.)
Furthermore, the standard needs to give guidance
as to how the situation contributes to a limit
value.
Using your example, exactly why should leakage
current be lower for an outdoor environment than
for an indoor environment?
This is not an easy question, although everyone
knows that leakage current must be lower for the
outdoor environment. If 0.5 mA leakage current
through the body is deemed acceptable in an
indoor environment, why is that same current
through the body deemed unacceptable in an
outdoor environment? We have been taught that
the current through the body is the "dangerous"
parameter. So, if 0.5 mA body current is
acceptable, why does this change with environment?
We presume the presence of moisture makes a given
situation more dangerous. Yet, the current is
established by the product, not by the environment.
Some would suggest that the presence of moisture
reduces the resistance of the contacts. I would
agree. Yet, still, the current is established
by the product (as a current source), and is
largely independent of the body resistance, with
or without moisture.
According to Whitaker (1939), the dc body
resistance drops significantly under wet
conditions. This is probably due to improved
electrical contact with the skin.
Nevertheless, leakage current is largely a
constant-current source, and is therefore
independent of body resistance.
If we assume 0.5 mA from a 120-V source, then the
source resistance is 240 kohms.
Using Whitaker's data, the nominal dry body
resistance is 11.45 kohms, and the nominal wet
body resistance is 1.86 kilohms. For the dry
body, the current from a 120-V source through
250 + 11.45 kohms is 0.459 mA. For the wet body,
the current through 250 + 1.86 kohms is 0.4765 mA.
The current is nearly the same!
So, we still have the same question: Why should
0.5 mA be considered harmful in a wet environment?
There must be some data somewhere that explains
why the body current in a wet environment must be
less than that of a dry environment.
Best regards,
Rich
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