Patent GB-487666-A (GB)
Title: Improvements relating to protective systems for electrical networks
IPC: H02H7/28
Filed: 1936-09-21
Abstract: 487,666. Protective cut-out arrangements. MUTH, H., and ZIMMERMANN, K. Sept. 21, 1936. Nos. 25670, 25671, and 25672. Convention dates, Sept. 19, 1935, Sept. 27, 1935, and March 16, 1936. [Class 38 (v)] [Also in Group XXXVI] A protective arrangement for a network fed by two or more feeders each of which contains a transformer at the network end, has each transformer connected to the network through a fusible element or elements and the one or more elements is or are adapted to be ruptured by an additional device, which is so arranged as to be energized by a relay operating in response to the reverse current that flows from the network on the occurrence of a fault, whether in the transformer or outside it, as in a feeder, so as to isolate the feeder from the network in cases where the reverse current is inadequate of itself to rupture the fusible element. Fig. 1 shows a net 1 supplied from a main 4 and bus-bar 2 over feeders 3 each supplying two transformer substations 7 connected to the net by fuses 9 and to the feeder 3 by switches 8. On fault at 11 or 12, the feeder-switch 5 opens on overload, and the fuse 9 of the feeder is ruptured by the reverse current relay closing the circuit to an additional heating coil 9b. The arrangement may be modified to include Merz-Price protection of the bus-bars 2 themselves. Thus, each contact 14, Fig. 3, shortcircuits the trip relay 15, the contact 17 of which short-circuits the trip coil 16. Contact 14 is also acted upon by a further coil 20a, the coil being repeated for each feeder 3 connected to the bus-bar, and being energized by closure of contacts 35 by a winding 34 in a residualcurrent circuit 33 which contains currenttransformer secondaries 5, one for each feeder 3 (only one primary is shown), and current-transformer secondaries 4b, one for each line 4 feeding the bus-bars. Where two or more sets of bus-bars are connected to a net and fault current may flow from one set to a fault by way of the other set, the fault must be cleared first by the switches on the supply side, and accordingly the reverse-current relays must have a time lag in operation. The fuses 9 may additionally be rupturable by operation of relays or devices responsive to internal faults or reverse-currents due to internal faults, in the transformers themselves, for example by operation of Buchholz relays, or by operation of the switches 8. Fig. 4 shows a form of fusible cut-out suitable for the elements 9. A conducting bridge 43 is soft-soldered at 44, 45 to the bent ends of conductors 41, 42 on which heating coils 46, 47 are wound in series such that when 46, 47 are energized, the solder meets and the bridge falls or is magnetically repelled. In a three-phase application, Fig. 7 (not shown), the free end of each coil 46 is connected to conductor 41 and the free end of each coil 47 to a contact, the latter contacts being all bridged, for example, by a reverse-current relay. Contacts in parallel therewith may be closable by a Buchholz relay or the latter may act on the same contacts as the reverse-current relay. Fig. 8 shows a modification with a heating coil 71 for igniting combustible material 69 which may either develop heat for melting the solder or create a pressure to break the soldered connection mechanically. The poles 62, 64 are concentric cylinders separated by insulation 65 and soldered to the annular flange 66 of the bridge 67. As alternative arrangements, fuses of low nominal rating may be arranged in parallel and a reverse-current operated switch connected in series with one of them, or such a switch may shunt a simple fuse, Figs. 5, 6 (not shown). Specification 12682/12, [Class 38 (v)], is referred to. The Specification as open to inspection under Sect. 91 described the following : a modification of Fig. 8 had the outer cylinder formed as a magnet coil and a central core provided. To prevent the fault-current in the sound feeders from tripping them out, each switch 5 had a release coil and release relay as in Fig. 3, the contact 14 being under the action of a heavy-current, short-time relay and of a thermal relay. The latter was energized by a transformer when a short-circuit was removed from the primary by operation of a further relay of lower current setting. The former relay would carry the heavy overload current of the faulty feeder and would disconnect it at once, whereas the sound feeders which would be carrying a relatively light overload, would trip out after a time delay due to the thermal device sufficient to allow the fuses in the faulty feeders to melt. This subject-matter does not appear in the Specification as accepted.