spring throws the starter jaw into its fully retracted position  where it remains until the next start. When the starter jaw first  engages the engine jaw, the motor armature has had time to  reach considerable speed because of its high starting torque.  The sudden engagement of the moving starter jaw with the  stationary engine jaw would develop forces sufficiently high  enough to severely damage the engine or the starter were it  not for the plates in the clutch pack that slip when the engine  torque exceeds the clutch-slipping torque. The engine selector switch must be positioned and the starter  switch and the safety switch—wired in series—must be  closed before the starter can be energized. Current is supplied  to the starter control circuit through a circuit breaker labeled  “Starter, Primer, and Induction Vibrator.”  [Figure 5-7]  When  the engine selector switch is in position for the engine start,  closing the starter energizes the starter relay located in the  engine nacelle area. Energizing the starter relay completes the  power circuit to the starter motor. The current necessary for  this heavy load is taken directly from the master bus through  the starter bus cable. All starting systems have operating time limits because of the  high energy used during cranking or rotation of the engine.  These limits are referred to as starter limits and must be  observed or overheating and damage of the starter occurs.  After energizing the starter for 1 minute, it should be allowed  to cool for at least 1 minute. After a second or subsequent  cranking period of 1 minute, it should cool for 5 minutes. In normal direct cranking action, the internal steel gear clutch  plates are held stationary by the friction of the bronze plates  with which they are interleaved. When the torque imposed by  the engine exceeds the clutch setting, however, the internal  gear clutch plates rotate against the clutch friction, allowing  the planet gears to rotate while the planetary carrying arm  and the jaw remain stationary. When the engine reaches the  speed that the starter is trying to achieve, the torque drops  off to a value less than the clutch setting, the internal gear  clutch plates are again held stationary, and the jaw rotates at  the speed that the motor is attempting to drive it. The starter  control switches are shown schematically in  Figure 5-7 .  5-6 Figure 5-7.  Starter control circuit. Off 1 2 3 4 Bus Starter bus Starter bus cable Master bus Ignition boost switch Primer switch To induction vibrator To primer  Starter relay To prop deicing relay To feathering pump relay Firewall junction box Engine selector switch Start switch safely switch Starter primer and induction vibrator Starter motor (engine No. 1)