Hi Xing Weibing:
> We can read from table 5A of IEC60950:1999:
>
> Touch current limit for accessible parts and circuits not connected
>
> to protective earth: 0.25 mA
>
> question 1 : How does it (0.25mA) come from and what it is based on?
For grounded (Class I) equipment for the home, the UL leakage current
is 0.5 mA. (This is NOT a 60950 limit.)
UL reasoned that if 0.5 mA arises from both real and stray capacitance,
then, for double-insulated (Class II) equipment, the current should be
one-half of 0.5 mA because the standard requires the same insulation
to be applied again (i.e., the second half of double insulation), thus
halving the capacitance. Half of the capacitance results in half of
the current, therefore 0.25 mA.
UL carried this concept from their generic double-insulation standard
into IEC 60950.
The UL reasoning is faulty because in real life the two capacitances
need not be equal. And, this is certainly not the case for reinforced
insulation.
Furthermore, the current, 0.5 mA, is deemed acceptable in terms of
the safe current that can be applied to the body.
Therefore, the 0.25 mA limit is not related to the effects of the
current on the body, but presumes a product construction comprised of
double insulation where each insulation system provides exactly the
same capacitance.
Best regards,
Rich
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