Ans. The current coils are usually wound for 5 amperes and the pressure coils for 110 volts.
Fig. 2,301.—Alternating current low voltage circuit closing low voltage relay, for 600 volts or less. The contacts are similar to those of the circuit closing overload type except that they are inverted. As long as the pressure is normal the contact cone is held above the contacts. When the pressure falls below one half normal, the cone and plunger rod drop and close the contact. This relay does not pick up its own plunger. The plunger rod is pushed up by hand after the pressure circuit is established. Low voltage relays are generally used in connection with a low voltage release or shunt trip coil on an oil switch or a circuit breaker. They are used in connection with motor booster sets to prevent a disastrous speed of the booster which might result from the loss of alternating current power. They are also sometimes used for indicating purposes.
Ques. What refinement is made in the design of relays and why?
Ans. Care is exercised to reduce to a minimum the volt ampere load imposed by the relay on the current transformer to permit the use of un-stranded meters and relays upon the same transformer.
The use of circuit opening relays to cut out the trip coil of an oil switch during normal operation, has been described, and in the short time that the trip coil is in circuit, it does not affect the accuracy of the instrument readings. This practice, however, does not apply in the case of curve drawing meters, voltage compensators or other devices which have in themselves sufficient load for separate current transformers. In this connection it should be noted that to obtain accurate instrument and meter readings; the current transformers should not be loaded beyond certain limits which depend upon the volt ampere load and power factor of each of the connected devices.
Fig. 2,302.—Condit type K circuit breaker with shunt trip and no voltage attachment. The shunt trip is usually applied as an auxiliary to other types of trip. It consists of a fine wire coil which is mounted as a self-contained part of the breaker and which when energized, trips the circuit breaker. It is used to open the breaker from some distant point, and the coil is arranged to be connected across the line. The coils are so arranged that the circuit breakers will operate on a voltage 25% above or 25% below normal. The shunt trip coil is not intended to remain across the line and should be only momentarily energized. The no voltage trip, receives energy from a high resistance or fine wire coil which is arranged to be placed directly across the line, but in contradistinction to the shunt trip type, in which the coil is momentarily energized to trip the breaker, the no voltage coil is constantly energized and a decrease or failure of pressure trips the breaker. It can be used as a remote control device the same as the shunt trip. Its general use, however, is to cause the circuit breaker to open when the voltage of the line fails from any cause. Its use is recommended on all motor circuits, as it affords an additional protection against accidents, for if the voltage should fail, the breaker immediately opens, and before the machine can start again the attendant must close the breaker. It will not work for the protection of storage batteries or of motor generator sets charging storage batteries, as, when the voltage of the generator fails, the voltage of the battery still maintains its full value. The action of the coil is independent of the direction of flow of current; it simply allows the breaker to stay closed as long as the voltage is on the line and opens the breaker when the voltage on the line ceases. No voltage circuit breakers are normally so adjusted that they will not release until the voltage approaches 50% of normal.
So great is the variety of combination used and the variations of these factors in their several combinations at different loads and settings, that special consideration of each arrangement is advisable.