How fast can a shunt trip molded case circuit breaker act ?

[bdn2004]

A proposal from a vendor suggests CT's and a relay on the secondary side of a 750kVA transformer that is giving us a very high arc flash level. I'm assuming this work as follows... 1) sense the arc flash current (appx 8000A) - 2) shunt trip the circuit breaker.

Is this right?

 

[dpc]

Depends on the configuration. Tripping a primary breaker is preferable, since that will provide protection for the secondary side breaker. If you shunt trip a main breaker on the low side, incident energy at the main breaker will still be high since it can't fully protect itself. Also, for a typical low voltage switchboard, especially group-mounted, the main breaker is generally ignored for arc flash calculations at the switchboard due to concerns regarding arc-flash propagation. No harm in tripping both high side and low side.

Also, you'll need to find room to mount the CTs. Not always as simple as it seems.

Cheers,

Dave

 

[bdn2004]

That's a good idea - trip both breakers. And yes the proposal is to trip the primary breaker. And we the main is excluded in the arc flash for that panels.

This is a 1200A Eaton type NGH circuit breaker on the primary. If it could trip in it's instantaneous range it would trip at max 0.05 seconds according to the TCC.

Is that the trip time that can be used for the breaker opening time for the shunt trip ? )(+ CT error + relay sensing time for total time)

 

[jraef]

Yes, the trip time is the time it takes the trip function to ACT on a fault level event. You still have the opening time of the breaker.

The concept is being used a lot for Arc Flash Incident Energy mitigation now. The idea is that since you can't do anything about the opening time, you must set the Inst. Trip to a very low level so that the total clearing time is effectively reduced because you started the reaction time sooner. But there is a caveat; the level is so low that it will nuisance trip on a lot of every day tasks. So the way this is implemented is in what's being referred to as "Maintenance Mode", where a breaker has TWO settings for the Inst. Trip, one for normal operation and another for when someone is performing tasks that need a lower incident energy, the "maintenance mode" if you will. The MM setting is being triggered via an input to the breaker, for example a door switch, key switch or even a laser safety scanner near the floor that recognizes that someone has waled in front of the gear.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[dpc]

If the MCCB is tripping via the remote relay and shunt trip, the TCC for the MCCB isn't quite accurate, since it's based the breaker tripping via the internal trip coil. But times will be similar - it's fast. I'd consult with breaker manufacturer. It the breaker is tripping internally, then the TCC time should be accurate - .05 sec is reasonable.