Shunt Trip with Ground fault sensor 3

[JJHorak]

Can anyone tell me how fast a shunt trip device opens a 480V breaker, typically anyway? I assume it varies with design. Do any designs reliably trip in < 5 cycles? Can anyone send a link to a shunt trip spec sheet showing trip times?

I have a collection of 480V MCCs fed by 480V power switchgear. The power switchgear feeders have ground fault trips. I am trying to determine if a ground fault sensor + a shunt trip scheme in the MCC buckets can operate fast enough to trip ahead of the ground fault trips in the power switchgear. The pwr swgr feeder grounds trip in the 0.15-0.25sec range, but could be slowed down if we wanted to accept loosing coordination with the bus ties.

 

[PHovnanian]

Timesabroom, post the 480V breaker model and part numbers.

It will also depend on the ground fault relays you select for this application.

 

[dpc]

I would expect the shunt trip solenoid to pull in within a cycle. Then the breaker still has to trip. This would be the maximum clearing time shown on the TCC for the instantaneous trip.

So adding two cycles to the max instantaneous clearing time should be conservative. I'm sure the breaker manufacturer can provide actual timing data.

For ground fault sensors, some breakers can be fitted with low energy shunt trip coils that may operate a little faster.

 

[rhatcher]

Timesabroom, I would think that the fastest time that you could expect a breaker to trip is the time delay setting of the relay controlling the shunt trip. Another way to say this is: For a ground fault sensor (relay) that is connected to a shunt trip device on a 480V molded case breaker, the relay setting is equal to the trip time. The time for the shunt trip to trip the breaker mechanism and for the contacts to open and interrupt current are included in that time.

Here is a link to a bulletin describing the Square D 'Ground Censor' ground fault system that is of similar type to what you will find available to install on your MCC. It shows time delays (trip times) of 0.1 seconds (6 cycles) and up.

Theoretically, individual MCC breakers will serve a much smaller load than a feeder breaker so the ground fault pickup current will also come into the picture when considering coordination.

http://ecatalog.squared.com/pubs/Ci...t Protection Equipment/Type GC/0931DB0101.pdf

 

[JJHorak]

Thanks for the responses.
I cannot give make/model; too many. The facility has ~7-800 loads being fed out of ~50 MCCs. There is no ground fault sensing in the feeders coming out of the MCCs, but there is ground fault sensing in the upstream power switchgear. A few times they have had ground faults in loads take out the whole MCC.

The comments and link told me a lot. Thanks. It gives me some feeling how fast a good shunt trip can be. It appears they can be faster than the upstream switchgear.

However, one more question: Some molded case breakers (the facility has lots of these) have external trip mechanisms that bolt onto the front of the breaker. A solenoid operated arm flips the switch on the front of the breaker. It is not clear to me how fast such devices are. Any hints?

 

[rhatcher]

Timesabroom, the external mechanism that bolts to the front of the breaker and 'flips the switch on the front of the breaker' is a motor operator. It is used to remotely open or close the breaker electrically instead of a person operating the breaker handle. This is for normal (no fault) remote open and close. The motor turns a screw (worm gear) that pushes the handle down to open or up to close the breaker. This may take a second or more, depending on the breaker.

The devices normally used to open the breaker in a fault (other than the trip unit) are the shunt trip or the undervoltage trip. This are internal to the breaker and operate by tripping the mechanism open, this giving a very quick operating time.

 

[ScottyUK]

Most of the bolt-on actuators are slow to open and fast to close. If you observe their operation they motor open and spring close - I believe this is to allow their use in synchronising operations.


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If we learn from our mistakes I'm getting a great education!

 

[JJHorak]

Thanks again to all. Now I can look half smart in front of my customer. I guess I owe all of you some of the $ they will pay me for how much I know.

 

[rhatcher]

Thanks Timesabroom, but we only gave you two cents worth. If you want to return them you can send one me in the US and one to ScottyUk in England.