What is the meaning of NO, NC and COM on the panel boards?

NO - Normally Open, NC - Normally Close, Com - Common

No = Normal open , it's mean magnatic locK normal open

NC = Normal clos ,it's mean magnatic lock normal close

COM = Comman , it's mean ground connection

NC - NORMALLY CLOSED

NO - NORMALLY OPENED

COMM - COMMON

NC- normally close NO- Normally open C- Common

NO or NC connections are given according to the requirement of end user.

If the system which we are connecting the relay module function normally on closed circuit connection and stops when the circuit is open we use NC connection. If there is any trigger to relay module the connection become closed and the system shut down.

Will control your relay output throw it

NO: Normally open it means the circuit is open until you activate the relay to close it for a specified time then it will back to Normally Open.

NC: Normally Close it means the circuit is close until you activate the relay to open it fro a specified time then it will back to Normally Close.  

COM: Common is used in both situation NO,NC

NO - Normally open (In this case the device is is normally open condition when it is in idle state. and as soon as there is trigger. The circuit closes and and intrusion signal is sent.) This technique cannot handle for cable cut situation. 

NC - Normally Closed (In this case the device is is normally closed condition when it is in idle state. and as soon as there is trigger. The circuit opens and and intrusion signal is sent.). This technique is useful for the situations where the system need alert for cable cut or broken.

Com: Common and it used in both the above mentioned situation. because there is normally one common for for 2-4 zones. and each zone is assigned to separate device.