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.
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
TCM has been "traditionally" (a term meaning we didn't have any other choice back then ... 🤔) been provided by some special electromechanical relays, some extra resistors and the use of some of the circuit breaker auxiliary contacts 52a and 52b.
In the case of TCM schemes based on opto inputs in stead of the relay coils, 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.
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:
- loose wire terminations
- 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.
It is also important to understand why TCM has an operating delay of typically around 400 milliseconds, perhaps as low as 300 milliseconds.
Part of the reason is that there is always some delay of the 52a and 52 b contacts changing state and so you can have a transient state where the monitoring may consider the trip circuit is faulty. True, but hardly 400 milliseconds worth.
The more significant reason is quite simple.
One of the monitoring coils and associated resistor of the typical TCM circuit is connected effectively in parallel to all the protection trip contacts.
Those contacts will short that coil until the fault is cleared and the protection function resets. Hence there is potential for the TCM to operate for slow opening breakers or slow fault clearance and or protection reset overall.
Hence we have to give opportunity for the fault to be cleared and the protection reset.
In other words, the TCM monitors the circuit, but Circuit Breaker Fail detection is a different story and typically has a time delay of around 400 milliseconds to force the back-trip.
Since TCM is an alarm condition (and not generally a cause for immediate CB trip), we can avoid too many false alarms by holding off the TCM alarm for the period of CBF.
Given all that as the principles, and given a good TCM is being used, the detailed wiring diagram to the TCM is CRITCIAL to make sure that all possible tripping circuits and associated wiring are covered.
Implementing some form of TCM using "just any" device (relay or opto based) in the system may not give you the alarm you need and expect! .
Some vendor diagrams are wrong and dangerous when used with multiple tripping contacts in parallel ☹
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: