short circuit capacity of MCB

[incognito1]

how to choose a MCB from 3kA, 4.5kA MCB, 6kA MCB and 10kA MCB for domestic application? i have seen 10kA MCB are used for industrial applications. but why industrial applications need 10kA MCB instead of 3kA, 4.5kA MCB, 6kA MCB

what is the thumb rule of selecting short circuit capacity of a MCB.

 

[krisys]

Normally the industrial system has stronger source with higher capacity transformers and concentrated loads. Hence, higher kA MCBs are required.

I heard that even higher rated (may be 21kA) MCBs are being used in some places. So the distinction is obvious.

 

[incognito1]

could u pls give me more information for the reason why we need higher kA rated MCB for industrial?

is there any technical reference guide from that i can get a more clearer idea abt this?

 

[Paulusgnome]

The MCB should be selected so that its KA rating is at least equal, and preferably higher than the (calculated or measured) prospective short-circuit fault current (PSSC) of the supply.
Industrial installations tend to have power supplies with high PSSC, hence the need for higher-rated circuit breakers. Not so with residential supply networks which often have a PSSC of 2KA or less.

 

[wbd]

The kA rating of the mcb needs to be greater than the available short circuit current at the location the breaker is installed. What is the size of the transformer and voltage for the circuit you are looking at? The maximum available fault current would be the transformer full load amps divided by the transformer impedance. Of course that would be right at the transformer terminals and ignores the impedance of the conductors to the panel.

This is why in industrial facilities the main breaker and main panel breakers may be rated at 42kA and then as you farther downstream the rating of the breakers can decrease to 25 or 22kA.

Typically the lowest rated mcb's I see are 10kA with an occasional 5kA. For a residential service fed by a 25kVA transformer 120/240V, the fault current is around 7kA, so typically the panels and breakers are 10kA rated.

 

[incognito1]

how can i calculate and get the fault current value as 7kA for a 25kVA transformer?

 

[cuky2000]

See the enclosed link:

http://www.cooperindustries.com/con...ch_Lib_Short_Circuit_Current_Calculations.pdf

 

[Mbrooke]

Doesn't earth fault loop impedance and perspective fault current testing decide?

 

[waross]

how can i calculate and get the fault current value as 7kA for a 25kVA transformer?

Knowing the voltage is a big help. Actually knowing the voltage is almost mandatory.
ARE YOU SURE THAT YOU SHOULD BE DOING THIS?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

This strikes me as one of those cases where if you have to ask...

Please find the assistance of someone locally who understands the local requirements. To me the consideration of anything less than 10kA can only be done if one's cranium is in a location where the sun don't shine.

Sorry, but it sounds like you're in over your head in an area where there's no room for trial and error.

Infinite bus on the high-side and you're looking at more than 13kA. Why are you questioning 7kA?

 

[Mbrooke]

The above reply is offensive and was uncalled for. In the IEC world selecting an interrupting rating less than 10ka is done all the time and is actually a very intelligent question. Given enough distance and right wire size perspective fault current under 10ka is very possible even for an industrial application. I do not believe the OP is closed minded to any degree. Googling earth fault loop impedance and perspective fault current is a good place to start.

 

[davidbeach]

Maybe it's one of those IEEE/ANSI vs. IEC things then. In the IEEE/ANSI world there's essentially nothing less than 10kA for installation in a panelboard. There are DIN rail mountable supplemental overcurrent devices for use in control panels that might have lower ratings (and some much older devices with no labelled rating that have to be assumed as being 5kA). I'm very surprised that any manufacturer would go to lengths of having four different devices of the same nominal rating with different interrupting ratings all less than or equal to 10kA; seems like a spare parts nightmare. But if they really do, good for them. Around here the utilities will go to great lengths (including extra lengths of secondary conductor) to keep single family residential services to less than 10kA, but you'll never get any other number from the utility; thus 10kA it is. Multifamily may go higher, next step is generally 22kA.

 

[ScottyUK]

Not often we see less than 6kA MCBs these days, and 10kA is normally offered when industrial manufacturers re-badge their mainline products for the domestic market. There are still retrofit types designed to repace rewireable fuselinks which are only good for 3kA, and some of the really early MCBs have only 1kA or 2kA breaking capacity as I found out fairly recently on an old panelboard made by Crabtree in the UK. That panelboard is getting changed ASAP.

 

[Mbrooke]

MCBs for the European and Asian markets come in a variety of interpreting ratings less than and well over 10ka. TT earthed supplies greatly reduce fault current, and in any domestic installation you are required to test and know the loop impedance coming from the DNO (POCO). Each breaker also has its own magnetic trip threshold (B, C, D, ect) curves and short circuit ratings are listed as Icu and Ics rather than one fixed listing:

http://www.electriciansforums.co.uk...l-forum/50977-how-can-i-take-breaker-ics.html

If a domestic supply has a fault current less than 10ka it does not make sense installing an MCB (miniature circuit breaker) of a higher interrupting capacity. Manufactures go to the length of having different devices for various technical reasons as having a one size fits all creates far bigger nightmares than potential inventory logistics. For example, the high magnetic trip thresholds of North American circuit breakers designed to cover all possible scenarios have been blamed for many fires and are the reason why arc fault circuit interrupters were mandated by the National Electrical Code NFPA70:

http://paceforensic.com/pdfs/Circuit_Breakers_The_Myth_of_Safety.pdf

http://paceforensic.com/pdfs/newsletter/KeepingPace-37.pdf

http://paceforensic.com/pdfs/newsletter/KeepingPace-15.pdf

http://paceforensic.com/pdfs/newsletter/KeepingPace-7.pdf

The above findings have been confirmed by UL and presented to the NEC code making panels:

http://www.ul.com/global/documents/library/white_papers/BreakerMitigationofArcFaults.pdf

http://newscience.ul.com/wp-content...lity_to_Mitigate_Parallel_Arcing_Faults_1.pdf

http://newscience.ul.com/wp-content...lity_to_Mitigate_Parallel_Arcing_Faults_2.pdf

The theory is that while a single magnetic trip level can cover all loads, the impedance of a short circuit itself along with the none sinusoidal current draw (low RMS current) from sporadic sputtering results in increased incident energy at a fault when relying on thermal trip instead of magnetic trip because thermal trip relies on a accumulated heating (inverse time) rather than an instantaneous (immediate) effect. A waveform may have 150amps peak, but with parts of the sign wave missing the current will "appear" much lower to the bimetal. In the IEC this is where various time current curves and magnetic pickups come in. A circuit breaker is selected for the inrush of the load and then the installer must make sure the loop impedance is low enough by selecting the right wire size to meet the specified disconnect times in IEC60364 or as Im thinking BS7671. Here is a paper regarding disconnect times and the various MCBs/conductor impedance in meeting them:

http://espm.co.uk/Max Zs tables 17th Ed.pdf

I apologies this is not the best paper, but typical MCBs for a domestic consumer unit:

https://www.tlc-direct.co.uk/Technical/DataSheets/Wylex/WylexTech.pdf

In terms of perspective fault current here is how the test is carried out:

https://www.youtube.com/watch?v=KOyLsi1GFqg

https://www.youtube.com/watch?v=e-v9LcuTQNM

 

[Mbrooke]

And oh, page 16 (actual number, not the viewer) of a Crabtree (UK) industrial catalog showing both 6ka and 10ka breakers
(3ka is also mentioned being available):

http://www.coronabd.com/download/Crabtree-Industrial Circuit Protection.pdf

Remember that IEC breakers are rated differently in terms of short circuit interpreting capability so that may also play a role. But the point I am trying to make is professional electricians and engineers install circuit breakers below 10ka every day outside of North America even in industrial and commercial properties while still being to code.

 

[waross]

In North America a typical residential breaker may have a rating of 15 Amperes. The instantaneous trip level may be 10 times this or 150 Amps. This will be the same whether the breaker has an interrupting rating of 5 kA, 10 kA or 22 kA.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

What waross said. Trip characteristics and interrupting ratings are independent of each other in any breaker I'm familiar with. If a 10kA breaker is adequate and I install a 22kA (in the same family) instead I've only spent a bit more money, but I haven't changed the trip characteristics at all.

The arc fault breakers add electronics to detect the signature of the arcing fault, but the thermal and magnetic tripping characteristics remain essentially the same.

On the other hand, if the interrupting rating affects the trip characteristics then it probably does make sense to try to optimize the selection and have lots of options. Where it doesn't then high enough is good enough and it doesn't make much sense to deal with lots of options.

Sounds like another one of those areas where both sides of the ANSI/IEC divide find what's done on the other side to be completely weird. Somehow the same physics results in very different engineering.

 

[ScottyUK]

David / Bill,

Over here we have three trip classes for MCBs: B, C, and D respectively corresponding to the magnetic element operating at 5x, 10x and 20x the rated current of the breaker. This has no relationship to the breaking capability of the MCB. The low breaking capacity MCBs are confined to domestic and small light commercial installations where the marginally lower cost proves attractive to the cut-throat world of domestic and light commerical installations. As networks are reinforced and larger transformers appear on the system the domestic fault levels are tending to rise, although in almost all cases they are ultimately backed up by an HRC service fuse provided by the utility.

 

[Mbrooke]

I never said magnetic pickup determines interrupting rating or visa versa. It does not and they are sepeprate. Rather I was giving an example of why manufactures stock different versions of the same thing. I am very well aware of North American trip curves considering I have read those documents posted.

 

[Mbrooke]

What waross said. Trip characteristics and interrupting ratings are independent of each other in any breaker I'm familiar with. If a 10kA breaker is adequate and I install a 22kA (in the same family) instead I've only spent a bit more money, but I haven't changed the trip characteristics at all.

I know A 10ka, 22, 65ka all have the same tripping characteristic. But you said the same thing as I did, a 22ka breaker costs more than a 10ka breaker. So if you have a large number of consumer units where the typical fault current is say 5ka it makes sense for manufactures to stock a cheaper 6ka in addition to 10ka instead of only 10ka or say only 16ka and cover everything. Yes its less logistics, but more cost to the customer.

The arc fault breakers add electronics to detect the signature of the arcing fault, but the thermal and magnetic tripping characteristics remain essentially the same.

Correct, however (AIC aside) the magnetic trip component has gone down over the years in single pole 120 volt residential breakers, but you can only go do down so far, hence the electronics found in AFCIs. In the early days of AFCI development the idea was to simply have a standard thermal magnetic breaker with a known magnetic trip level of 75amps. 75amps was chosen as being the lowest anticipated level of fault current in a dwelling (500amps available fault current at the panel plus a long circuit length, say 150 feet). While such a breaker would provide parallel arc fault protection, it would nuisance trip on motor and tungsten inrush. Thus, a solution was created where instead to relying on a coil at 75amps, electronics would monitor the current waveform starting at 75amps. Thus, discrimination could be achieved. A motor starting with a 120 amp inrush would not trip because the waveform is mostly sinusoidal, but if someone drove a nail into NM, the electronics would see the sporadic/distorted sine wave and trip mimicking the advantage of a low magnetic trip breaker.


Of course 15 years latter AFCI were required to also look for series arc faults which has nothing to do with parallel arc faults (whole other ball game), but it gives you the the idea behind the history.

Similar philosophy in the IEC, however its up to the electrician to select the right pickup and then make sure the system can pass the required current to trip the breaker magnetically.

Sounds like another one of those areas where both sides of the ANSI/IEC divide find what's done on the other side to be completely weird. Somehow the same physics results in very different engineering.

Well, also keep in mind whats at play. 230 volts has more hazards and is less forging than 120 volts, so it does play a role in the design.