270 Wenrich — A Study of the Time Interval in a 



The circuit of the tripping element is completed when the 

 armature is held down l>v the magnet and then only after the 

 tdock has unwound a predetermined amount. This contact, in 

 the tripping circuit, effected by the unwinding of the clock, is 

 made in many different forms. The magnet may be made to 

 operate for any definite current depending on the distance 

 between the poles and the armature. The time between the 

 release of the clock mechanism and the making of contact 

 in the tripping circuit is not determined or affected by any 

 particular armature setting, but rather by the amount of 

 unwinding. Here, then, we have no inverse time element. 



The amount of unwinding for any particular release may be 

 varied and easily adjusted for any specified time to trip. If 

 the relay is set to release the clock mechanism at 200 amperes 

 and the unwinding adjusted to make contact in 2 seconds after 

 the release, the circuit will open in 2 seconds provided the 

 line current remains at 200 or more for 2 seconds. 



If, however, the line current falls below 200 before the 2 

 seconds are up, the armature goes back and hence no contact 

 can be made to open the circuit. This is not a desirable relay. 

 While it has the important feature of operating on both D. C. 

 and A. C, it has no resetting element. After each time the 

 contact is made, the clock must be wound. It gives little 

 assurance of reliability in the clock mechanism stopping to 

 unwind, if, during the predetermined time for tripping, 

 the line current falls below the value necessary to effect a 

 release. 



Perhaps the most popular relay with an adjustable inverse 

 time element is the bellows type. Here the actuating mechan- 

 ism consists of an iron core in a solenoid compressing a bellows 

 in its upward movement. The bellows is attached to the upper 

 end of the iron core. There is an adjustable needle valve in 

 the bellows for controlling the escape of air. A retarding 

 force is furnished by the air pressure in the bellows. The 

 inverse time element is controlled by the adjustable valve. 

 The tripping circuit is completed by a ring on the iron core 

 engaging two contact pieces in its upward motion. The time 

 element adjustment is furnished by the distance between the 

 engaging ring on the iron core and the contact pieces. For over- 

 load adjustments suitable weights are placed on the engaging 

 ring. This is both a D. C. and A. C. relay. A variation from 

 this particular form is the substitution of a dashpot for the 

 air bellows. This shows that the ordinary plunger circuit 

 breaker may be modified by attaching to the iron core a bel- 

 lows or dashpot, thereby getting an inverse time element 

 attachment. The tripping circuit is generally separated and 

 energized by a storage battery. In this investigation, the aim 



