PLC Wiring standards

[clin]

Can anyone tell me if there is a standard laid down as to how interlocking is to be handled by PLCs? Interlocking between forward and reverse contactors for example.

 

[ctolbert]

clin:

I have never heard of any real written standard. However, I do latching circuits in plc's all the time. If you are speaking of ladder design; it is laid out just like a standard latching circuit in any motor control diagram or by a set or reset command (which is dictated by the type of plc instruction set available to you).

For contactors or the like, I always use a mechanical interlock. You have the command structure with a failure to open (I.E. stop) and the mechanical interlock to prevent simultaneous operation of both forward and reverse direction.

For process controls, I always use a watchdog timer to do a safty shutdown if there is a come loss or time out. A lot of the process safeguards are dictated by code (NFPA 70 for furnaces as an example.)

Can you elaborate more on what you are looking for?

Carl

 

[rajeshpalande]

hi i am Rajesh
regarding interlocking of forward and reverse logics in PLC one must take into account the signals from encoder or tacho fitted on to the motor. With the help of encoders/tacho/resolvers we can get the zero speed signal and after which PLC can activate the change direction output.
regards