Design And Construction Of Automatic Change-over For Three Phases

3.6     OPERATION OF TNMER RELAY
Timer relay are based on the electromagnetic induction principle which can only be applied where the actuating quantity or quantities A.C, they cannot be operated by D.C, since we are primarily concerned with an A.C system. This may be good place to restate a basic principle which should be kept in the back every operations mind.
Relay operate for the reason only
(i)    The AC quantity to which the relay respond has reached the setting for which the relay is calibrated
(ii)    The timer relay contactor are closed some mechanism mean. This could be when time of change from utility supply to generator is reach in the panel or by someone poking the contactor either accidentally of intentionally and sometimes removing or replacing a timer relay cover will cause a relay to operate.
3.7     PRINCIPLE OF OPERATION
The change-over panel consists of two (2) 100A circuit breaker which serves as a means of isolation and at the same time a protective device. The circuit breaker A is used to feed in the utility supply to the system while circuit breaker B is used to feed in the generator set supply to the system. Also in the panel, there are six (6) contactors, which are electromagnetic device that are design to carry current of difference capacities. Contactor A is load contactor that carries the utility supply to the load while B is equally a load contactor that carries the generator set supply to load. Contactor C is a control contractor which equally serve as relay the contractor is also been in operated with a relay timers monitor the function of the utility power contractor  or load contractor. The stop button is connected to supply from the red ohase through the normally close (NC) of the stop button to the contractor. Contractor C is equally meant to carry DC supply through the contractor for the purpose of supplying the contractor F.
Contractor D serves the same function as that of C but is only monitor the generator set load contractor with the help of the timer on it. The relay timer on contractor D helps in relaying the operation of contractor D helps in relaying the operation of contractor B so that the generator set will not be loaded before it maintain stability.
    Contractor E is mean to discharge the work of the kick starter on the generator set and to equally control work of operator D when the generator must have given out an output.
    Contractor F is a DC contractor of the 12v. it is used to start the kick starter of the generator set. It is connected through the
P= IL VcosѲ
But it is 3 Phase
... P=3L VcosÏ´
P= power supply in watts
Where
IL= load current in Ampere
V= supply voltage in volts
Cos = power factor (p.f) but p.f ... cosec 0.8
P = 50KVA
IL=?  
V = 415V (3phase)
... IL =
Since we cannot set material for exactly 86.9A, therefore we decide to choose for 100A and also for future extension.