38 Chapter 2 • Cable joints and switchgear
Fig. 2.11 Termal magnetic type
Fig. 2.12 Magnetic type
Operation on short-circuit: Te magnetic flux is concentrated by the iron plate at the end of the bimetal strip. Te iron plate sometimes has a few turns of conductor around, as indicated in the sketch. In the event of a short-circuit, the magnetic flux becomes strong enough to attract the iron armature with enough force to activate the tripping mechanism, thus bypassing the time delay.
2.7.3 Magnetic type circuit-breaker
In this type of circuit-breaker, fig. 2.12, both overload and short-circuit protection is provided magnetically.
Operation on overload: When the current exceeds the rated value, the flux set up by the solenoid is strong enough to overcome the tension of the spring and draw the piston into the solenoid. Only when the piston is inside, or partly inside the solenoid, will the flux be strong enough to attract the armature with enough force to activate the tripping mechanism.
Time delay: Time delay is afforded hydraulically by the time taken for the oil to be forced past the piston as it is pulled inwards. Te speed at which it moves and the depth that it must be before tripping takes place is dependent on the magnitude of the overload current, i.e. it has an inverse time/current characteristic.
Operation on short-circuit: In the event of a short-circuit the flux set up by the solenoid is strong enough, regardless of the position of the piston, to attract the armature with enough force to activate the tripping mechanism instantaneously, thus bypassing the time delay.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134