Hi Kendall,
On 20/4/1996 you wrote:
> I was discussing UL 1950/CSA950/EN60950 clause 2.11, Limited Power
> Source with a colleague, and we were looking at the Table 9 limits for
> maximum output current Isc (1000/Uoc) and maximum VA (250). If Uoc is
> 20V, then Isc is 1000/20 = 50A max. However, 20V X 50A = 1000VA.
> It seems like if the maximum allowable VA is 250 and Uoc is 20V, then
> the maximum current is limited to 250/20 = 12.5A.
Please note that the purpose of Table 9 is to determine the value of the
overcurrent protection device that must be provided. These are not the
allowable power and/or current limits of a limited power circuit.
1. The <1000/Uoc and <250VA limits with the overcurrent protector BYPASSED.
2. Uoc is open cct voltage. When you load the cct to max current, the output
voltage will usually drop and the VA will be less than Uoc x Isc.
3. The <250VA limit is another, separate, requirement. If the VA in Item 2
above exceeds 250VA, then 250VA becomes the limit.
4. After all the above tests, then you must provide a fuse (last column of
Table 9). Using your example, with a 20V cct, you will need to provide a 5A
fuse. So, no, you will never get a continuous 1000VA or 12.5A.
Hope this clarifies (somewhat) the question.
Best regards,
Egon Varju
CSA Pacific Region
PS: I had an argument, er... disagreement, er... discussion, on this very
subject with Rich Nute recently. You may soon be hearing from him too ...
:-)
