Instantaneous Relays.—The so called instantaneous relays operate almost instantly on the occurrence of the abnormal condition that they are to control.

There is of course a slight time element comparable with that of an overload circuit breaker, but for practical purposes, the operation may be considered as instantaneous.

Fig. 2,307.—Electric circuits of Condit type "A" relay. The construction is described in fig. 2,309. As here shown, the relay is not in operation, but should the current passing through the coil be of sufficient value to cause the lower movable half of the magnetic circuit to approach the upper stationary half of the circuit, the relay will be transformed from an ordinary electromagnet into a repulsion motor. The contact will short circuit the brushes of the armature and thus cause it to revolve, the speed of rotation being dependent on the amount of current flowing to a predetermined point, and thereafter the speed of rotation of the motor remains constant irrespective of the current value. Time adjustment: This is obtained by varying the distance through which the contact travels, provision being made whereby adjustment can be made as close as .1 of a second. Current adjustment: This is obtained by means of a calibrated spring. Standard relays are calibrated at 6, 8, 10, and 12 amperes, the coils being designed to carry five amperes continuously, with a temperature rise not exceeding 86° Fahr. Power to operate relay: The relay requires twenty volt amperes for its operation at full load; the influence of this type of relay on the ratio and phase angle of current transformers is small.

Time Limit Relays.—Under this classification there are two sub-divisions.

Fig. 2,308.—Characteristic curves of Condit type A selective relay. Curves 1, 2, 3, and 4 show the time variation of this relay with different settings at the various current values. The relay may be adjusted to trip the switch at any point represented between curves 1 and 4. This relay is a combination of an inverse time limit relay and a definite time limit relay. The combination of the characteristics of the two types are seen in the curve, the first part of which is inverse, and the latter part definite from a point of three or four times full load current. This combination of features being desirable as, for instance, in transmission work, particularly where it is necessary to use circuit breakers set selectively, as, due to the inverse feature of the curve, the relays can be set so that on a moderate overload, they will require the proper length of time to operate, and at the same time will operate quickly enough on heavy short circuits to prevent damage to the distribution system or its apparatus. Due to the definite feature of the latter part of the curve, the relays of the varying circuit breakers when once set to operate at different time values will never operate simultaneously irrespective of the value of the short circuit current, thus tending toward continuity of service.