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Author Topic: Why does the main distro trip, and not the "sub-distro"?  (Read 633 times)

Miguel Dahl

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Why does the main distro trip, and not the "sub-distro"?
« on: September 18, 2019, 12:55:19 pm »

I've experienced this sooo many times, that I finally have to ask someone.

A classic setup. 230V 3PH. One main distro being fed 63A, out of this comes a 32A which is feeding a smaller sub-distro. Someone forgets to weather-proof an extension cord or a SOCA or whatever, it starts to rain, and the next day at get-in there's no power.

Main 63A distro has tripped. Not the sub-distro. Sub-distro has GFCI on every outlet. Why?

(I believe it's called a GFCI (not very steady in english when it comes to this). Just a little 30mA "breaker" on every outlet which trips if you have a leak somewhere. Can press the button on every outlet on the sub-distro and the breaker trips, isn't this a GFCI?)

The experience I remember best was and american band coming to our festival (EU) so they had transformers with them for the pedal boards. Someone plugged in their pedal board and the whole audio-side of the stage lost power, even though the transformer was hooked up to a 6x16A sub-distro with (what I believe is called) GFCI's. We lost power at the main distro.
« Last Edit: September 18, 2019, 12:58:34 pm by Miguel Dahl »
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Nathan Riddle

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Re: Why does the main distro trip, and not the "sub-distro"?
« Reply #1 on: September 18, 2019, 01:00:12 pm »

I've experienced this sooo many times, that I finally have to ask someone.

A classic setup. 230V 3PH. One main distro being fed 63A, out of this comes a 32A which is feeding a smaller sub-distro. Someone forgets to weather-proof an extension cord or a SOCA or whatever, it starts to rain, and the next day at get-in there's no power.

Main 63A distro has tripped. Not the sub-distro. Sub-distro has GFCI on every outlet. Why?

(I believe it's called a GFCI (not very steady in english when it comes to this). Just a little 30mA "breaker" on every outlet which trips if you have a leak somewhere. Can press the button on every outlet on the sub-distro and the breaker trips, isn't this a GFCI?)


Why in your case? Not sure. But I'll continue to add 'noise' to the convo :)

Common household GFCI is max 5mA to protect life & limb. There are industrial ones rated for more.

I've seen 1200A 3PH main breakers trip due to a 60A 3PH AC breaker not tripping due to short. That is due to the 60A breaker going bad.

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Miguel Dahl

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Re: Why does the main distro trip, and not the "sub-distro"?
« Reply #2 on: September 18, 2019, 01:08:42 pm »


Why in your case? Not sure. But I'll continue to add 'noise' to the convo :)

Common household GFCI is max 5mA to protect life & limb. There are industrial ones rated for more.

I've seen 1200A 3PH main breakers trip due to a 60A 3PH AC breaker not tripping due to short. That is due to the 60A breaker going bad.

Maybe it wasn't a ground-fault, but just a straight up short. But also then why didn't the second distro trip..

Here it's normal with 30mA for a 16A in the equipment we use, and higher the more more A's you connect. For instance a 125A (or whatever it is) has 100 something mA. Can't remember the number.
« Last Edit: September 18, 2019, 01:11:40 pm by Miguel Dahl »
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Nathan Riddle

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Re: Why does the main distro trip, and not the "sub-distro"?
« Reply #3 on: September 18, 2019, 03:03:58 pm »

Maybe it wasn't a ground-fault, but just a straight up short. But also then why didn't the second distro trip..

Here it's normal with 30mA for a 16A in the equipment we use, and higher the more more A's you connect. For instance a 125A (or whatever it is) has 100 something mA. Can't remember the number.

GFCI = ground fault circuit interrupter. Your earlier description is correct.

I highly doubt ground-fault or arc-fault or leakage current due to water caused the main breaker to trip.


The experience I remember best was and american band coming to our festival (EU) so they had transformers with them for the pedal boards. Someone plugged in their pedal board and the whole audio-side of the stage lost power, even though the transformer was hooked up to a 6x16A sub-distro with (what I believe is called) GFCI's. We lost power at the main distro.

Also re-reading your edit with experience.
It sounds like something is miswired. I'd go through the entire power system with a fine-tooth comb or whoever is licensed to do so.
Make sure the connections (wire nuts, buss bars) are tight and not high resistance too.
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Stephen Swaffer

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Re: Why does the main distro trip, and not the "sub-distro"?
« Reply #4 on: September 18, 2019, 04:15:17 pm »

Every break/fuse/OCPD has a characteristic trip curve.  The most common breakers are inverse time delay-meaning they quick faster the larger the overload.  Some breakers are thermal-typically with a slower trip curve, some are magnetic which is typically your "fast" trip. 

Over current coordination is the science of sizing both the ratings and the trip curves so that the branch circuit (final OPCD) trips first, then feeders back to the source as needed.  Doing the math isn't always practical or possible on smaller systems.

My suspicion-your breaker for the main distro is a different brand/series breaker that has a little different trip curve than the sub-distro.  Not necessarily good or bad-often to protect solid state devices a faster trip is desirable-with dynamic loads a slower might be preferable to reduce nuisance tripping.

Curves should be available on manufacturers web sites.
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Miguel Dahl

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Re: Why does the main distro trip, and not the "sub-distro"?
« Reply #5 on: September 18, 2019, 05:32:50 pm »

Every break/fuse/OCPD has a characteristic trip curve.  The most common breakers are inverse time delay-meaning they quick faster the larger the overload.  Some breakers are thermal-typically with a slower trip curve, some are magnetic which is typically your "fast" trip. 

Over current coordination is the science of sizing both the ratings and the trip curves so that the branch circuit (final OPCD) trips first, then feeders back to the source as needed.  Doing the math isn't always practical or possible on smaller systems.

My suspicion-your breaker for the main distro is a different brand/series breaker that has a little different trip curve than the sub-distro.  Not necessarily good or bad-often to protect solid state devices a faster trip is desirable-with dynamic loads a slower might be preferable to reduce nuisance tripping.

Curves should be available on manufacturers web sites.

We use C. But C here and C there, it often trips at the first leg. This is what I don't understand. It may be that the feeder into the distro which isn't ours isn't C, but maybe A.  But i has till happened when we connect the feeder to our main distro. This is not just "one company's distro" but a general.
« Last Edit: September 18, 2019, 05:36:09 pm by Miguel Dahl »
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ProSoundWeb Community

Re: Why does the main distro trip, and not the "sub-distro"?
« Reply #5 on: September 18, 2019, 05:32:50 pm »


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