Use of MCB's

[pavement]

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

 

[CJFlatters]

You ideally need specific test data from Terasaki if you want to look at both discrimination and cascading issues.

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

 

[ScottyUK]

The TemBreak series is a MCCB range, not an MCB range. The two items of equipment are very different in their ratings and capabilities. MCCB curves can be plotted in exactly the same way as relay curves on log-log graphs. The curves for the particular MCCBs in question are available from Terasaki's website if you register.

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,
I'm getting a great education!

 

[Bung]

Terasaki, Schneider, ABB, all the big manufacturers publish extensive guides for discrimination, cascding, application etc of their own products. IEC standards offer some guidance if you wantto mix products.


Bung
Life is non-linear...

 

[pavement]

Yes, sorry the MCB was a typo, the reason for the choice of Terasaki was due to having literature and having requested their in house discrimination software, guess it just being lazy is suppose on my part.The reason behind the question was detemine a minimum grading margin as such between successive devices, as none of the usual PRAG manuals dedicate any time on the use of LV MCCB devices.

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?

 

[Sanman1966UK]

Discrimination is only achivable by using different frame sizes of devices, and can only be verified by using the same maufacture throughout your system. As opposing manufactures will not provide you with approval, as these values are provided by test only.

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