Hello from San Diego:
Mike Royer asks: > Where do you put the AC mains fuse relative to the "X" capacitor used for > the > EMI filter? Depends. Everything (wires, circuit boards, X caps, Y caps, switches, etc.) must be protected against the effects of overcurrent. If the part is not protected by a fuse in the equipment, then it is protected by a fuse (or circuit-breaker) in the wall. The real issue is, in the event of a fault, whether the fault current is sufficient to operate the fuse (or circuit-breaker) before the fault causes an unacceptable condition such as a fire. So, if a fault in the X capacitor can blow the fuse or circuit-breaker in the wall, then the construction is acceptable. Or, if the X capacitor shorts, then blows open and clears the circuit (without operating the fuse or circuit-breaker in the wall), then the construction is acceptable. If the fault does not operate the fuse or circuit-breaker in the wall, and causes a fire, then you will need to install a local fuse that will operate before the fire is started. This could mean that you would need a fuse in series with the X capacitor. > How does the "accross the line" safety rating of the capacitor affect this > answer? It doesn't. Fuses provide protection in the event of a fault. X caps are evaluated to operate normally in the presence of continuous ac voltage with superimposed transients. This is what distinguishes X caps from other caps. The X caps are not evaluated with induced faults. > Specifically. Would you put an X cap in the unfused area of the AC inlet > like the EMC fellow wants? What about a "Y" cap? See preceding comments. The comments apply equally to both X and Y caps. I agree with other comments answering these questions. If you are using a discrete EMC filter, then I would put the fuse on the supply side of the filter. If you are using a unified power inlet/EMC filter, then you must rely on the fuse/circuit-breaker in the wall. Best regards, Rich ------------------------------------------------------------- Richard Nute Quality Department Hewlett-Packard Company Product Regulations Group San Diego Division (SDD) Tel : 619 655 3329 16399 West Bernardo Drive FAX : 619 655 4979 San Diego, California 92127 e-mail: [email protected] -------------------------------------------------------------
