>---------- >From: [email protected] [SMTP:[email protected]] >Sent: Tuesday, June 24, 1997 12:29 PM > >I am wrestling with the concept of clearances as they apply >to our -50Vdc power circuits in a telecom application. >Looking at UL 1950 (3rd edition), Clause 3.4.1 says that >the >"connections" to a centralized dc power system are subject >to the requirements for primary circuits in accordance with >Cl 3.2. This is the >clause that relates to the power cordage, terminals, strain >relief, bushings, and such physical stuff. OK, no >problem. Annex NAB offers no useful guidence for this at >all.
The intent of this Subclause is to require the same means for connection for centralized dc powered equipment as is required for mains powered equipment. Meaning, field wiring terminals and other power connection methods must comply with the same that apply to ac powered equipment. The basis for these requirements is in the high fault currents that are available from dc power systems. > >Clause 3.4.1 says nothing about treating the centralized dc >power CIRCUIT as a PRIMARY CIRCUIT for clearance >considerations. More specifically, the NOTE under 3.4.1 >defines the dc powered system as a SECONDARY circuit. I am >wondering if we should be using Table 3 for clearances at >all. What is the right interpretation about -50Vdc power >with >respect to creepage and clearance? This is because there was no intent to make these requirements identical to those for primary circuits. It was recognized that, while there are large fault currents available from dc power systems, the shock hazard from the voltage alone is relatively low. The appropriate clearance and creepages in a dc power circuit are those used for secondary circuits and at the working voltage for the circuit, EXCEPT at the means of connection to the source. > >Is -50Vdc power SELV maybe? >And earthed as well because the plus return connects to a >master ground bar or equivalent. This has been a matter of much debate. It was recognized that many or most centralized dc power systems could meet the requirements for SELV, but there was no guarantee that all of them would include at least basic insulation through an isolating transformer. Indeed, there was some scuttlebutt that some dc power systems were derived from autotransformers. There are some additional marking requirements, if a piece of equipment is intended to connect dc power systems for those power systems to be treated as SELV. Refer to the North American Anneces. > >I am also wondering if a power disconnect device [Clause >2.6] is >required in a centralized dc power circuit. I see no >reference for this. This is sticky because, as you know, >switches or breakers to handle 50 Vdc at 10 amps are quite >large. There are upstream feeder fuses, but fuses are not >on the >short list of approved disconnect devices in clause 2.6.2. A power disconnect device would still be required, but I don't believe the 3.0 mm contact separation would still be required. You should contact UL and CSA directly to get a reading on this. If a centralized dc power system exists, it will most certainly have suitable branch circuit protection, or the installation would be in violation of the USNEC and CEC. Fuses are not considered disconnect devices by simple virtue of there being no switching capability inherent in their design. There are, however, fused pullout switches that would meet the requirement of 2.6.2.. >Can anyone please point me in the right direction? > >Mike Donnelly >AG Communication Systems >Phoenix, AZ > > Mike Donnelly Regards, Peter L. Tarver Nortel [email protected]
