in fire and gas system what is the meaning of....

Line monitoring is detection of a break or a short.

EDIT: OK actually four states:
* Open contact
* Closed contact
* Open wiring
* Shorted wiring

I've seen this is done with a two-resistor network, one in parallel with the contact and on in series with the contact. The input itself is really an analog input that detects the four different voltage states via the resistor network.

Hope the nickel-tour explanation makes some sense.

so it is like fail safe i/o in the plc. right????
is there any standard like SIL??

Exactly.

As far as standards, I couldn't say for sure. That gets a little far afield for the work I do. There are, however, a lot of brilliant people on here who I'm sure will jump in later and help us out.