The NH system is classified as a plug-in fuse system and is made up of the following: fuse base (possibly with connection covers and phase cover), fuse inserts with contact blades, device for replacing the fuse inserts (low-voltage plug-in handle). Maintenance and servicing must be carried out by qualified personnel. NH fuse inserts of operating class gTr are used to protect transformers. The current-time characteristic curve is specially adapted to the transformers to be protected and results in optimum selectivity compared to NH fuse inserts of operating class gG. 1.3 times the transformer nominal current can be passed through the fuse for at least 10 hours. If the current is at least 1.5 times the transformer nominal current, the current is switched off within 2 hours. VDE 0636 Part 201, IEC 60269-1 and -2.