Selective Tripping

higher current pickup. provided the fault is on the part of

the mechanism trip latch by moving against trip paddles

the line protected by the breaker having the lower setting.

fastened on the tip shaft. Looking at the breaker from the

Hence if a fault occurs in any part of the electrical

right, counterclockwise rotation of the trip shaft causes the

system, only the breaker nearest the fault will trip. In

breaker to trip. Clockwise rotation resets the mechanism

order to reduce the possibility of damaging the equipment

latch. The manual trip button and overload devices all

and to provide maximum safety to the operator, the

operate in this fashion to trip the breaker. The movement

overload caused d by a fault is removed in a minimum

of the striker arms of each of these, when activated,

amount of time by selective tripping.

should move from 1/32 to 1/ 16 inch beyond the point at

producing current up to five or ten times the breaker

which tripping occurs. This is what is meant by "positive

rating are removed in a matter of a few cycles less while

tripping."

currents in excess of this value are removed

instantaneously For the exact characteristics and setting

c. Selective Tripping. Selective tripping is obtained

each in a selective system m see coordination charts

when the breakers in the electric al distribution system are

figure 5-5 and 5-6 When malfunctioning of the power

arranged on the basis of a progressive series of time and

sensor trip is indicated the trouble should be traced to

current pickup. This will allow the breaker having the

one or more of the four components involve and that

shorter time setting and the lower pickup to trip before the

component should be replaced as a unit.

breaker having the longer time setting and the

5-9