main breaker tripping before branch circuit

[supergeek]

... it might appear that adequate "energized work" procedures were not being completely followed.

Any comments or "lessons learned" you'd like to share?

"Procedures.....?"

Actually, in hindsight, it was only a #12 wire, but the end had about 1/2" of copper showing. Then Murphy's Law kicked in and that little end wedged itself between the enclosure and C phase. For some reason, it seemed like pulling the wire out really fast would eliminate any danger. Yeah, it wasn't exactly an OSHA-approved technique or anything. So next time, cutting any bare copper off the wire and taping the ends will be in order. And I thank everyone who took the time to read and respond to my breaker question as well!

 

[templdl]

All breakers in series with that fault will see the fault. If the available fault current were 10ka and there was a bolted fault which would allow the entire 10ka to be seen be the breakers. Then the issue is what would the magnetic calibration of those breakers be and would that affect any reasonable coordination.
The residential branch breakers that I'm familiar with my be calibrated in the neighborhood of 6-7x the breaker rating at best. Please note that there is a calibration allowance when one looks at a trip curve that allows the breaker to be calibrated much higher. The main breaker most likely is calibrated at 10x its rating +-20%. It has also been my experience that the average breaker will be calibrated of the high side, which yields fewer rejects in the manufacturing testing process.
With that in mind a 100a main breaker would have a mag. cal. of 1000a. Now apply a bolted fault in one of the branch circuits which may cause up to 10ka of fault current to flow instantaneously and seen by the branch circuit and main breaker. Because the current rises instantaneously there is little chance for a branch breaker with a lower magnetic calibration to react quick enough to clear the fault before the main trips. This is not popular but never the less a very common occurrence.
With smaller distribution systems is impractical to design breaker that will coordinate under such circumstances.
With larger breakers the have more withstand very short times delays are possible in electronic trip breakers that create a very brief delay to allow a down stream breaker to clear the fault. There are also electronic trip breakers that have the ability to communicate. The down stream breaker can tell the upstream breaker that it sees the fault and will trip.
But remember, the kaic rating of a breaker has nothing to do with the neither magnetic trip neither calibration nor coordination. It only has to do with the breaker's ability to clear the fault and still pass the applicable UL test requirements

 

[supergeek]

main breaker tripping before branch circuit

Anyone else ever have problems with main breakers that trip, even though the problem is clearly with the branch circuits? Most recently for me, some wires were being pulled out of a sub-panel when something arced from c phase to ground (the bare end of one of the wires being pulled out.) The arc happened on the line lug ahead of this panel's main. Instead of the incident tripping the 3 phase 125 amp breaker feeding this panel, the 800 amp main breaker of the I-line panel tripped. In other words, this panel was being fed from an I-line section. Instead of just tripping the branch circuit in the I-line, it took out the I-line's main breaker. Any thoughts on this would be much appreciated!

 

[cpal]

Now would be a great time for you to web search for information on Series Rated OCPD (not that this is the problem)

The sequence in which OCP clears in a system is not always according to Hoyle.

Up stream devices (generally ) are capable of clearing higher fault levels (kA +) The fault that was presented most likely was at the maximum clear rating or above for the branch braker and the upstream device say fault within it's pick up window.

This happens more often then you may expect.

Keep in mind that my explanation is generic and incomplete , but the subject is understandable with some moderate reading.Try
http://www.bussmann.com/apen/topics/selcoord/
I know it is a fuse mfg but the info is good


Charlie

 

[NoVA Comms Power]

... Any thoughts on this would be much appreciated ...

While I think you already have your answer on breaker tripping ... I'm curious ...

... it might appear that adequate "energized work" procedures were not being completely followed.

Any comments or "lessons learned" you'd like to share?

 

[jim dungar]

This has nothing to do with short circuit interupting ability. An AIC rating only deals with preventing the breaker from failing violently or becoming shrapnel. The difference between AIC ratings of the same family of breakers is often nothing more than more glue and stronger rivets (i.e Square D FA/FH).

The problem seen by the OP is caused by a lack of coordination. Short circuits typically cause circuit breakers to operate in their instantaneous (magnetic trip) region.

If the upstream break has a lower trip setting than the down stream break it will trip first. The solution is to adjust the upstream breaker using settings chosen by a qualified individual.

However, if the fault was sufficient to cause the upstream breaker to enter it's instantaneous region then it will often (but not always)unlatch/trip before the downstream device.

 

[kingpb]

While I think you already have your answer on breaker tripping ... I'm curious ...

... it might appear that adequate "energized work" procedures were not being completely followed.

Any comments or "lessons learned" you'd like to share?

I agree, this topic should be moved to "Safety"