what is the difference between Ith & Iu rating in MCCB. Particularly on SA

what is the difference between Ith & Iu rating in MCCB.
Particularly on SACE ABB Circuit Breaker?

Question Posted / dalton

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what is the difference between Ith & Iu rating in MCCB. Particularly on SACE ABB Circuit Break..

Answer / mazed

Extract form appendix K of standard IEC 60947 -2
Voltage
• Ue = Rated operational voltage
• Ui = Rated insulation voltage
• Uimp = Rated impulse withstand voltage
Current
• In = Rated current
• Ith = Conventional free –air thermal current
• Ithe = Conventional enclosed thermal current
Short -Circuit
• Icm = Rated short circuit making capacity
• Icu = Rated ultimate short circuit breaking capacity
• Ics = Rated service short circuit breaking capacity
• Icw = Rated short time withstand current
Trip unit
• Ir = Adjustable overload setting current
• 1.05xIr = Conventional non –tripping current
• 1.30xIr = Conventional tripping current
• Isd = Short time delay tripping setting current
• Ii = Instantaneous tripping setting current

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what is the difference between Ith & Iu rating in MCCB. Particularly on SACE ABB Circuit Break..

Answer / mazed

CCB Ith & Ir Acronym Meaning
05/02/2010 9:16 AM

We are planning to replace our Chiller with bigger capacity.

The motor rating of old chiller is 375 kw whhle the new one
is 570 kw, all at 380 volts, 3phase , 60 hz. The suppllier
required us to a circuit breaker with minimum ampacity of
911 amps.

our existing Circuit Breaker is SACE ABB, Iu =1250, Ue =660
Volts, 3 poles. Their is a rating of Ith @ 45 deg C of 800
Amps maximum and Ir = 6300 max. My questions are:1. What is
the Ith & Ir means. 2. Can i use ourold MCCB with the rating
of 1250 Amps Iu?

Please all extert here help me.

My best regards to all.
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* #4 "Re: What is the Ith & Ir at MCCB stand for?" by
JRaef on 05/02/2010 7:01 PM (score 5)

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#1
[Comment] Re: What is the Ith & Ir at MCCB stand for?
05/02/2010 2:42 PM

Ith is the thermal current rating of the breaker. The more
the motor current exceeds this, the quicker the breaker will
trip. I'm unsure of the other abbreviations, but they may
represent the magnetic trip rating, which is nominally
instantaneous.

If the motor requires 911 A, it is likely to trip this
breaker either 1) right away if the starting current exceeds
6300 A, or 2) in a short time if the running current exceeds
800 A. So I don't think this combination will work.
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Dalton
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#2
In reply to #1
[Comment] Re: What is the Ith & Ir at MCCB stand for?
05/02/2010 2:59 PM

Thanks, but how about the Iu (1250 Amps) which i believed is
the nominal current rating. Our MCCB rating Iu = 1250 Amps &
Ue is 660 volts. then their is anothr rating which is Ith =
800 max @ 45 Deg C. any idea on this Sir?
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#3
In reply to #2
[Comment] Re: What is the Ith & Ir at MCCB stand for?
05/02/2010 4:15 PM

My idea on that is exactly as before. If the thermal rating
Ith of 800 A is exceeded by the 911 A motor, the breaker
will trip (sooner or later).

However, if the ambient temperature at the breaker is less
than 45°C, the Ith will be somewhat higher. This may get you
by, but it looks really borderline. Reduced-voltage and
unloaded starting will also help.
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Anonymous Poster
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#5
In reply to #2
[Comment] Re: What is the Ith & Ir at MCCB stand for?
05/03/2010 5:16 AM

ASK ABB for their catalogue on MCCBs select yourself you
will find all inputs required you could download the
catalogue from ABB web site.

good luck

murali

Ex power user
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Dalton
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#6
In reply to #5
[Comment] Re: What is the Ith & Ir at MCCB stand for?
05/03/2010 8:46 AM

thanks. this an old model MCCB from ABB. SACE SN1250 model.
Installed last 1986 by the German Contractors here in KSA.

All I want to know is if could still use this MCCB. But i
think the best solution now is to buy a new MCCB.

Rgards to all.
Reply
[Rated a 'Good Answer' with a score of 5] 5
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#4
[Comment] Re: What is the Ith & Ir at MCCB stand for?
05/02/2010 7:01 PM

Extract form appendix K of standard IEC 60947 -2

Voltage

* Ue = Rated operational voltage
* Ui = Rated insulation voltage
* Uimp = Rated impulse withstand voltage

Current

* In = Rated current
* Ith = Conventional free –air thermal current
* Ithe = Conventional enclosed thermal current

Short -Circuit

* Icm = Rated short circuit making capacity
* Icu = Rated ultimate short circuit breaking capacity
* Ics = Rated service short circuit breaking capacity
* Icw = Rated short time withstand current

Trip unit

* Ir = Adjustable overload setting current
* 1.05xIr = Conventional non –tripping current
* 1.30xIr = Conventional tripping current
* Isd = Short time delay tripping setting current
* Ii = Instantaneous tripping setting current

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