Patent GB-447979-A (GB)
Title: Improvements relating to leakage current actuated apparatus, particularly applicableto neutral conductors subject to voltage fluctuation
IPC: H02H3/16
Filed: 1934-11-28
Abstract: 447,979. Protective cut-out arrangements. BULLEN, A. G., 29, Newstead Road, Liverpool. Nov. 28, 1934. No. 34141. [Class 38 (v)] Electrically-tripped switches.-A leakage protective system has the operating- magnet winding centre-tapped to a point of the neutral conductor remote from the point at which the latter is permanently earthed and the ends of the winding are connected to earth electrodes which are separated from one another. One of the latter is connected to the casing of apparatus or a conduit system to be normally maintained at earth potential and from which leakage current is to be conducted by part of the winding to the neutral, whereby a p.d. between the latter and earth causes current to flow differentially through the halves of the winding. On breakdown between a live conductor and the casing or conduit, however, the leakage current predominates in one half of the winding. The consuming apparatus is supplied over the neutral and one phase line B from a remote transformer F having its star point earthed, and the apparatus casing and conduit S are earthed at D. The neutral is connected at X to a conductor connected to a mid-point tap of a magnet, the two coil ends of which are connected to the conduit S and an earth electrode D<1>, the two coils A, C being oppositely-acting on an armature 10, so that current flow due to the normal potential of the neutral above earth (as the result of unbalance in the transformer F) divides equally among the coils A, C and leaves the armature unaffected. On leakage between the live phase and the conduit or casing, the current mainly traverses the coil C to the neutral at X and, neglecting the small flow from S via D, D<1> to X, unbalances the magnet and causes 10 to be attracted. The contacts 3 controlling the supply, and contacts 5 in the coil circuit, are controlled by a member 21 a rounded part 19 of which is urged by spring or gravity against a roller 17a on a bar 17 on two knife-edges 18. The ends of the bar 17 are held respectively by springs 22 and a projection 16 on a spring-controlled lever 14 pivoted at 14a. The attraction of the armature 10 acting through arm 12 on lever 14 removes the restraint of the latter and allows 19 to slip under the roller 17a to open contacts 3, 5. The other end of 17 may be lifted by an overload coil 4 and plunger, or by a bimetal strip, or the normal restraint may be provided by a device as described in Specification 406,333, the arrangement providing protection against stray leakage currents. The invention is applicable also to D.C. systems having an earthed point but liable to excess p.d. to earth at another point. The Specification refers also to the use of a resistance or condenser in the circuit between X and the coil C for limiting the current. The Provisional Specification states also that the wax thermostat described in Specification 406,333 may be located in the iron of the leakage relay, so as to be heated by the heating of the coils or by eddy currents set up in the spring of the thermostat ; also that it may be located in a pocket in an unlaminated iron member in a gap in the magnetic circuit of the relay, so as to be heated by the heating effect of eddy currents in the unlaminated iron.