I have a input module come with a DDC. I found that the module would be no response sometime. When I measured the wire of some Dry contact point, I found there is 15V AC. I don't know where induced this voltage, but I think my input module is sensitive to it. Do anyone here can advise some methods to solve the problem? Thank you.

 

We have solved this problem by installing a pull-down resistor from the input terminal to ground. You can determine the value something like this:
24V / .016A = 1,500 ohms
24V x .016A x 2 = .769W

Install a 1,500 ohm 1 watt resistor to each input point you want protected in a 24 volt system. You can use the same method for other voltages:
120V / .016A = 7,500 ohms
120V x .016A x 2 = 3.84W

For a 120 volt system install 7,500 ohm 5 watt resistors. This has solved many noise problems and it doesn't matter if the noise is caused by induction, electrostatic (capacitive)or leakage from triacs or transistors.

Good luck.