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