Difference between Short circuit rating and Ultimate breaking rating (ICU)

[Muhammad Umer ansar]

I have a Three phase MCB with the following data mentioned:
Short circuit symetrical current= 65KA
ICU= 100 KA
breaker rating= 32A
class C.
I am confused between short circuit and Icu.

 

[paulengr]

I have a Three phase MCB with the following data mentioned:
Short circuit symetrical current= 65KA
ICU= 100 KA
breaker rating= 32A
class C.
I am confused between short circuit and Icu.

It’s obvious.

The long term trip rating is 32 A. As in after 3 hours at 33 A you should trip but never at 32 A for a fully rated breaker. In a distribution panel unless it’s 100% duty derate to 80%. But since 32 A is a nonstandard (miniature MCCB size) that does not apply.

You get a standardized thermal curve from that point. The higher the current the faster it trips. The curve based on your description (sounds like a FAZ breaker) is specified in UL 489. Often you see IEC terms on the breaker and tables so this is confusing to read. The number printed on the handle and what we refer to as the breaker “rating” is the long term trip setting or 32 A. ALL other trip settings are based on this number.

The class C rating in UL 489 for MCCBs means the instantaneous trip setting is fixed at about 10-20 times the rating or 320-640 A for this breaker.

However the breaker cannot interrupt ANY current. Between 640 and 65,000 A there isn’t any problem. Above 65,000 A it can weld shut or even be ripped apart and explode from the magnetic force exerted on it. This limit is the allowable interrupting current or AIC. This is the 65 kA cutoff.

So as far as how it works the thermal curve (32 to 320 A) is designed for overloaded conditions. It is typically a mechanical issue such as overloading a motor. The instantaneous trip is for an electrical fault such as a short circuit or an arcing fault. However depending on the actual conditions things might not fall into nice neat categories like this. For instance breakers must be sized much higher than the load to avoid nuisance tripping due to starting currents on motors and transformers, so those are protected against overloads in another means.

You need to calculate or be given the available short circuit current and this must be less than the breaker AIC or you can’t use a breaker. You will need to look at series ratings for the breaker (say putting it in series with a bigger breaker or a fuse) or using a different device. Generally 65 kA is the highest rating available.

 

[electrofelon]

It’s obvious.

The long term trip rating is 32 A. As in after 3 hours at 33 A you should trip but never at 32 A for a fully rated breaker. In a distribution panel unless it’s 100% duty derate to 80%. But since 32 A is a nonstandard (miniature MCCB size) that does not apply.

You get a standardized thermal curve from that point. The higher the current the faster it trips. The curve based on your description (sounds like a FAZ breaker) is specified in UL 489. Often you see IEC terms on the breaker and tables so this is confusing to read. The number printed on the handle and what we refer to as the breaker “rating” is the long term trip setting or 32 A. ALL other trip settings are based on this number.

The class C rating in UL 489 for MCCBs means the instantaneous trip setting is fixed at about 10-20 times the rating or 320-640 A for this breaker.

However the breaker cannot interrupt ANY current. Between 640 and 65,000 A there isn’t any problem. Above 65,000 A it can weld shut or even be ripped apart and explode from the magnetic force exerted on it. This limit is the allowable interrupting current or AIC. This is the 65 kA cutoff.

So as far as how it works the thermal curve (32 to 320 A) is designed for overloaded conditions. It is typically a mechanical issue such as overloading a motor. The instantaneous trip is for an electrical fault such as a short circuit or an arcing fault. However depending on the actual conditions things might not fall into nice neat categories like this. For instance breakers must be sized much higher than the load to avoid nuisance tripping due to starting currents on motors and transformers, so those are protected against overloads in another means.

You need to calculate or be given the available short circuit current and this must be less than the breaker AIC or you can’t use a breaker. You will need to look at series ratings for the breaker (say putting it in series with a bigger breaker or a fuse) or using a different device. Generally 65 kA is the highest rating available.

I do not believe you answered the question.

 

[ron]

I have a Three phase MCB with the following data mentioned:
Short circuit symetrical current= 65KA
ICU= 100 KA
breaker rating= 32A
class C.
I am confused between short circuit and Icu.

Can you share a link to the datasheet for the circuit breaker?
I suspect the answer lies in this ABB pdf document as the Icu is a characteristic I've seen related to IEC standards. The difference between the 65kA and 100kA is how many cycles into the fault and magnitude of current is the breaker exposed. Typically symmetrical is 3 cycles.

https://search.abb.com/library/Download.aspx?DocumentID=9AKK107046A8858&LanguageCode=en&DocumentPartId=&Action=Launch&DocumentRevisionId=A

 

[Jraef]

Interrupting Capacity (65kAIC) means at fault currents up to 65,000 amps, the breaker will SAFELY interrupt the fault at least once and (per UL 489 testing procedures) be capable of being re-closed at least once and open one more time at the rated IC. After being reclosed and suffering a second fault current event, even at a lesser value, it should not be re-used again and be replaced. Most mfrs will tell you to replace it ASAP after the first fault event just to be sure, since the second one will result in an unexpected shutdown.

ICU is an IEC terminology implying that the breaker CAN interrupt 100,000A, but may destroy itself in the process and may cause collateral damage, meaning NOT "safely" and not able to be used again. That term has no meaning or acceptable value here in North America.

 

[electrofelon]

.

ICU is an IEC terminology implying that the breaker CAN interrupt 100,000A, but may destroy itself in the process and may cause collateral damage, meaning NOT "safely" and not able to be used again. That term has no meaning or acceptable value here in North America.

Surprised there is even a term for that . Doesn't seem like that should even be a thing.

 

[Muhammad Umer ansar]

It’s obvious.

The long term trip rating is 32 A. As in after 3 hours at 33 A you should trip but never at 32 A for a fully rated breaker. In a distribution panel unless it’s 100% duty derate to 80%. But since 32 A is a nonstandard (miniature MCCB size) that does not apply.

You get a standardized thermal curve from that point. The higher the current the faster it trips. The curve based on your description (sounds like a FAZ breaker) is specified in UL 489. Often you see IEC terms on the breaker and tables so this is confusing to read. The number printed on the handle and what we refer to as the breaker “rating” is the long term trip setting or 32 A. ALL other trip settings are based on this number.

The class C rating in UL 489 for MCCBs means the instantaneous trip setting is fixed at about 10-20 times the rating or 320-640 A for this breaker.

However the breaker cannot interrupt ANY current. Between 640 and 65,000 A there isn’t any problem. Above 65,000 A it can weld shut or even be ripped apart and explode from the magnetic force exerted on it. This limit is the allowable interrupting current or AIC. This is the 65 kA cutoff.

So as far as how it works the thermal curve (32 to 320 A) is designed for overloaded conditions. It is typically a mechanical issue such as overloading a motor. The instantaneous trip is for an electrical fault such as a short circuit or an arcing fault. However depending on the actual conditions things might not fall into nice neat categories like this. For instance breakers must be sized much higher than the load to avoid nuisance tripping due to starting currents on motors and transformers, so those are protected against overloads in another means.

You need to calculate or be given the available short circuit current and this must be less than the breaker AIC or you can’t use a breaker. You will need to look at series ratings for the breaker (say putting it in series with a bigger breaker or a fuse) or using a different device. Generally 65 kA is the highest rating available.

You are saying that at 33A the breaker will trip. Then from 33 to 320A( found out with Class), breaker will work as an overload with inverse time tripping. And then from 320 to 640A, it will work for short circuit protection as an instantaneous tripping feature.The max current breaker can bear is 65KA. Then what will happen any current between 640 and 65KA occurs on system?

 

[Muhammad Umer ansar]

It’s obvious.

The long term trip rating is 32 A. As in after 3 hours at 33 A you should trip but never at 32 A for a fully rated breaker. In a distribution panel unless it’s 100% duty derate to 80%. But since 32 A is a nonstandard (miniature MCCB size) that does not apply.

You get a standardized thermal curve from that point. The higher the current the faster it trips. The curve based on your description (sounds like a FAZ breaker) is specified in UL 489. Often you see IEC terms on the breaker and tables so this is confusing to read. The number printed on the handle and what we refer to as the breaker “rating” is the long term trip setting or 32 A. ALL other trip settings are based on this number.

The class C rating in UL 489 for MCCBs means the instantaneous trip setting is fixed at about 10-20 times the rating or 320-640 A for this breaker.

However the breaker cannot interrupt ANY current. Between 640 and 65,000 A there isn’t any problem. Above 65,000 A it can weld shut or even be ripped apart and explode from the magnetic force exerted on it. This limit is the allowable interrupting current or AIC. This is the 65 kA cutoff.

So as far as how it works the thermal curve (32 to 320 A) is designed for overloaded conditions. It is typically a mechanical issue such as overloading a motor. The instantaneous trip is for an electrical fault such as a short circuit or an arcing fault. However depending on the actual conditions things might not fall into nice neat categories like this. For instance breakers must be sized much higher than the load to avoid nuisance tripping due to starting currents on motors and transformers, so those are protected against overloads in another means.

You need to calculate or be given the available short circuit current and this must be less than the breaker AIC or you can’t use a breaker. You will need to look at series ratings for the breaker (say putting it in series with a bigger breaker or a fuse) or using a different device. Generally 65 kA is the highest rating available.

Oh got it!
One more thing, if MCCB provides both Overload and short circuit protection then why we use separate Overload Relay in Star delta starter in combination with MCCB? Why cant we use Overload feature of MCCB instead of using separate overload?

 

[don_resqcapt19]

Oh got it!
One more thing, if MCCB provides both Overload and short circuit protection then why we use separate Overload Relay in Star delta starter in combination with MCCB? Why cant we use Overload feature of MCCB instead of using separate overload?

The setting or operation point of the overload protection in a MCCB does not comply with the requirements of 430.32. It will not open at a current that will protect the motor from damage if the motor is overloaded. In general the MCCB prevents overload damage to the conductors, but not to equipment connected to those conductors.

 

[Jraef]

Oh got it!
One more thing, if MCCB provides both Overload and short circuit protection then why we use separate Overload Relay in Star delta starter in combination with MCCB? Why cant we use Overload feature of MCCB instead of using separate overload?

Outside of North America the MCCB is frequently used for motor overload protection, provided you use one where the thermal setting is adjustable.

Here in North America we have rules about how an MCCB can be used that limit that possibility of having adjustable thermals. But if you follow all the rules, you CAN in fact use a version of MCCBs that are called Motor Protection Circuit Breakers (MPCB), so long as it is a motor BRANCH circuit, meaning nothing else on that circuit besides the motor. It’s not yet very common though.

 

[Muhammad Umer ansar]

You are saying that at 33A the breaker will trip. Then from 33 to 320A( found out with Class), breaker will work as an overload with inverse time tripping. And then from 320 to 640A, it will work for short circuit protection as an instantaneous tripping feature.The max current breaker can bear is 65KA. Then what will happen any current between 640 and 65KA occurs on system?

can you help me in that @don_resqcapt19 @Jraef
@electrofelon

 

[don_resqcapt19]

can you help me in that @don_resqcapt19 @Jraef
@electrofelon

For the motor overload, take for an example a 3 phase 30 hp, 480 volt motor. Under the NEC we would typically use a 80 to 100 amp MCCB for the branch circuit short circuit and ground fault protection. The full load current for this motor would be approximately 35 amps.

The branch circuit breaker does not protect the motor from overloads as its long time trip is more than twice the full load current of the motor. The branch circuit breaker is sized that large because a smaller rated breaker would likely trip on the motor starting and inrush current. As a result of the use of these higher rated breakers to permit the motor to start, the NEC requires a separate overload protective device to protect the motor from overloads. For this motor the typical setting for the overload device would be 125% of the full load current of 35 amps or a setting of 43.75 amps.

The use of a 45 amp breaker that would provide reasonable overload protection would be very unlikely to permit the 30 hp motor to start. The use of a breaker with a high enough rating to permit the motor to start, will not provide any overload protection for the motor.

 

[NewtonLaw]

I have a Three phase MCB with the following data mentioned:
Short circuit symetrical current= 65KA
ICU= 100 KA
breaker rating= 32A
class C.
I am confused between short circuit and Icu.

UL 489 standard gives fault current ratings in symmetrical amps and they assume a certain X/R ratio range that if the system impedance falls within that range your circuit breaker will operate successfully one time at that level. The X/R ratio affects the asymmetrical fault level and while these X/R ratios may change drastically due POCO system updates the symmetrical fault duty levels may stay the same. UL states that ratings outside there allowances (X/R ratios they have taken into account) must be addressed by the designing engineer but they are not clear as to how you are supposed to know this without getting both the symmetrical and asymmetrical fault duties from the POCO.

So to your question, I suspect the Icu = 100 kA, is the peak asymmetrical fault rating at an X/R of 1.5.

IEC/EN 60947-2 defines the ultimate short-circuit breaking capacity (Icu), also known as the breaking capacity, according to a specified test sequence.

Since Icu is an IEC standard term, someone with more background would be more helpful.