Using 1.5 sq mm Cable for 230V AC and 12V DC Together Over 100m—Is It Safe?

Dear sir
In a four cable 1.5 sq mm can we use two wires for 230 v single phase AC and other two wire for 12 v DC over a distance of 100 meters
Will there be any problem.

You don't specify a load power, check the voltage drop for the various voltages and loads against cross sectional area and distance.
My guess is that you will be in breach of safety regulations doing this, having a high voltage and extra low voltage together in the same cable would pose a definite safety issue.

Thank you sir,
Please go through the schematic AC siren may take not more than 2 amperes.The relay is a DC relay. Please reply. Will there be any technical problem other than safety issue.

Use the calculator here
http://www.calculator.net/voltage-drop-calculator.html
Note that they specify a one way voltage drop, so need to multiply result by two.
You will need the wire diameter, rather than cross sectional area,
cheers,
Richard

Dear Sir
what should be done to compensate the voltage drops.Is it enough to increase the source voltage.or amperage should be increased.

A heavier cable is the best way, if you cannot get lower current sirens/relays.
Pushing up the input voltage to get the desired operational voltage is not really practical.
If the voltage drop is less than 10%, then the devices will probably be OK, I can't say for sure since you have supplied no data. If the area where you intend to use the equipment suffers from low mains voltage frequently, then you may have used up any margins for the equipment. I suggest you firstly find out what the minimum operational requirements are for the devices used, and work from there.
Richard