Signal Placement

[PJW]

I want to know how to place the signal in layouts after decided the signal aspect (ex :position of first signal)

Signalling The Layout for an MAS solution broadly comprises:
a) Analysing the implications of the braking characteristics and headway specification,
b) Determining the most suitable form of signalling to utilise on each portion of the route
c) Deciding on options for placing certain key signals to protect junctions etc
d) Considering all the operational moves etc which are required
e) Juggling the signal positions to best fit the various constraints
f) Specifying the signal profile and adding route boxes to define every route
g) Defining the lie of points, adding any traps needed, numbering etc
h) Dividing up the train detection sections appropriately, identifying, depicting overlaps etc
i) Adding explanatory notes where necessary, particularly re method of working, interfaces etc.

You actually asked where to put the first signal, presumably as a basis to define the positions of all the others in that direction, determined by the signal spacing calculated earlier. This is rather over-simplistic (although it is indeed on the correct lines); it is more of a matter at looking where the critical areas are, tentatively sketching an option for that area and seeing what implications it has for elsewhere; then seeing whether another option taking something else as a fixed point would give a better solution overall.

So there is no 100% clear answer:
1. If there is a major station with pointwork immediately beyond the platform ends, then platform starters are essential and basically give you some fixed places around which you must develop the remainder of your solution.
2. Sometimes though it is less obvious; if there is a junction area then need to consider whether to place the point work
a) within the overlap length of a protecting signal or
b) beyond that length;
indeed will the junction protecting signal be close to it at all.

Basically you are trying to balance
a) Safety - protecting junctions etc
b) Operational flexibility to operate the train service pattern required
c) Economy- provide enough but not excessive quantities of signalling.

I've tried my best to explain, at least from a UK mainline perspective, in the Study Pack on the DVD that all applicant for the exam should be sent. There is a lot of it to read because I have tried to explain thoroughly. The above is the outline, but if you want more detail then I suggest you need to wade through the DVD.