Some contactors have auxilary NC contacts. If you're installing control centre for motor, you should have normally closed (NC) contact for protecting motor's power circuit from casual short circuit (one contactor should be switched off while other is working). 
<ul><li>But what to do, if your contactors have no auxilary contacts? You can solve this problem pending on your fantasy, if only your wiring wouldn't cause short circuit. But let's see main solvation of this problem: you push button SB1, the current begin to feed contactor KM1, putting the motor to spin to left; NC contact on SB2 is protecting circuit against short circuit. What will be, if we push SB2? Pushing SB2, it's NC contact will turn off the motor spin to left circuit and put current to gain through contactor KM2 putting the motor to spit to right.
<ul><li>Both buttons SB1 and SB2 have mechanic constraint between NO and NC contacts, which are protecting motor's circuit against casual short circuit.</li></ul></li></ul>
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