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TCS, or rather TCM as referred to in IEEE C37.2, is a common requirement for increasing overall confidence in the reliability of a tripping circuit.

Wiring of the TCM is important There are "degrees" of trip circuit monitoring:

• tripping voltage is healthy
• trip wiring is operational whilst circuit breaker is closed
• trip wiring is healthy prior to closing
• combinations of the above

A faulty Trip Circuit is potentially a disaster waiting to happen .. hence it is important to have confidence that tripping will work BEFORE closing the circuit breaker AS WELL AS whilst it is closed.

The trip circuit can become faulty for a variety of reasons:

• rodent eating through wires
• trip supply failure
• circuit breaker trip coil open circuit
• faulty circuit breaker auxiliary contacts not changing to correct positions
• isolating links left open

None of these are self-evident until there is a real power system fault necessitating a trip , or at the next maintenance program.

If you close with it faulty, you can't trip when a power system fault actually occurs or even just as a controlled manual opening !
If it fails whilst the CB is closed, an alarm can be raised to prompt early intervention prior to the next power system event.
In either case, that may necessitate isolating the particular circuit breaker on both sides by opening multiple other circuit breakers until the problem is corrected.

It is to note that TCM does not monitor the operation of the protection device contact itself.  This can only be done with physical device testing.

However, the detailed wiring diagram to the TCM is CRITCIAL to make sure that all possible tripping circuits and associated wiring are covered. 

Applying TCM as part of "just any" IED in the system may not give you full coverage if there are other IEDs that may also be required to trip the CB.

In the case of TCM schemes based on opto inputs, the vendors usually have some example schemes in their instruction manuals which offer various features and advantages, but the overall principle is the same:

TCM uses a continuous, but small "trickle" current (well below the circuit breaker trip coil minimum operating current) passing trough the trip circuit wiring to and from the protection relays and through the trip coil to prove a continuous circuit..
Of course this also requires some "fail safe" considerations of a failure or problem in the TCM circuitry itself to prevent an inadvertent trip of the circuit breaker.

This PDF shows the correct wiring for multiple tripping IEDs and the positioning of the TCS function at the "bottom" of the chain of protection contacts.

A more extensive review of TCS is provided here:  

Extra Notes: