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Difference Between Ith & Iu

04/09/2017 5:34 AM

Dear all

Can you help me to know the difference between Ith & Iu rating in MCCB?

we have an MCCB with Iu = 125 Amps and Ith = 32 amps max

I know,Ith is the thermal current of circuit breaker in this value the circuit breaker

will trip.but what about Iu,in the other word what will happen when current value

reaches to 125A

thanks

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#1

Re: differaence between Ith & Iu

04/09/2017 7:28 AM

The circuit breaker has two trip modes, one thermal and one magnetic. It responds slowly to marginal overcurrents and quickly to significant overcurrents.

"Thermal magnetic circuit breakers[edit]

Shihlin Electric MCCB with SHT

Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions. The thermal portion of the circuit breaker provides a time response feature, that trips the circuit breaker sooner for larger overcurrents but allows smaller overloads to persist for a longer time. This allows short current spikes such as are produced when a motor or other non-resistive load is switched on. With very large over-currents during a short-circuit, the magnetic element trips the circuit breaker with no intentional additional delay.[6] "

https://en.wikipedia.org/wiki/Circuit_breaker#Thermal_magnetic_circuit_breakers

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#2

Re: differaence between Ith & Iu

04/09/2017 9:03 AM

If you meant " the difference between Ith and Icu" then Icu (kA rms) = rated ultimate short-circuit breaking capacity.

This performance expresses the maximum value of the short-circuit current that can be broken by the circuit-breaker.

That means after such an elevated current interruption case the MCCB cannot be reused unless tested for its suitability.

According to IEC 60947-2 the following definitions are available:

In=Rated current; Ith=Conventional free-air thermal current; Ithe =Conventional enclosed thermal current;Icm = Rated short-circuit making capacity

and in a short-circuit case:

Icu=Rated ultimate short-circuit breaking capacity; Ics =Rated service short-circuit breaking capacity;Icw =Rated short-time withstand current

As trip unit:

Ir=Adjustable overload setting current

Isd=Short time delay tripping setting current

Ii=Instantaneous tripping setting current

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#4
In reply to #2

Re: differaence between Ith & Iu

04/10/2017 1:40 AM

Dear sir

good day

thank you very much for your reply

I mean Iu=uninterrupted current and not "Icu"

thanks

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#6
In reply to #4

Re: differaence between Ith & Iu

04/10/2017 10:33 AM

IEC 60947-2 sais:

Rated uninterrupted current (Iu): The rated uninterrupted current of an equipment is a value of current, stated by the manufacturer, which the equipment can carry in uninterrupted duty.

In=Maximum uninterrupted current withstood at ambient temperature, given without abnormal temperature rise

Iu depends on ambient temperature and altitude.

That means In it is Iu for a constant ambient temperature[usually 40oC] and sea level.

See-for instance:

https://library.e.abb.com/public/475ef3d72a0a249dc1256cbf003cad2f/ITSCE-604050012.pdf

Section 5

For example let’s take Sace S2 R125 Circuit-breaker with thermomagnetic releases.

Iu=125 A at 40oC but 145 A at 10oC and only 101 A at 70oC [see 5/34]

Ith depend also on temperature.For circuit-breakers with thermomagnetic releases will be 0.7-1*Iu

For circuit-breakers with electronic releases Iu and Ith are almost constant.

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#3

Re: Difference Between Ith & Iu

04/09/2017 5:48 PM

<...what will happen when current value reaches to 125A...>

Simply, the breaker will operate and disconnect the load/fault.

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#5

Re: Difference Between Ith & Iu

04/10/2017 9:56 AM

It is a breaker. It will take action when the current is 125 amps due to magnetic breaker action, but will long-term tolerate only 32 amps due to thermal part of breaker.

Use some common sense, and read up on the manual, and you will find out.

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#7

Re: Difference Between Ith & Iu

04/10/2017 12:50 PM

Here is what it really means.

Manufacturers of molded case circuit breakers have two basic categories of design; the rating of the current carrying components (CCCs) such as the contacts, terminals and connectors inside, and then the trip components (TCs) such as the thermal sensors, magnetic sensors etc. For most mfrs, they cannot afford to have their machines make a different set of CCCs for every value of TCs, so they make them in certain groups. So for example you may have one set of CCCs for 150-250A maximum, to which is applied TCs that might be 150A, 175A, 200A 225A and 250A. Here in North America we just call them "Frame Sizes" for the most part, but even that is not a totally accurate depiction when you are looking at 150A and below, because the frame itself may have different CCCs inside. So I actually prefer the IEC nomenclature for this, because Iu tells you exactly what is inside of your breaker.

In your case the Iu = 125A is telling you that the CCCs in your breaker are all made to handle 125A. But then the TCs, the thermal trips in particular, are rated for only 32A, meaning this is a 32A breaker, regardless of the fact that the parts inside could handle more. The extra capacity of the CCCs is essentially wasted in this case, because the 32A trip setting will not allow that breaker to carry more than 32A continuously. So if you bought a 60A breaker of that same brand the same would be true; 125A CCCs, but maximum 60A running current without tripping.

In some cases, mfrs do offer different Iu ratings in that frame size, often 60 or 63A then 125A. For whatever reason, your brand chose to not have two different values in the CCCs. It means nothing to you however because you really can't use it, so it's just interesting information. Your mfr may also offer the 63A as a different frame size as well, in which case the 32A Ith would be redundant, but may be offered in both frames so that they mount equally with 125A frame breakers in a switchboard configuration.

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#8
In reply to #7

Re: Difference Between Ith & Iu

04/10/2017 1:50 PM

Translation: If thou beest not a journeyman, take a hike, and pass on forward the worketh to a true man of journey.

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#9
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Re: Difference Between Ith & Iu

04/10/2017 3:06 PM

I imagined the voice of John Cleese from Monty Python and the Holy Grail when I read that.

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#10
In reply to #9

Re: Difference Between Ith & Iu

04/10/2017 4:04 PM

Thank you. With temperance and some forbearance, I will be here all week.

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#11
In reply to #7

Re: Difference Between Ith & Iu

04/11/2017 5:26 AM

Dir sir

I am very grateful to you and thank you very much.

Best regards

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#12

Re: Difference Between Ith & Iu

04/11/2017 5:41 AM

Dear,

I think, by Iu, MCCB Manufacturer is indicating maximum rating available in that frame & by Ith, MCCB Manufacturer is indicating maximum current this MCCB of yours is meant to carry continuously beyond which it would mean overcurrent and trip as per I-T Characteristics of MCCB.

Hope this answers your query satisfactorily.

Regards,

Jignesh

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