Very much concur with Mr. Nute for ITE and CTE stuff. My employer makes all manner of power conversion stuff. Approx 20% of my time is for direct customer support or supporting the documentation requirements of the Conformity Assessment Body's engineer that is looking at the customer's stuff. Suffice to say, have seen much weirdness for employer's products where used in industrial equipment. The component power supply is one of the principle instruments intended to provide galvanic isolation and to provide other forms of protection from shock and fire. Circuits and materials that are incorporated in the end-use installation of the CPS, typically intended to increase safety margin, may decrease the margin.
1. UL508 and 508A, depending on the OVC and operating environment and other installation peculiarities for your equipment, can have some significant requirements not seen in ITE. The tables in UL508A for SCCR will kill many ITE-certified CPS. 2. Fusing of the mains input should be part of the component power supply's construction. Recommend that additional current interrupt components not be installed at input to CPS unless the conditions of acceptability or installation instructions for the power supply clearly indicate requirement for input fusing. Type tests for an ITE CPS are typically performed to verify there is no fire or shock hazard that would result from a fault condition. Type tests for a typical CPS rated for OVC II/III must be performed with a 20A (or greater) distribution breaker, lo-Z, and 'stiff' mains. So your input wiring and connection components must support the unit's least favorable abnormal/fault condition. NFPA70 (mostly article 310) and NFPA70E has ratings for this stuff. 3. Current limiting components on the output of a power supply, as others indicated, must have the margin to tolerate the conditions of the least favorable fault conditions that were observed during the assessment of the CPS. This information would be available to the CAB engineer via the TRFs that support the units certification. Do NOT use a CPS where the CAB engineer cannot obtain a copy of the certification report. 4. Most CPS are not intended for powering motors of significant power levels. Nollaig Shona Daoibh, Brian From: Richard Nute [mailto:ri...@ieee.org] Sent: Tuesday, December 23, 2014 1:57 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] fusing outputs of small power supplies Hi Bob: You have three issues to deal with: 1) Regulatory - standards and codes; 2) Safety; 3) Reputable manufacturer. For regulatory, see IEC/EN/UL/CSA 60950-1, Tables 2B and 2C. These two tables define a NEC Class 2 circuit. Class 2 circuits are doorbell circuits; the wires can be run without mechanical protection and a fire enclosure, and can be accessible. Your specs, 24 volts and 4 amps, are very close to the 100 VA limit. You will exceed the 100 VA limit with any load that exceeds 4.17 amps, let alone short-circuit (Table 2B). So, you need to go to Table 2C to select a fuse value. Since your output voltage is more than 20 but less than 30, the maximum output fuse value is 100/24 (assuming the supply is less than 250 VA before the fuse). You are stuck with a 4 amp (or less) fuse to achieve Class 2 (which is desirable for avoiding further regulation of your wiring). Fuses and equivalent operate under fault conditions to prevent fire. To determine an effective fuse value, test - without overcurrent protection -- under short-circuit to see what overheating and possible fire occurs. Then, select a fuse rating that will prevent the overheating and fire. Since the power supply manufacturer doesn't know this stuff, I would select another manufacturer who does. You may have a power supply that will catch fire or cause an electric shock despite the fusing. Good luck, and have a happy holiday, Rich From: Bob LaFrance [mailto:b...@creare.com] Sent: Tuesday, December 23, 2014 10:40 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] fusing outputs of small power supplies Greetings, I am curious to know what guidelines to use when fusing power supplies. I have a machine with some small 4amp 24v power supplies. I have placed breakers on the input side of supplies. I don't know if the current limit circuits within the power supply can be expected to protect output wiring. I am mainly concerned with NFPA 7 & NEC, but I would like to hear UL or IEC thoughts on the subject. The power supply manufacturers I have asked don't seem to know - that struck me as very odd. Many years ago I worked for a manufacturer of motor drives. We developed a software implementation of a motor overload relay and got UL 508 blessing. I am looking for similar arrangement. Thank you and Merry Christmas. Bob N9NEO - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
