Circuit Breaker Instantaneous Settings

[Humble2000]

I have a breaker with three instantaneous settings on it. I don't understand the concept behind this...
Why do I need to adjust all three settings for motor protection?
This is a thermal magnetic molded case circuit breaker.
I appreciate your responses.

http://www.sea.siemens.com/circuit/product/CircuitBreakers/Instruction Guide JD & LD Frame Rev D.pdf

 

[wbd]

The 3 settings are so you can adjust each phase independently. The instantaneous setting is for short circuit protection whereas the thermal is for overloading. This is where the thermal magnetic phrase comes from: thermal for long term overloading and magnetic for short term (short circuit). so, the instantaneous is protecting from short circuit of the cables feeding the motor. This should be set to allow the motor to start (above the starting current) but below the cable damage limits.

 

[jraef]

Please rephrase your question slightly. Are you asking "Why do I have adjustable magnetic trip settings?", or "Why are there separate setting for each phase?", or both questions?

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Humble2000]

Well, If the purpose oof this breaker is to protect the motor and the cable. why I have three seperate settings?
I mean only one setting would be enough.
What will be the application of setting those adjustments differently?
Thanks JRaef.Com I read the FAQ731-376

 

[davidbeach]

You have three settings because you have a cheap breaker. It costs more to link all three together so that you only have one setting to make.

 

[waross]

For motor faults, the breaker will most likely respond to the lowest setting or the second lowest setting.
If one pole is set too low, it may occasionally trip the breaker on the first quarter cycle inrush current, depending on the point in the cycle that that pole is energised.
respectfully

 

[Humble2000]

Thank you very much. I appreciate all responces.

 

[dpc]

Agree with David - the three separate adjustments are a manufacturing convenience - not to provide more adjustablity options.

 

[EddyWirbelstrom]

If the breaker is current limiting, then the Inst setting should be above motor inrush current which is approx 10xFLC. Allow for +/-20% tolerance gives an Inst setting of 12.5*FLC.

 

[victa1288]

For some high efficiency motors, the Inst setting should be above 20FLC in order not to cause nuisance tripping during starting. Asymetrical inrush current is of the concern for some fast reacting breakers.

 

[jraef]

When used as a main circuit breaker on a motor controller where there is also a single phase transformer connected to the load side of the breaker, it is sometimes necessary to have the 2 phases feeding the transformer adjusted for slightly higher instantaneous settings than the one feeding just the motor because of the aditional transformer inrush. 99.99999% of the time this is not an issue however because if it is just a Control Power Transformer, the size is usually so low that the inrush is relatively insignificant. But on one or two occasions I have had to tweak two of the phase trip settings in order to make the breaker hold in on startup.

JRaef.com
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