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rpsamel
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Normal MCCB vs. Current-Limiting MCCB

10/08/2008 2:31 AM

I would like to know what is the difference between normal MCCB & a current limiting MCCB?

Is every MCCB a current limiting MCCB or there is some special technology involved to make it currnt limiting?

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JRaef
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#1

Re: Normal MCCB vs. Current-Limiting MCCB

10/09/2008 12:34 AM

No, not every MCCB is Current Limiting (CL).

There are more than one type of CL MCCB. The main ones are fused and fuseless. In a fused MCCB, the case of the breaker has fuse holders and fuses are inserted that, although much higher than what the MBBC trip settings are set at, will act to limit the total amount of let-through current if it exceeeds the MCCB rating.

In a fuseless CL MCCB, the current power path inside the MCCB ismanipulated so that the magnetic forces that surround conductors are used to provide a mechanical advantage in the case of a high current fault so that the contacts of the breaker are opened faster than the simple mechanical springs can act. The current is fed through a horseshoe shaped set of copper solid conductors so that the magnetic fields surrounding them end up opposing each other. So the higher the fault current, the FASTER the contacts will be forced open, effectiviely limiting the peak value of the let-through current.

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rpsamel
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#2
In reply to #1

Re: Normal MCCB vs. Current-Limiting MCCB

10/09/2008 1:00 AM

Thanks JRaef for clarification.

Does the normal MCCB always breaks the current at the set value? Or there is some sort of current limiting in normal MCCB also?

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JRaef
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#3
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Re: Normal MCCB vs. Current-Limiting MCCB

10/09/2008 12:25 PM

All breakers trip based on current settings, within a tolerance. SO to that extent all circuit breakers are sort of "current limiting", but that is not what the terminology is used to describe.

"Current Limiting" is in reference to Fault Current, not normal operating current, and what the MCCB does in the case of a fault down stream of it . Available Fault Current is a calculation based upon the utility's generating capacity, the line impedance between that and the load, plus the impedance of any other transformers and current limiting devices in between. So for example, an installation near to a large sub-station may have an Available Fault Current of 65,000A. When a fault happens, ALL 65kA will attempt to flow to that fault, along with all of the magnetic forces that go along with it. If the fault happens down stream from your MCCB, the MCCB will be the first device in the circuit to attempt to stop that 65kA. That is why MCCBs have an "Interrupting Capacity" as part of their rating. If the breaker is rated for 65kAIC or higher, it can do the job.

But what if there is another smaller MCCB downstream of that one, rated for only 35kAIC? By the time the up stream MCCB clears the fault, the "Let Through" current may have reached 45kAIC and the smaller MCCB is destroyed, possibly with a lot of colateral damage and fire. With a current limiting breaker up stream, the available fault current as seen by the smaller MCCB may have been limited to 30kA and the smaller MCCB would have survived.

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SATURN
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#4
In reply to #1

Re: Normal MCCB vs. Current-Limiting MCCB

04/07/2014 10:17 PM

Hi ,

But to what level of prospective fault current does a current limiting MCCB limits the fault current ? Suppose if a 80KA SC current withstand fault limiting type MCCB is used , i would like to know this MCCB would limit the fault current to what level of fault current? are there any characterisitc curves available for this

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Von Marry
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#5
In reply to #4

Re: Normal MCCB vs. Current-Limiting MCCB

11/16/2015 4:22 AM

Hey,everyone.

I met a problem a SLD notices its incomer breaker is 200A MCCB which will met current limiting type..A current limting capability of a kind of ABB MCCB ,what does current limiting type mean?

I never figure it out ? I saw a Tmax specific let-through energe curve of T4 MCCB ,Did it told me the answer?

Look forward,Guys.
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PWSlack
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#7
In reply to #4

Re: Normal MCCB vs. Current-Limiting MCCB

03/04/2024 8:56 AM

The <...MCCB...> doesn't do that; the fault impedance does that. All the <...MCCB...> does is open after the time specified in its characteristic curve based upon the fault impedance.

Curves are published within British Standard 7671, among other places, to which every Qualified Electrician in the UK will have had more than a little exposure. Similar publications are applicable elsewhere in the world.

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Anonymous Poster #1
#6
In reply to #1

Re: Normal MCCB vs. Current-Limiting MCCB

12/11/2015 3:11 AM

I would like to know did the ABB Tmax MCCBs is all the current limiting type ?and who would like to offer the right answer for my question?

Actually ,I never give up to find this specifical parameter in its Tmax PDF.But nothing i found.Anyone offer me a help?

Thanks a lot for coming reply.

Marry

December 11,2015
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Anonymous Poster (1); JRaef (2); PWSlack (1); rpsamel (1); SATURN (1); Von Marry (1)

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