If a fuse or circuit breaker is used, then the short circuit current limit is actually 1000 divided by the open circuit voltage, per table 2C - since this is no longer an inherently limited power source. This gives more headroom on the short circuit current requirement (though important to note that the measurement must be taken with the overcurrent protection shorted), but the fuse rating requirement may not allow this to be LPS. The use of a 5A (or lower) rated fuse on a circuit that requires 8A certainly could lead to nuisance tripping. Max VA without the protection must be limited to 250 VA or less also.
If a fuse or CB is not used, then not only must the 8A short circuit current limit be met, the max VA under any load condition cannot exceed 5 times the open circuit voltage, per table 2B. Scott Aldous Compliance Engineer Advanced Energy Tel: 970-407-6872 Fax: 970-407-5872 From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, May 04, 2007 2:44 PM To: [email protected] Cc: [email protected] Subject: Re: limited current circuits in IT equipment Hello Brian, Under IEC 60950-1:2005 clause 2.1.1, bare parts of a SELV circuit may be accessible to the operator. Limited current circuits need not be at SELV voltages. They are specified as a circuit that doesn't have the current capability such that there is hazardous energy. As such, they are a separate category of circuits that may be accessible to the operator. The situation may be slightly different in the United States. UL 60950-1 matches IEC 60950-1 on this matter. However, the National Electrical Code has a different requirement. It basically requires you to limit accessibility to Class 2 circuits. This is not to be confused with Class 2 under IEC 60950. NFPA 70 Article 725 gives the limits for Class 2, which are basically the same as those for an Inherently Limited Power Source as described in 60950-1 Clause 2.5. If your 10 VDC, 8 A contact is going to connect to building wiring or other systems, it may need to be an Inherently Limited Power Source. At 8 amperes, it can be done, but it will be difficult. The 8 ampere limit has to be the short circuit current. If you have a fuse or circuit breaker to limit current, the maximum value is 5 A as shown in Table 2C. If you have jump started a car or changed the battery, you have probably been exposed to 12 - 14 volts with plenty of current capability without any harm. Many people ignore the proper order for connecting and disconnecting terminals to avoid sparks and fires, but the voltages are normally safe. (I know that I am opening my self up to comments as well as risking an off-topic discussion, but the analogy was what came to mind.) Ted Eckert American Power Conversion/MGE http://www.apc.com/ The items contained in this e-mail reflect the personal opinions of the writer and are only provided for the assistance of the reader. The writer is not speaking in an official capacity for APC-MGE or Schneider Electric. The speaker does not represent APC-MGE's or Schneider Electric's official position on any matter. "Brian O'Connell" <oconnellb@tamura corp.com> To Sent by: <[email protected]> [email protected] cc Subject 05/04/2007 03:06 limited current circuits in IT PM equipment A customer would like a 10Vdc/8A contact to be exposed to the end user. I would like to say no because this would not meet the requirement for an LCC ckt. But as I read from the sacred scrolls of 60950-1, it was noted that if the exposed part is not at a Hazardous voltage (reliably SELV) or energy level, that there is no requirement for it to be LCC if the device is Class I construction and there is no cap bridging the reinforced insulation. Did I miss something ? Does this seem ok ? thanks, Brian - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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