What if the energy flows in both directions, what is the end or the beginning?  How long is the cable in question?  AC or DC?

One possible advantage could be an overload protection at the end of the cable means it might be in close proximity to the device.  EG a lathe in a work shop with thermal and over current protection

psychicwarrior said:

what (if any) advantages are there for positioning overload protection at the end of a cable ?

Usually convenience - a couple of examples...

1. Industrial motorised equipment subject to occasional mechanical overload - it's common to put separate electrical overload protection next to the machine, so the operator can reset it as soon as the mechanical cause has been removed - no need to make the operator go to wherever the DB is (or give them access, where mistakes could take out other machines and affect production).

2. An unfused spur on a domestic ring final circuit - relying on the plug fuse at the far end to prevent overload removes the need to introduce an extra protective device at the start of the spur - which would often somewhere where it would be inconvenient to put (and/or maintain) a protective device .. e.g. a JB under the floor or the back of an existing socket where it wouldn't be aesthetically pleasing to have to stick in another device.

   - Andy.

Does positioning affect Ib <= In <= Iz for any part of the cable ...

psychicwarrior said:

Does positioning affect Ib <= In <= Iz for any part of the cable ...

as long as there are no branches before the OPD (i.e. so the OPD sees all the current flowing through the cable it's protecting) then you should be OK.

   - Andy.

so one can take the max. current capacity of the cable for the conditions  (so long as fault protection is met at the origin) upto the point of the overload protection...

in my earlier travels as newbie, there was an example that caught me out ... it was a 50A mcb protecting a run of 2.5T&E to a 1 gang  skt bs1363   ...

(similar to the spur example mentioned earlier)

all are effectively relying on overload protection at the 'far end' so to speak  (unless fuses count as fixed load scenarios )