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Topic Title: Ics Vs Icu in MCCBs, ACBs
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Created On: 09 March 2019 04:49 pm
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 09 March 2019 04:49 pm
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Selk

Posts: 29
Joined: 21 April 2018

What is the significance of Icu rating in IEC breakers? From what I have read,

Icu - Cannot be to put back into service after interrupting the fault at the rated Icu Short circuit current
Ics - Can be able to put back into service after interrupting the fault at the rated Ics Short circuit current.

Since If I use the Ics rating, even after breaking the fault current, the breaker can be in service again. then what is the need of Icu rating? However if I use the Icu rating for selecting the breaker and after breaking the fault current, it can not be in service again, I may use fuse here. I cannot understand the use of Icu?

How Icu and Ics differs for different voltages?
 09 March 2019 06:13 pm
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

Since If I use the Ics rating, even after breaking the fault current, the breaker can be in service again. then what is the need of Icu rating? However if I use the Icu rating for selecting the breaker and after breaking the fault current, it can not be in service again, I may use fuse here. I cannot understand the use of Icu?


Say a breaker had an Ics rating of say 10kA and a Icu rating of 16kA and you installed it to protect a circuit that has a PFC of say 10.5kA at the DB and 8kA at the far end of the circuit. Only faults on the first short section of cable would exceed the Ics rating - all faults further downstream would be below Ics - so leave the breaker serviceable. If the cable route was such that faults were unlikely over the first section (because it was in a protected environment say) you might choose to take the risk of a damaged breaker in the very unlikely event of the fault on the first section, thus saving the cost of a more expensive (or possibly not available) breaker. Either way you have the reassurance that the fault will be safely cleared (PFC <= Icu).

Yes, fuses can be a feasible alternative in many cases (and often used to backup circuit breakers as fuses often have much higher breaking capacities) and often lower initial cost.

- Andy.
 09 March 2019 06:31 pm
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broadgage

Posts: 3170
Joined: 07 August 2007

Consider also installations with two LV transformers each feeding one end of a busbar, with the bus coupler normally open.

An MCCB would normally only to have to break the fault current from a single transformer.
Under exceptional or abnormal operating conditions with the bus coupler closed the MCCB might have to break the combined fault current of both transformers.

Having to replace the MCCB after such a rare event that might never occur, is probably fine.

Likewise, consider a large installation with a grid supply and a large standby generating set.
Not NORMALLY paralleled, but might be BRIEFLY used thus to avoid loss of supply when changing from one supply to the other.
Having to replace the MCCB after a short circuit that occurred during the few minutes a year of parallel operation is arguably acceptable.
 11 March 2019 07:19 am
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mapj1

Posts: 12039
Joined: 22 July 2004

There is a continuous distribution of possible fault currents depending where and what the fault is.
But, above the larger of the two currents - ICU, the breaker is not even guaranteed to open safely, it may weld shut and not open, or it may explode and shower the area with hot metal and burning plastic. This is not that likely, and there is some margin, but this is a rating never ever to be exceeded - if the ICU cannot be met, then larger devices are needed, or some additional energy limiting must be provided by upstream devices, usually HRC fuses, that remove the supply before enough heating has occurred within the breaker to destroy it. Such 'death or glory' fuses are often in service for decades, and never operate.

Less severe faults operate the breaker as the designers intended, but will burn away at the arc catchers and the contact surfaces themselves.
In reality, being OK to be reset after just one trip is not that helpful, as quite often folk will try to reset onto the same fault. Even at fault levels that are half or quarter of the ICS, some damage is done each time, and the breaker will still fail after a few operations if the fault is not removed..

-------------------------
regards Mike
 11 March 2019 09:28 am
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CMK3PD

Posts: 92
Joined: 23 September 2016

I'm fairly involved with switchboard testing. We would normally allow a device to open twice at Icu ratings, and probably a third time on single phase at 60%, but that is as far as we would go. The test requirements of Icu is Open - Close - Open, so they are tested with 2 open's
We have in the past seen devices fail dramatically when pushed to their limit, it's not a pretty sight.
However many manufacturers now have Icu=Ics, so technically those should be capable of Open- Close-Open-Close-Open, but...
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