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Operation of circuit breaker - does it require a pulsed or sustained trip signal?
4

Operation of circuit breaker - does it require a pulsed or sustained trip signal?

Operation of circuit breaker - does it require a pulsed or sustained trip signal?

(OP)
For a typical distribution level circuit breaker, how long does the trip signal from the associated protection relay have to be maintained before CB tripping is insured (i.e. the operation becomes irreversible)? Is a pulse trip signal all that's required or does it need to be maintained for a sustained period of time? Also, is this standard to all CBs or does it vary between models?

RE: Operation of circuit breaker - does it require a pulsed or sustained trip signal?

Assuming you are referring to an external trip signal from a protective relay, it would be considered a pulse, but the length of the pulse is going to depend on the type of trip coil you are interfacing with, often determined by the type of breaker we are talking about.

If it is a Molded Case or Insulated Case Circuit Breaker, the pulse will have to be long enough to energize the coil of a solenoid that is operating the trip mechanism, called the "shunt trip" coil. That length will vary, but the longer the better because in the shunt trip mechanism, the coil circuit passes through a Normally Closed set of auxiliary contacts that change state immediately when the mechanism moves. So even a sustained signal is automatically removed from the solenoid coil once the desired effect has taken place (this is done to prevent burning out the solenoid coil). Too short of a signal, i.e. milliseconds, and the coil may not fully develop the necessary flux to operate though. Only the breaker mfr will know the minimum signal duration to ensure that it operates, but if you give is a one second pulse, you will be fine no matter what.

If we are talking Power Breakers, i.e. VCBs or ACBs, the they often use an impulse trip circuit that is backed up by a charged capacitor to store the trip coil energy. So in that case they will have a much shorter pulse length requirement, i.e. 3ms minimum, but also a much shorted MAXIMUM signal length, because there is an electronic firing circuit facilitating the faster trip time, and that circuit can over heat. So I have seen typically a maximum signal duration of 1 second on those, maybe less (GEs ED trip coil is where I got those values by the way). YMMV

"Will work for (the memory of) salami"

RE: Operation of circuit breaker - does it require a pulsed or sustained trip signal?

(OP)
Thanks for the very comprehensive answer jraef. I have contacted the breaker manufacturer as you advised. If no answer is forthcoming I will assume a maximum duration of 1 second.

RE: Operation of circuit breaker - does it require a pulsed or sustained trip signal?

In general, protective relay contacts are not designed to interrupt the highly inductive trip coil current. The signal must be maintained until the auxiliary switch has interrupted the current. Safest bet is to maintain the signal until the condition causing the protective trip is removed. Too short a signal (or a bad auxiliary switch) will cause the relay contacts to fail.

RE: Operation of circuit breaker - does it require a pulsed or sustained trip signal?

I agree with the answers above, but I'd like to add an additional comment: the relay should be monitoring the breaker status via an auxiliary contact, so it should know when the breaker is no longer shut and end its command to trip.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
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RE: Operation of circuit breaker - does it require a pulsed or sustained trip signal?

The standard Schweitzer trip logic will assert the trip signal for at least nine cycles after the initiating signal (for instance an overcurrent element timed out - 51P1T) starts. I will stay asserted as long as the initiating signal is asserted. It will also latch in until an unlatch trip condition (ULTR) is true. ULTR is typically asserted when the relay pickup is not longer true (for instance NOT 51P1). ULTR sometimes is set to assert when the breaker 52a auxiliary switch opens.

The breaker local control circuit will have a 52a contact in series with the trip coil circuit to de-energize the trip coil when the breaker opens.

The short answer to your question is to maintain the trip signal for at least nine cycles to assure that the trip coil operates.

RE: Operation of circuit breaker - does it require a pulsed or sustained trip signal?

GE's UR series relays have monitored output contacts. They can be set up to monitor trip current. the normal mode of employment is for the contact to close on assertion of an element, either protection or control, and to latch it until the current drops. The standard electrical operator has an 'a' contact in series with the trip coil, so when the breaker opens, the contact opens, and current drops to zero, therefore the relay notes the current loss and opens the output contact.

In other applications, as noted above, output can be a one-shot with several cycles of duration to allow the breaker sufficient time to open.

old field guy

RE: Operation of circuit breaker - does it require a pulsed or sustained trip signal?

xnuke,
Why protective relay should be monitoring the breaker status?
protective relay will reset after breaker( main contact) open and primary current extinguish.

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