Hello from San Diego:
Bill Jackson asks whether standards specify the location of fuses in relation to the power switch. Standards usually only specify that a product must be safe in the event of any fault within the product. A fuse provides protection in the event of a fault. The location of a fuse in the circuit depends on the fault for which the fuse must provide protection. With regard to the primary circuit, a fuse is usually located to provide protection against a fault in any primary circuit component. To do this, it must be the first component in the circuit. Often, this construction is not possible. For example, the EMI filter is often the first component in the product. Therefore, the EMI filter is protected by the fuse in the wall, while all the other components are protected by the fuse in the product. Also, as has already been mentioned, some designers prefer to have the fuse after the switch so that the fuse can be replaced with the switch off. In this case, the switch is also protected by the fuse in the wall. (On the other hand, I remember a product in which the switch would occasionally fail -- so the fuse was ahead of the switch.) Placing the fuse after the switch is not necessary with the "touch-proof" fuseholders. These fuseholders are designed such that neither terminal is accessible during fuse replacement. 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] -------------------------------------------------------------
