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.
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? Is -50Vdc power SELV maybe? And earthed as well because the plus return connects to a master ground bar or equivalent. 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. Can anyone please point me in the right direction? Mike Donnelly AG Communication Systems Phoenix, AZ Mike Donnelly
