Patent GB-629736-A (GB)
Title: Improvements in or relating to temperature-responsive devices for the protection of polyphase electrical systems
IPC: H01H71/16
Filed: 1947-11-13
Abstract: 629,736. Protective cut-out arrangements; thermal switches. REYROLLE & CO., Ltd., A, YOUNG, A. F. B., GEESON, C.T., and GRAY, W. Nov. 13, 1947, No. 30211. [Class 38(v)] An arrangement for the over-current and phase-unbalance protection of polyphase systems, utlizes a thermal relay having a number of bimetallic elements whose temperatures vary in accordance with the currents in different phases or conductors. In Fig. 1, three bimetallic elements A-C are heated by coils A<SP>1</SP>-C<SP>1</SP> energized in accordance with the currents flowing in the three phases of the supply. The three elements are fixed at one end to a coupling member or shaft D, which is free to rotate, and at their other ends elements A, C carry contacts A<SP>2</SP>, C<SP>2</SP>, whilst the element B has its other end located between rollers B<SP>2</SP>, B<SP>3</SP>, which prevent any lateral movement of this end. When the three elements are heated to the same extent, the bending of the elements A, C, which tends to close either the contacts A<SP>2</SP>, A<SP>3</SP> and C<SP>2</SP>, C<SP>3</SP> or contacts A<SP>2</SP>, A<SP>4</SP> and C<SP>2</SP>, C<SP>4</SP>, is counterbalanced by the bending of the element B which turns the shaft D so that the resultant movement of the contacts A<SP>2</SP>, C<SP>2</SP>, is zero. In the event of unbalanced conditions, for example over-current in the phase in which coil A<SP>1</SP> is connected, the movement of the shaft D is not sufficient to cancel the movement of the free end of the strip A so that contacts A<SP>2</SP>, A<SP>3</SP> or A<SP>2</SP>, A<SP>4</SP> close to energize the trip circuit of a circuit-breaker. Protection against balanced over-currents is provided by an arm F having a contact F<SP>1</SP> which co-operates with fixed contacts F<SP>2</SP> to complete the trip circuit when the shaft D has been turned sufficiently by the element B due to the excess current in coil B<SP>1</SP>. The arm F may be bimetallic so as to compensate for changes in ambient temperature which would otherwise affect the over-current protection. As the elements A-C are similar, automatic compensation for changes in ambient temperature is obtained for the phase-unbalance protection, The coils A<SP>1</SP>- C<SP>1</SP> may be associated with an iron core and a contact-carrying armature so that instantaneous protection may be obtained electromagnetically under heavy short-circuit conditions. In a modification, Fig. 6, two bimetallic coils H<SP>1</SP>, H<SP>2</SP> are connected at their outer ends by a common coupling member H<SP>5</SP> and cups H<SP>7</SP> H<SP>8</SP>. The inner end of coil H' is attached to a fixed abutment L, whilst the inner end of coil H<SP>2</SP> is attached to a rotatable shaft J having a contact-carrying arm J'. When the coils H', H<SP>2</SP> are heated to the same extent no movement of the shaft J occurs, but when heated to different temperatures the shaft turns and contacts J<SP>2</SP>, J<SP>3</SP> or J<SP>2</SP>, J' are closed. An arm H<SP>6</SP> attached to the cup H<SP>7</SP>, cooperates with contacts L<SP>2</SP> to afford overcurrent protection, ambient temperature compensation being provided by a bimetallic coil L<SP>1</SP>. If this latter arrangement is used to protect a three-phase system the heating elements H<SP>3</SP>, H' for the coils H<SP>1</SP>, H<SP>2</SP> are supplied from current transformers X<SP>1</SP>, Y<SP>1</SP> associated with two of the three phases. In this case the heater H<SP>3</SP> is energized by a current proportional to the vector sum of the currents in the two phases and heater H<SP>4</SP> is energized, in effect, by currents proportional to the arithmetic sum of the currents in the two phases.