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GMIEE (Electrical) (OP)
16 Oct 08 10:39
How does one determine the instantaneous trip setting of an MCCB? A software I am using wants me to input the instantaneous trip setting of an MCCB. There is no manual setting that I see though. Isn't this a factory determined level that should already be in the software?
dpc (Electrical)
16 Oct 08 11:39
Smaller MCCBs often have fixed instantaneous trips.  Larger breakers generally have adjustable instantaneous trips.  Determining the best setting is part of the overall coordination of the system.  

If you are trying to model an existing system, you may need to make some assumptions as to what the actual settings are.

For MCCBs on transformer primaries or used on motor protection, the instantaneous must be set high enough to avoid tripping on inrush current when the equipment is energized.    
Bilegan (Electrical)
16 Oct 08 13:45
You can determine the instantaneous trip setting of your MCCB if you know the full load current/rated current of your individual or total loads. Assuming, after having performed your calculation and considered the inrush current, safety factors, etc...and come out with a 220 amperes current of your static load, the 220 amperes is NOT the instantaneous trip setting of your MCCB.

To protect your static load you have to select your MCCB setting from NEC article 240, sub-section 240.6.(A), the instantaneous trip setting of your MCCB as per NEC should be 225A. Next, you have to select your MCCB ampere frame size as well from the catalogue of MCCB manufacturer, either select a 225AF/225AT or 250AF/225AT.

Check also the sub-sections 240.6.(B) & 240.6.(C) of the NEC.   


Murphy's Law - If anything can go will.

rcwilson (Electrical)
16 Oct 08 14:33
Bilegan - I think there is a misunderstanding about the standard amp rating of a MCCB and the instantaneous trip.  A 225 A rated circuit breaker will have an instantaneous trip unit that trips at 5-10 times its trip rating ( 1125A - 2250 Amps for a 225 A breaker.)  The 225 A MCCB can carry  225A without tripping.  (Of course code says to derate it to 80% of that for continuous loads.)

Some  MCCB's have fixed instantaneous settings, usually about 10 times their rating.  (The typical 20 A breaker trips on instantaneous around 200 A.) Large breakers with adjustable trip units may have an adjustable range of 2-10X.

GMIEE - if you can't find the data on the actual circuit breaker stat with a settign of 10X the amp rating.  Whether this a "good" guess (conservative in terms of safety for example)  depends on what analysis you are doing.

sibeen (Electrical)
16 Oct 08 19:59
As pointed out the smaller range of mccb have fixed instantaneous trips, but these also come in different flavours.

The 'standard' curve for mccb is the C curve. With this curve the instantaneous trip is set to approximately 7 to 10 times In (it may vary across manufacturers)

Other common curves are:

B curve - instantaneous trip is set to approximately 3 to 5 times In.

D curve - instantaneous trip is set to approximately 10 to 14 times In.

You need to check individual manufacturers spec sheets for the exact ratings of their offerings. Some manufacturers also have other curves for specialist applications.
dpc (Electrical)
17 Oct 08 10:42

This may be standard in IEC land, but in the US, MCCBs generally have one IT range, determined by UL standards.  Motor Circuit Protectors , which are only IT, do have a selection of ranges.   
Bilegan (Electrical)
17 Oct 08 11:43

rcwilson, yes there is, I misunderstood the question of GMIEE. I am thinking for the standard trip rating, not the instantaneous. Thank you for your correction.

Sorry for the lapse.


Murphy's Law - If anything can go will.

mayanees (Electrical)
17 Oct 08 16:45
This seems like a good time to make this post, since the topic is instantaneous trip settings...
My application is on a Medium Voltage system, say 15kV, and the development of the appropriate settings for relays that may be called on to energize groups of transformers.
The method is the same as the MCCB example: Determine the amount of expected inrush, and set the relay to allow that level, but not higher. The other factors to look at are: the available capacity - to make sure you don't set it so high that the system couldn't deliver enough power in a fault condition to reach your setting; and the upstream OCPD settings.

davidbeach (Electrical)
17 Oct 08 16:56

Quote (mayanees):

Determine the amount of expected inrush, and set the relay to allow that level, but not higher.
Boy, you must be good.winky smile

Most of us use a margin of error because we can't accurately predict the actual inrush current.

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