What is Trip circuit supervision relay(TCS,95)? Circuit diagram of TCS and working


You all know, how important a trip circuit is for a circuit breaker. if for instance, following problems will come in trip circuit, breaker will not trip when required or on fault.

  1. Faulty trip coil, generally a breaker has provided with two trip coils
  2. Failure of auxiliary supply to trip circuit, in most cases, it is D.C. supply provided by stationary battery.
  3. Fault in trip circuit i.e. open and short circuit
  4. Faulty breaker auxiliary contacts

in all these cases, breaker will not trip so it becomes necessary to continuously supervise the trip circuit of breaker.

Thank God, there is a relay for this . relay called TCS (Trip circuit supervision relay). Generally it is represented by number 95 which is ANSI code.

The relay supervise the trip circuit of the breaker and gives an alarm if any of the below abnormality found.

  1. Faulty trip coil, generally a breaker has provided with two trip coils
  2. Failure of auxiliary supply to trip circuit, in most cases, it is D.C. supply provided by stationary battery.
  3. Fault in trip circuit i.e. open and short circuit
  4. Faulty breaker auxiliary contacts

but what about the relay becomes defective ? Yes, the TCS relay gives alarm if it becomes defective or faulty.

below is the picture of a TCS relay(Make: ABB)

ABB TCS Relay

Wiring diagram/working of TCS relay

Before we understand how TCS relay works, we first have to understand the trip circuit of circuit breaker ..

Once we knew how a breaker trip circuit is ? Than we will be able to understand how TCS relay supervises the breaker trip circuit.

Below is trip Circuit of breaker.

Breaker trip circuit

Here what you see above, trip circuit has two parallel contacts i.e. TNC switch and Trip contact and than it is in series with a breaker auxiliary contact (52 a, Normally open when breaker is off) than there is trip coil of breaker.

Trip circuit is given supply 24 volt DC by stationary batteries.

TNC Switch mean Trip and close switch ..if you want to trip and close manually than you can do this by this switch.

Trip contact is nothing but the NO(Normally Open) contact of Master trip relay..Master trip relay is the realy which pick up when any of the protection relay get activated or pick up.

Under fault condition, when any of the protection relay becomes activated or pick up than master trip relay pick up, resulting in closing of trip contact( No becomes NC) and 52a contact is automatically closed when breaker is close. Than relay trip coil gets 24 volt DC and it pick up because both the trip contact and 52a is closed.

Than Trip coil trips the breaker.

Wiring diagram of TCS relay

Now you have seen above Trip Circuit of the breaker. Now see below how TCS relay is connected in the trip circuit of the breaker and supervises the trip circuit.

Wiring diagram of TCS relay

Above is the wiring diagram of TCS relay wired to trip circuit of the breaker.

you see, one block made by dotted lines in the above pic, one block is the the breaker ( shown 52 a and 52 b, breaker auxiliary contacts and a trip coil)

And another block made by solid line is ABB TCS relay.

Also in the TCS relay, you see there is a current injector that injects 2 to 3 mAmp in the trip circuit and a detector in the realy checks that small current flowing through the circuit or not?

Working of TCS relay

TCS relay continuously injects few mA ( 2 or 3 mA) in trip circuit and measures it. when trip circuit is healthy than current injected by relay flows through trip circuit and TCS gives healthy signal. during an abnormality, current does not flow and TCS gives alarm.

below is TCS relay when circuit breaker is off or in tripped condition. this circuit is known as pre-close supervision.

Path of the current injected by TCS relay (2mA) is shown.

Pre-close supervision

below is TCS relay when circuit breaker is ON or in closed condition. this circuit is known as post-close supervision.

Post-close supervision

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