Ques. Describe the time mechanism of a definite time limit relay.
Ans. It consists of an air dash pot, and an air diaphragm or equivalent retarding device connected to the contact mechanism.
Ques. How does it operate?
Ans. In some designs, when the contacts are released, they descend by gravity against the action of the retarding device thereby making contact a definite interval after the occurrence of the abnormal condition.
Fig. 2,309.—Condit type "A" selective relay, designed for use with circuit breakers where selective or discriminating action is required. The circuits and connections of this relay are illustrated in fig. 2,307, and its characteristics in fig. 2,308. In construction, the relay consists of a special motor with a short circuited armature and a split field. Under normal conditions, the fields are separated from each other and the motor armature does not revolve. The force tending to pull the two faces of the field together is opposed by a spring, the compression of which determines the number of amperes necessary to cause the relay to begin operation. The motor structure performs the whole work and the motor itself un-meshes and meshes the gears without the aid of any external device.
Ques. How does the inverse time limit type operate?
Ans. The actuating and contact mechanism is attached directly to an air bellows and in operation tends to compress the bellows against the action of a specially constructed escape valve in the latter.
