# Breaker Iu vs In



## grover (Sep 27, 2007)

I'm used to seeing Iu or In, or the two identical, but I recently came across a molded case breaker that had an Iu=125A and In=100A, (adjustible from 75-100A). It's not a plug, it's built into the breaker and can't be changed beyond the pot. What, exactly, does this mean? As I understand it, Iu=125A means the contacts are rated for 125A... but with a 100A trip setting, why does that matter in the least? Why would they even bother labeling it? Am I missing something here?


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## Dark Knight (Sep 27, 2007)

grover said:


> I'm used to seeing Iu or In, or the two identical, but I recently came across a molded case breaker that had an Iu=125A and In=100A, (adjustible from 75-100A). It's not a plug, it's built into the breaker and can't be changed beyond the pot. What, exactly, does this mean? As I understand it, Iu=125A means the contacts are rated for 125A... but with a 100A trip setting, why does that matter in the least? Why would they even bother labeling it? Am I missing something here?


That is interesting. Who is the manufacturer of the breaker?


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## grover (Sep 27, 2007)

BringItOn said:


> That is interesting. Who is the manufacturer of the breaker?


It's a GE D125, 3-pole breaker. I couldn't find any literature on it; it doesn't appear to be that old, but it's hard to tell these things. There's nothing fancy about the application either, it's feeding an electrical panel with IT equipment.


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## Dark Knight (Sep 27, 2007)

Would it be possible that these are the normal and the continious duty operating points? I am shooting out of my hips so I am sorry if this looks like a stupid question.

Have to say I have never heard about these ratings Iu and In.

I am curious. Let me know when you find out.


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## Art (Sep 29, 2007)

iirc

Iu = ultimate or instantaneous (magnetic)

In = nominal or time dependent...if say set at 75, it will carry 75 forever, will trip at 80 in say x sec, 85 x/2, etc. (thermal)

so the circuit is sized for 125% of the load at max setting 100/125


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## grover (Oct 1, 2007)

[SIZE=10pt]I found these definitions in some random product specs, but it still doesn't really help clarify:[/SIZE]

Rated uninterrupted current Iu: it is the value of current which the device is able to carry for an indefinite time (weeks, months, or even years). This parameter is used to define the size of the circuit-breaker.

Rated current In: it is the value of current which characterizes the protection release installed on board of the circuit-breaker and determines, based on the settings available for the release, the protective characteristic of the circuit-breaker itself. Such current is often related to the rated current of the load protected by the circuit-breaker.


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