Mersen Form 600 fuses are the original current-limiting fuses, pre-dating all the standards. Their 500VDC 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.