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Author Topic: Which Breaker will trip  (Read 1693 times)

Nitin Sidhu

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Which Breaker will trip
« on: February 16, 2015, 12:45:52 am »

Basics 101. But i could not find an answer on google, and care not to experiment.

Hypothetical senario :

So, main 3ph distro has a 4pole 63amp MCB+isolator. Neutral is bonded here.
Feeding 3 single phase 3pole 20amp breakers.
Each 20amp breaker branching into 4 x 5amp breakers.

I have a power drop connected to one of the 5amp breakers which goes faulty, the hot cable comes in contact with the chassis and creates a ground fault.

Will the entire circuit trip, or will only the 5amp breaker feeding this power drop trip ? Or will all 4 5amp breakers on the phase trip ?

I ask, cause the grounding conductor is bonded at the main distro, and hence the entire circuit should fault.

Thank you.
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Mike Sokol

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Re: Which Breaker will trip
« Reply #1 on: February 16, 2015, 12:51:32 am »

Basics 101. But i could not find an answer on google, and care not to experiment.

Hypothetical senario :

So, main 3ph distro has a 4pole 63amp MCB+isolator. Neutral is bonded here.
Feeding 3 single phase 3pole 20amp breakers.
Each 20amp breaker branching into 4 x 5amp breakers.

I have a power drop connected to one of the 5amp breakers which goes faulty, the hot cable comes in contact with the chassis and creates a ground fault.

Will the entire circuit trip, or will only the 5amp breaker feeding this power drop trip ? Or will all 4 5amp breakers on the phase trip ?

I ask, cause the grounding conductor is bonded at the main distro, and hence the entire circuit should fault.

Thank you.

To be sure I would have to draw out the entire distro schematic and trace the various current paths. But I'm pretty sure only the 5 amp breaker feeding that power drop would trip. The other 5 amp breakers are outside of this fault current path, so they won't see any abnormal current flow and won't trip.
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Lyle Williams

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Re: Which Breaker will trip
« Reply #2 on: February 16, 2015, 04:20:55 am »

Only the 5A.  Assuming that all breakers have similar trip curves.
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Steve Bradbury

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Re: Which Breaker will trip
« Reply #3 on: February 16, 2015, 12:06:31 pm »

The one which is most inconvenient to get at or located behind a locked door is bound to trip first.  :(

Lyles comment about trip curves is important, as is the actual load and how it is distributed between circuits.

This link shows some examples of load current v trip time for MCBs

http://www.protekuk.co.uk/10ka-mcbs.pdf
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Jonathan Johnson

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Re: Which Breaker will trip
« Reply #4 on: February 16, 2015, 12:48:15 pm »

The comment about trip curves is important.

In general, the 5A breaker should trip. However, it depends on how quickly the breakers react.

For example, if your 20A breaker is magnetic, but your 5A breaker is thermal, the 20A breaker may trip first, because it's based on instantaneous current flow, where the 50A thermal breaker has to take time to heat up. That's an overly simplified example; in reality it's generally not so clear. Many breakers can handle a slight overload for a short period of time, to allow for inrush current when connecting a load.
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Re: Which Breaker will trip
« Reply #4 on: February 16, 2015, 12:48:15 pm »


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