HV circuit breaker - Trip Circuit Supervision - operating principle question

[milosz125]

Hello All,

I need to work out trip circuit supervision circuit. I attached example for such circuit (ABB example).

My question is why the external resistor R (bypass resistor) is only considered in CB (circuit breaker) open condition (via 52b contact) ?

If CB is closed 52a contact is closed (52b is opened) and current flows via RXMB and via CB Trip Coil. Why in that configuration external resistor is not needed? Does the unnecessary CB operation in that configuration occurs? I think it could be

Please give me explanation.

Kind Regards
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[Julius Right]

In my opinion RXMB1 relay has to assure the trip circuit is in order only when the breaker is open.
There are situations when the trip circuit is out of order and if the breaker is closed on short-circuited equipment it will not open and could enhance the damage.
But in the breaker closed position the trip contact is closed and the RXMB1 relay coil is short-circuited since no need to control the trip circuit integrity [if it is interrupted the trip coil drops-out and breaker trips any way.]

 

[ATSman]

Here is how I see the sequence of operation:

Breaker Open:
RXMB1 is energized thru resistor R, contact 52/b and breaker trip coil. As long as there is continuity in the trip coil (trip circuit intact) the NC contact of RXMB1 indicates a no-alarm condition. Conversely, if the trip coil opens up, the RXMB1 relay drops out on time delay and indicates an alarm condition.
Breaker Closed:
RXMB1 remains energized thru contact 52/a and the breaker trip coil. Still a no-alarm condition.
Trip Condition:
If a fault occurs the trip contact from the protection relay closes to energize breaker trip coil thru contact 52/a and breaker trips. Trip coil de-energizes thru 52/a open contact. During the momentary trip time when RXMB1 coil is shorted, the time delay feature of RXMB1 prevents it from dropping out and indicating an alarm.

To avoid confusion, the trip contact is in the protection relay and independent of the rest of the control circuit. The value of resistance for R is chosen low enough to pick up relay RXMB1 but high enough not to pick up the trip coil and trip breaker.

Westinghouse provided a simpler version in their older LV air frame breakers (DB's, SPB's) by wiring the Open indicating light in series with the breaker shunt trip coil. With the breaker open, the lit Open light indicated that the shunt trip coil circuit was intact.

 

[milosz125]

Thank you for your replies.

But in the breaker closed position the trip contact is closed and the RXMB1 relay coil is short-circuited since no need to control the trip circuit integrity [if it is interrupted the trip coil drops-out and breaker trips any way.]

Maybe it is not clear enough for me but if CB is in closed position and trip circuit integrity is corrupted so trip coil drops - out. But is that lead to CB trip? I think to trip CB you need to excite trip coil to make CB trip, so when trip circuit is broken somewhere you cannot do that (to trip CB).

Here is how I see the sequence of operation:


To avoid confusion, the trip contact is in the protection relay and independent of the rest of the control circuit. The value of resistance for R is chosen low enough to pick up relay RXMB1 but high enough not to pick up the trip coil and trip breaker.

Yes to value of resistance R is due to conditions you mentioned above. But why you do not need resistance R in CB closed state (contact 52a closed). I think in that state trip coil could be undesirably picked up as well.

It is just not clear for me why these two states of CB (CB opened , CB closed) are not equal from TCS (Trip Circuit Supervision) point of view: I mean why resistor R is not utilized in both CB states?

Greets