G&W’s Power Assisted Fuse (PAF®) offers current limitation to systems with continuous current ratings through 630A and up to 38kV. This makes the PAF ideal for applications beyond the ratings of conventional current limiting fuses and for economical alternatives to conventional expulsion, vacuum and SF6 fuses which are not current limiting. The PAF is a commutating form of current limiting device where the continuous current is carried by a continuous copper bus bar path. This path is opened under overcurrent conditions to introduce a parallel mounted current limiting fuse. The PAF can be mounted indoors or out. Metal enclosed PAF fuses are available with enclosures, cable terminations, bus connections, supports and enclosures.
Traditionally, the current limiting fuse has worked well as overcurrent protection on systems with normal continuous currents up to 200A. Their current limitation capability, speed of operation, compact size and low cost, make them ideal add-ons to existing installations. Current limitation is a major benefit because it yields a significant reduction in the magnitude of the let-through current. This can lead to substantial savings by reducing damage to the faulted equipment. The damage limitation capabilities of current limiting fuses may prevent the secondary catastrophic failure of oil filled enclosures. For systems rated above 200A, circuit breakers and expulsion fuses are most commonly used. Though able to withstand higher continuous current, these devices are not current limiting and are relatively slow fault interrupters, therefore permitting the damage of higher let-through currents to occur. The application of a PAF for protection of under-rated circuit breakers can provide significantly improved protection at a substantial cost savings over replacement of those circuit breakers. Also, for applications where available fault currents have increased due to expanding power requirements, simply replacing the circuit breakers may not be adequate protection for other under-rated equipment on the system.
A large cross section copper conductor carries the continuous current. Upon occurrence of a short circuit current, a sensing element initiates triggering of a cutting device placed at strategic intervals along the copper bus. This creates multiple gaps in the bus. The cutting devices are similar to those that have been developed for military and space applications in which long shelf life and reliable operation are prime requirements. The arc voltage across the gaps is used to transfer the short circuit current to a parallel mounted current limiting fuse. The fuse element melts in the conventional manner, interrupting the current without venting of flames or gases. At short circuit current levels, current limitation is provided within the first half loop of fault current and prior to the first current peak.