Abstract:
A power breaker apparatus comprises a permanent fuse and resistors connected in parallel to the permanent fuse through a connecting conductor and is connected between a power source and a load to break a current supply to the load when a fault current is passed. At least external surfaces of the parallel resistors near the connecting parts of the connecting conductor are coated by an insulating coat.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a power breaker apparatus which quickly detects an abnormal current to break a power supply to a load when the abnormal current is fed by a short-circuit of the load. 
     As shown in FIG. 1, the conventional power breaker apparatus comprises a permanent fuse PF and resistors R connected in parallel to the fuse which are connected between a short time delaying main breaker B and a plurality of shunt breakers F 1  to F n . 
     The permanent fuse PF suddenly increases its resistance when fault current is passed. The relation of a resistance R PF  of the conventional permanent fuse PF and the parallel resistance R is R PF  &lt;R whereby the ratio is controlled to pass all of the current through the permanent fuse PF in the normal state. 
     For example, when a short-circuit is caused in the circuit of the shunt load F 1 , the fault current is passed through the permanent fuse PF to increase suddenly the inner resistance of the permanent fuse to be several hundreds times to the initial resistance whereby the fault current is reduced and the limiting current is fed through the parallel resistor R to the shunt breakers F 1  and accordingly, the shunt breaker F 1  is actuated. 
     FIG. 2 shows the current limiting condition of the fault current. When the fault current I pros  is fed, the inner resistance of the permanent fuse PF is suddenly increased to perform the current limiting operation to control the peak value of the current to I pfc . The resistance of the permanent fuse PF is greater than the parallel resistance R whereby the fault current I pfr  is continuously passed through the permanent fuse PF and the resistor R to operate the shunt breaker F 1 . 
     The conventional power breaker apparatus has the above-mentioned structure and the presumed short-circuit current maximum peak value I c  is limited to the current limiting maximum peak value I pfc  resulted by the permanent fuse PF. However, at the current limiting operation, there is a problem which causes an abnormal voltage between the parallel resistor R and the connecting conductor so as to damage the parallel resistors by arcing. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a power breaker apparatus which prevents arcing by coating at least both ends of the parallel resistors with an insulating coat so as to overcome the above-mentioned disadvantages. 
     The present invention is to provide a power breaker apparatus which comprises a permanent fuse and resistors connected in parallel to the permanent fuse through a connecting conductor, and which is connected between a power source and a load to break a current supply to the load when a fault current is passed, wherein at least external surfaces of the parallel resistors near the connecting part of the connecting conductor are coated by an insulating coat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of an electric system of a power breaker apparatus; 
     FIG. 2 is a graph of current limiting characteristics by a permanent fuse; 
     FIG. 3 is a schematic view of the power breaker apparatus; and 
     FIG. 4 is a partially broken view of a parallel resistor connected in the power breaker apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the power breaker apparatus of the present invention will be illustrated. 
     In FIG. 3, the reference numeral (1) designates a conductor in the power source side; (2) designates a connecting conductor; (3) designates heat radiation fins; (4) designates a current terminal for a permanent fuse; (5) designates a permanent fuse; (6) designates a current terminal of the permanent fuse; (7) designates heat radiation fins; (8) designates a connecting conductor; (9) designates a conductor in the load side and (10) designates a parallel resistor. 
     Referring to FIG. 4, the structure of the parallel resistor (10) is described in detail. 
     The reference numeral (11) designates a spacer which is preferably a copper spacer having suitable thickness or being formed by piling up suitable sheets of thin plates and which is disposed between the parallel resistor (10) and the conductor (1) in the power source side and between the parallel resistor (10) and the conductor (9) in the load side. The spacer is fastened by a clamp screw (15) and a nut (16). 
     An insulating tube (14) and an insulating washer (13) are disposed on the external part of the clamp screw (15) so as to give only the resistance of the parallel resistor (10) between the conductor (1) in the power source side and the conductor (9) in the load side. As the result, the clamp screw (15) and the nut (16) are completely insulated from the charging part. Moreover, both of the ends of the parallel resistor (10) are coated with an insulating coat (12) such as a silicone tape. 
     In said structure, the current supply in the normal state is fed from the conductor (1) in the power source side, through the connecting conductor (2), the permanent fuse current terminal (4) and the permanent fuse (5), the current terminal (6) in the reverse side, the connecting conductor (8) to the conductor (9) in the load side. 
     In the normal state, the resistance of the parallel resistor (10) is several hundreds times to the inner resistance of the permanent fuse (5) whereby the current is not passed through the parallel resistor (10). 
     If a short-circuit is caused in the shunt load, the short-circuit fault current I pros  shown in FIG. 2 is passed. The inner element of the permanent fuse PF causes phase conversion as solid-liquid-gas by the heating caused by the fault current whereby the current limiting operation is performed because of significant high resistance. 
     The current limiting peak value given by the permanent fuse is limited to I pfc  shown in FIG. 2. 
     Thus, the resistance of the permanent fuse (5) is higher than the resistance of the parallel resistor (10) whereby the fault current I pfc  is continuously passed through the parallel resistor (10) and the permanent fuse (5). 
     During the current limiting operation, overvoltage E fm  is generated. 
     
         E.sub.fm =I.sub.pfc ·R 
    
     wherein I pfc  designates the current limiting peak value and R designates the resistance of the parallel resistor. 
     The overvoltage E fm  is significantly increased when the short-circuit current is increased such as 200 KA, whereby there is a possibility arcing between the parallel resistor (10) and the connecting conductor (1) or the connecting conductor (9) or near the surfaces of ends of the parallel resistor (10). However, in accordance with the present invention, the arcing is not caused because the insulating coat is provided on the parallel resistor (10). 
     In said embodiment, the insulating coat (12) is provided only on both of the ends of the parallel resistor (10). Thus, it is possible to provide the insulating coating on all surface of the parallel resistor (10). 
     The connecting conductors (1), (9) are rigidly fixed in a box frame in which the power breaker apparatus is included. The angle of L-shape part is not always constant, because of processing errors and accordingly, there is a possibility that the distance between both of the conductors (1), (9) is not always constant and a space is formed between the conductor and the parallel resistor (10) or the conductor is contacted in slant with the end surface of the parallel resistor (10) to cause a point contacting condition instead of the surface contacting condition. 
     However, in accordance with the present invention, a conductive spacer (11) is provided whereby the parallel resistor (10) is always contacted with the connecting conductors (1), (9) in a surface contacting condition so as to prevent the arcing between them by forming a gap between them. 
     As described above, the parallel resistor of the power breaker apparatus of the present invention is improved to prevent the arcing whereby the power breaking can be perfectly attained to increase significantly the capacity for breaking, even though large short-circuit current such as 200 KA is passed.