Use of MCB's
Use of MCB's
(OP)
Any anybody give me some guidelines on discrimination when using MCB's as the PRAG manuals from the usual suspects dont mention anything about these devices for some reason. I was looking to use Terasaki TemBreak Series to protect a couple of 440 LV buses with have prospective asymmetrical peaks of 47kA and 37kA further downstream. The minimum fault levels for both buses are around the 309A mark.(phase-phase faults)
Any help in determing how to go about this would be much appreciated.
Many Thanks
Any help in determing how to go about this would be much appreciated.
Many Thanks






RE: Use of MCB's
Not familiar with the Tembreak PD's, if thet are MCB's their fault rating is likely to be in the region of 10 - 16kA (BS EN 60947).
If you have high upstream fault levels as mentioned you really need to look at the manufacturers data - according to IEC364 (cant comment on the NEC) you could use cascading by looking at the type and rating of the upstream device (if its a MCCB), or use an HBC fuse to limit the let-through energy to suit the MCB's rating.
_______________________________________
Regards -
Colin J Flatters
Consulting Engineer & Project Manager
RE: Use of MCB's
Purely out of curiousity, is there any reason why you specifically wish to use Terasaki's equipment? There are many other suppliers of good quality MCCB's out there.
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If we learn from our mistakes,
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RE: Use of MCB's
Bung
Life is non-linear...
RE: Use of MCB's
If for example, you have a bus which you want to protect that has a load current of 66A but a minimum prospective fault current of 309A due to phase-phase fault in a unearthed system, and a maximum of 33kA RMS what would would you generically set the MCCB in terms of the instantenous 'section' of the characteristic?
RE: Use of MCB's
As a general rule if you miss a frame size you will achive discrimination ie 63A mcb (160A mccb) 250A mccb (400 A mccb) 630A mccb etc... Frame size should not be confused with trip size.
Discrimination is good to have but is not a necessity, dont just increase your breaker size as the ecconomics need to be considered.
I agree with previous, that terasaki are one of the poorest devices to achieve discimination with as they trip too-ooo easy.
For a comercial building i would recommend Merlin Gurin,and for an industrial application I think ABB are good.
I did not really understand your aplication as i am a services engineer but i hope it helps