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Sinking and Sourcing are terms used to define the control of direct current flow in a load. A sinking digital I/O (input/output) provides a grounded connection to the load, whereas a sourcing digital I/O provides a voltage source to the load.
Two types of input cards exist, those that 'sink' current (also known as positive logic) and those that 'source' current (also known as negative logic).
Most common in Europe is the 'sinking' type of output, these will be used with the PNP transistor. Less common nowadays are output cards that 'source', these were popular in Asia and require the NPN type of transistor in order to operate correctly
Most common type of short circuit in the distributed wiring of large systems are ground faults.
Low side switching has the advantage that a fault just causes a load to stay on, nothing is destroyed.
If you short the output of a high side switch to ground, it often kills the high side driver pretty quickly.
Current limiting is possible, but it adds complexity and cost.