# Circuit Breaker Help



## JRaef (Mar 23, 2009)

Ratt1976 said:


> I have a NSX250F circuit breaker by Scneider Electric. The trip unit is magnetic only!
> 
> So I guess my question is does this circuit breaker provide overload protection or is it just short circuit protection? I look at the time vs I/Ir graph and it has a thermal withstand range. I am unable to find any information on thermal withstand. Is this the range at which the circuit breaker trips or breaks down?
> 
> Thanks


What is the application? Is it inside of a Square D motor starter built at the factory? Because that is the ONLY place that breaker can be used.


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## Big John (May 23, 2010)

The breaker does not have any overload protection. It's most likely a motor circuit protector and provides short-circuit protection in equipment where overload is handled by a separate device. 

"Thermal withstand" is just the temperature rise the breaker will tolerate during a severe fault and has nothing to do with protection ability.


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## Wirenuting (Sep 12, 2010)

JRaef said:


> What is the application? Is it inside of a Square D motor starter built at the factory? Because that is the ONLY place that breaker can be used.


Or an old Square D MCC.. 
I could use it.


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## Ratt1976 (Jan 31, 2013)

The breaker is upstream from a softstarter controlling a motor wired inside-delta


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## Ratt1976 (Jan 31, 2013)

Thanks, I think normally the breaker would be ok but since the softstarter and motor are wired inside-delta there is a possiblility of a high impedence short so I might consider getting a thermal-magnetic to replace the magnetic only.


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## JRaef (Mar 23, 2009)

Probably a good plan. If it doesn't have one, I would also make sure the breaker has a shut trip on it tied to a current sensing relaymwired so that if current flows in any phase when the soft starter is Off, it shunt trips the breaker, some soft starters come with that capability already, but you still need the ST in the breaker. The major down side to inside the delta connections is that you are one shorted SCR away from losing the motor if you have no way to detect the unrestricted current flow. The soft starter sellers usually avoid telling people that detail because they don't want to lose the sale.


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