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•G rLrlFig. 6The combined fuse NGH 5001A end capB fuse wireC glass tubeD rodE solderF helical springelements. Similarly the 10-pair strips NFL 3201 are of identical constructionto strips NFL 3211. The pairs are on 10 mm centres. Every installed 10-pairstrip occupies a vertical space of 105 mm.The contact springs, of tinned german silver, are assembled on plasticblocks with high insulation resistance to breakdown and leakage currents.The leakage current paths are at least 4 mm long, which suffices forindoor installation. Under the connecting screw is a slot for the wire. Thisarrangement greatly simplifies the wireman's work, as the end of the wireneed not be formed into a loop.The Protector MountingFig. 3 shows a protector mounting for five 10-pair strips. When carryingover-voltage arresters, the mounting at the same time serves as earth connection.As stated above, mountings are made for one. two and five 10-pairstrips. No waste of space is involved in placing several mountings one abovethe other. For instance, one 10-pair and two 20-pair mountings occupy thesame space as a 50-pair mounting. The mounting is made of nickel-platedbrass and has earth terminals at each end. It also carries fanning strips forthe jumper wires. The fanning strips are made of anodized aluminium. Theanodization provides satisfactory protection against corrosion and is electricallyinsulating.1000100101,iVsAB\nni0,1 0,2 0,5 1 101|11111\\\ •*Fig. 7 X 2150Fusing curve of combined fuse NGH5001Label Holder NBM 1111Every 10-pair strip can be fitted with a label holder as shown in fig. 4. Twospring catches permit the holder to be pushed into position without needingto be secured by screws. It is supplied with label and transparent protectingplate.Over-voltage Arrester NGA 3001This protector, illustrated in fig. 5, consists of two cylindrical metalelectrodes of a patented alloy which are pressed into a plastic tube so as toproduce the proper gap between them. The breakdown voltage is 700—1.000 volts d.c. As in all over-voltage arresters of this type, the strikingpotential increases by 10 to 20 per cent on steep wave front voltage surges.Its load capacity is greater than that of carbon arresters. Arrester NGA 3001withstands a large number of discharges of up to 0.2 coulombs, whereascarbon arresters are often short-circuited at 0.1 coulombs. In the event of apermanent arc being formed, the electrodes become heated. This causes theplastic tube to soften, whereupon the electrodes are forced together by themounting springs so that the line becomes earthed. The protector must then bereplaced.Combined Fuse NGH 5001This fuse, illustrated in fig. 6, is of a patented design. The end caps A andthe glass tube C are the same as in fuse NGH 22. F is a helical spring which,at point E, is attached by a solder of low melting point to the metal rod D.Also soldered to point E is an insulated resistance wire, which is first wounda number of turns around the rod and then continues in the form of fusewire B. At fairly low currents the rod heats up until the solder at point Emelts. Owing to the relatively large mass of the rod. a certain period elapsesbefore the break occurs. At high currents wire B melts very quickly, andbefore the solder. Fig. 7 shows the fusing curves of the solder (A) and theA fuse wire (fi). The curve of the combined fuse unit thus follows the fullvdrawn line. The resistance of this fuse is only about 4 ohms. The resistanceof the old type of heat coil was 8 ohms.17