Mersen Form 600 fuses are the original current limiting fuses, pre-dating all the standards. Their 500V DC rating (through 600A) is a useful feature for special fusing applications. Form 600 fuses provide a high degree of current limitation in AC and DC applications. types 1 and 3 are dimensionally interchangeable with class H and K fuses. types 4 and 5 have unique rejection dimensions and are not interchangeable.