Doug - The Note to Subclause 3.4.1 of CSA C22.2 No.950/UL1950 Third Ed. clearly states that dc. powered systems are considered secondary circuits. It is only the connections to the source of supply that are treated identically to primary circuits (for field wiring considerations).
However, there is an apparent problem with Table 18, in that Note 6 does say that dc mains circuits require the 1000Vrms type test. However, the Note to Subclause 3.4.1 certainly makes it clear that your input is a secondary circuit. As well, you should be aware of the May 14, 1993 Subject 1950 Bulletin to Industry and its Attachment C, which deals specifically with equipment of the type you mention. If the guidelines are followed, your input circuit is not only considered a secondary circuit, but a SELV circuit. Keep in mind, though, that the above is strictly applicable to North America, but other international third party safety agencies are in general agreement with the concept. Some discussion with those agencies may be needed to ensure you have their agreement on the issue. Your installation practices and instructions need to be up to snuff. It should also be noted that work in TC74, WG7 has recently accepted a proposal much akin to Attachment C. This proposal might make it into the First or Second Amendment to the Third Ed. of IEC60950. Peter L. Tarver Nortel [email protected] >---------- >From: Doug McKean[SMTP:[email protected]] >Sent: Tuesday, October 28, 1997 11:06 AM >To: IEEE Product Safety Technical Committee - >Subject: Hi-Pot Testing of -48 VDC Telco Central Office Equipment ... > >I'm having a hell of a time with UL. > >A piece of equipment I'm having tested is >powered with -48vdc from the power grid >in a CO. That power grid takes in AC from >the street, steps it down to 48 VDC rectified, >then passes it across the batteries, finally >distributing it out to the racks in the CO. > >That 48 VDC is SELV for there are fuses all >over the place in the CO right up to where >the equipment in the racks plug into it. > >Under UL1950 3rd Edition, Table 18, page 125, >I'm declaring that the 48vdc is secondary and >I asked to have my equipment tested operationally >at 500V. > >I have had 48vdc equip tested at another >lab and approved to UL1950 3rd Edition >under the same claim above. > >UL says no way. It must be tested under >the PRIMARY heading of Table 18 page 125. > >It is important to note here that I WANT >the hi-pot testing to be done. > >Well, now I'm told that if I demand this >to stay as I originally asked, then I only >get UL1950 2nd Edition until the year 2000. > >I don't understand. I've had this done >before without a problem. > >Also, the power supply people, more than >one, find this to be news to them. Most >of the hockey puck DC-DC converters out >there (that I've dealt with) aren't able >to withstand 1414VDC operational. > >Am I alone here? > >Are other mfrs of 48vdc and 24vdc equipment >intended for COs running into the same thing??? > >Regards, Doug > >
