Reply to: FWD>RE>>Limited Power Source Egon H. Varju wrote:
> On 20/4/1996 Kendal Wilcox 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. This is not quite true; the intent is *not* exclusively to determine fuse overcurrent protection value. The intent is that, if a limited power circuit is needed or required for a load and the source output circuit impedances (capacity) are insufficient to limit the output current (not inherently limited), a fuse *may* be used to create a limited power circuit. If a fuse is the limiting device of choice, then the source must not be capable of delivering more than the specified values of current and power. Because the fuse can be somewhat generic (though generally must have been evaluated as suitable for the purpose), no particular I^2*t figure is applicable; therefore, a limitation of output current is placed on the source. The reason for a maximum power value is somewhat more gray to me, but was probably a compromise on what was felt or known to present a reasonable ability to initiate a fire in materials common when the tables in the US and Canadian electrical codes were written (a long time ago). >1. The <1000/Uoc and <250VA limits with the overcurrent protector BYPASSED. Quite so. The limits apply to the source, as if the fuse and load were not there. >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. True again; especially of linear power sources, where the output voltage essentially drops to zero (though some fraction of a volt is still present and must exist in order to drive the current). Nonlinear loads, like switching power supplies, normally don't get close enough to 0 V to be thought of as a short circuit, before they shut down. >3. The <250VA limit is another, separate, requirement. If the VA in Item 2 >above exceeds 250VA, then 250VA becomes the limit. The last sentence isn't clear. I assume you mean that if the measured Uoc and Isc values are multiplied together, and that result exceeds 250 VA, a separate measurement is needed for power. The 250 VA limit is the power delivered into the Norton equivalent impedance (although, a resistive or active load is normally used) of the power source (maximum power transfer). >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. Up to 20 Voc, the limit for fuse rating is 5.0 A. Above 5.0 A, the maximum fuse size is based on calculation and the result can't exceed 1/10 of Isc. >Egon Varju >CSA Pacific Region Peter L. Tarver Northern Telecom, Inc. [email protected]
