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






RE: short circuit capacity of MCB
I heard that even higher rated (may be 21kA) MCBs are being used in some places. So the distinction is obvious.
RE: short circuit capacity of MCB
is there any technical reference guide from that i can get a more clearer idea abt this?
RE: short circuit capacity of MCB
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.
RE: short circuit capacity of MCB
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.
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
http://www.cooperindustries.com/content/dam/public...
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
ARE YOU SURE THAT YOU SHOULD BE DOING THIS?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: short circuit capacity of MCB
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?
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
http://www.electriciansforums.co.uk/electrical-for...
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_...
http://paceforensic.com/pdfs/newsletter/KeepingPac...
http://paceforensic.com/pdfs/newsletter/KeepingPac...
http://paceforensic.com/pdfs/newsletter/KeepingPac...
The above findings have been confirmed by UL and presented to the NEC code making panels:
http://www.ul.com/global/documents/library/white_p...
http://newscience.ul.com/wp-content/uploads/2014/0...
http://newscience.ul.com/wp-content/uploads/2014/0...
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%20Zs%20tables%2017th%20Ed.pd...
I apologies this is not the best paper, but typical MCBs for a domestic consumer unit:
https://www.tlc-direct.co.uk/Technical/DataSheets/...
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
RE: short circuit capacity of MCB
(3ka is also mentioned being available):
http://www.coronabd.com/download/Crabtree-Industri...
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.
RE: short circuit capacity of MCB
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: short circuit capacity of MCB
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.
RE: short circuit capacity of MCB
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.
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
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.
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.
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.
RE: short circuit capacity of MCB
I've seen, and continue to deal with, the results of people optimizing designs in the past to use just the bare minimum; in fact we paid contractors bonuses 20+ years ago to spend as little as possible on as tight a design as possible. The added O&M costs since then probably exceeded the first cost savings within a few years and we keep paying and paying. Useful projects get put off simply because a $10k project will require many times that in infrastructure upgrades simply to make the necessary space available.
If you're designing a manufactured product that's going to be produced thousands or millions of times then by all means make it as cost efficient as possible, optimize the heck out of it. Particularly if you aren't going to be responsible for ongoing maintenance of it. But if you're building something for the ages, the guys/gals that come after you to maintain it will curse your name for years to come if you optimize it to the nth degree and leave no wiggle room.
If you can use a 6kA today because you calculate 5.8kA and then the utility upgrades their system and then the value calculates out to 6.1kA, how much have you saved in the long run by installing 6kA breakers instead of 10kA? Not going to happen you say, the utility won't need to make that change; well something will change. Maybe the laws will change and indoor grow operations for personal medical or recreational use will become legal and the neighborhood power consumption goes up by 50% and the transformers all become overloaded and get replaced with larger transformers. Things happen and overly optimized designs become obsolete much quicker.
Having too many times come after the guy that picked just the "right" size/rating/layout over the years, I get a bit testy; "oh, not again". The original question, and some of the follow ups, remind me of the design "logic" that ended up with a single section 600A MCC (lowest vertical bus rating is 600A) with two size 1 starters and a 60A feed to the MCC. The other four cells in the MCC were forever useless because that 60A feed couldn't carry anything else. The owner then paid much more to get a couple of motors added than they would have if the feed to the MCC had been 200A originally. But 60A was a lower first cost. Bah.
RE: short circuit capacity of MCB
This is a really handy paper imo:
http://www.alabamapower.com/business/services/arch...
RE: short circuit capacity of MCB
The majority of the residential services are single phase (split-phase) 120/240 volt system. The 240 V secondary winding is center-tapped and the center neutral wire grounded. Most service transformers are pole mounted a less quantity are pad mounted unit.
See below a typical SC damping effect on the service entrance feeder.
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
Below is another approximate curve for a single phase residential service 120-240 Volts based in 25kVA & 50 kVA with 1.4% and 1.8% impedances respectively.
The SC provided by pole mounted transformers is usually lower for the same kVA capacity since those transformers have larger impedances than the pad mounted units.
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
RE: short circuit capacity of MCB
You may be able to correct the graph.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: short circuit capacity of MCB
can someone clarify why short circuit current rating getting decreased like this?
RE: short circuit capacity of MCB
At the service the impedance of the service conductors plus the transformer impedance limits the fault current.
At the branch panel the impedance of the branch circuit conductors plus the impedance of the service conductors plus the transformer impedance limits the fault current.
etc.
Bill
--------------------
"Why not the best?"
Jimmy Carter