Three phase main breaker to feed single phase panel

The breaker needs to be rated for it. Some are, some are not. Some breakers will trip too quickly with one phase missing (differential trip feature), some may never trip because they need power to the sensing elements to function properly. The only way to tell would be to get all of the model information from the breaker and contact the manufacturer.

i have seen some technicians who make looping for two poles of the breaker (connect them in series inaway that the line current will pass thro' both of them) and use the third pole as neutral. i don't know whether this will boost up the tripping of the breaker in case of s/c or not?

Do not forget that a neutral must never be interrupted!

Genarally it should not be given to lighting DB.Some time there is possiblity for giving alternate voltage 400volt it will create proublem for equipments as well as technician.There is the rule for lighting distribution conversion from 3phase to 1phase. That is 3 pole as incommer, 2 pole as phase segregation, then single pole for circuit.This compartment named as 7 segment DB.This is for safty purposes.

I have seen this done in our Plant some years ago. If the breaker is already in place or mounted in the control panel, may even cost more to dismantle and reinstall a new 2 pole breaker. Just leave the 3rd line unused, preferably the center taps.

A 120/240 3 phase is a Delta system. This usually has the 'wild leg' or 'high leg' e.g. which is approximately 208 volts to ground. 'A' phase to ground is 120 volts to ground, 'B' leg is 'wild leg' usually marked with orange marking paint or tape, and 'C' phase which is also 120 volts to ground. Therefore, you should have a qualified electrician make any and all connections within a panel. You should never break the safety plane, that is the area within the panel enclosure, unless you are qualified to do so. Please be extremely careful if you must break the plane, secure the power and wear protective clothing and approved gloves and face shield. I like talking to you and I would hate to lose you.

If the 3 pole breaker has overload protection you will have to connect two of the poles in series and use the third pole for the neutral.

If you are not breaking neutral (Single pole + neutral) then you will need to wire all 3 poles in series and you will need to checkout any neutral associated with the 3 pole breaker is rated for the single phase load. Many 3 pole + neutral devices had a reduced rating on the neutral which will not be suitable for single phase.

Not enough details have been provided to provide a valid answer. The breaker doesn't care if current (at or less than rated value) is going thru three legs or two legs or unfortunately even just one leg as in a low level fault situation. Some responders are confusing current imbalance detection features of some motor overload units with panelboard circuit breakers which do not have that feature. IMPORTANT FIRST POINT (yes I am yelling) What is the service voltage that is feeding this 3 pole breaker? Is it actually 120/208V 4 wire? Is it 120/240V center tapped delta 4 wire? Is it standard 240 delta corner grounded or possibly ungrounded? Until this is known any comments are bordering on the dangerous side.