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jonald
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Current Liminting MCCB Coordination

06/01/2011 9:00 PM

I have read that a current limiting fuse can achieve coordination by selecting 2:1 ratio of its ampere from upstream to downstream.

What if we use current limiting MCCB? Can we achieve coordination too?
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kvsridhar
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#1

Re: Current Liminting MCCB Coordination

06/01/2011 10:47 PM

Yes, one manufacturer claims discrimination with a 2.5:1 ratio. Another company claims 1.6:1 ! Apparently even better than fuses.

ECT201

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pnaban
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#2
In reply to #1

Re: Current Liminting MCCB Coordination

06/01/2011 11:12 PM

Instead of depending on manufacturers the authorities(BS/IEC/NEC/IEEE/VDE/NEMA etc) should lay down standards to be complied with.

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kvsridhar
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#3
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Re: Current Liminting MCCB Coordination

06/01/2011 11:24 PM

i have always felt that the standards (IEC in my case) specify the minimum level that an equipment must satisfactorily achieve. The designer must strive to do better than that. So, in this case, if i was a system designer, i would choose the newer MCCB which gives me a better ratio (1.6:1) thereby further reducing the stress on my system, and is also a more economical solution perhaps.

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pnaban
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#6
In reply to #3

Re: Current Liminting MCCB Coordination

06/02/2011 5:40 AM

For fuses we consider pre-arcing I^2t values.For CBs tripping characteristics of both drawn in the same graph paper will be useful.

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kvsridhar
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#7
In reply to #6

Re: Current Liminting MCCB Coordination

06/02/2011 6:57 AM

All good manufacturers of current-limiting MCCBs give this data in two ways:

1. As discrimination and cascading tables with the larger MCCB and smaller MCCB, etc.

2. Also as curves on log-log graph.

However, at the maximum short-circuit level, both will open under contact repulsion, though only the downstream one will trip and clear the fault. The upstream one may open momentarily, and reclose. This momentary opening actually helps in restricting the let-thru energy.

Yes, but when the contacts reclose in that upstream MCCB, the tips are in molten state. The company's expert assured me that they will not weld. i hope so

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PWSlack
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#4

Re: Current Liminting MCCB Coordination

06/02/2011 3:53 AM

British Standard 7671.

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kvsridhar
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#5
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Re: Current Liminting MCCB Coordination

06/02/2011 5:28 AM

i am intrigued. i would have thought that discrimination is not something that can be specified in a standard, since a cost-conscious user may choose cascading for example. Do you mean BS7671 really specifies this ? It would be very interesting to learn, for me as well as many others on CR4. May i request you to spare a little of your time and tell us more on this ? Thank you.

i would hasten to assure you that i am sincerely in the quest for knowledge, and that this is not a "Oh yeah? Prove it to me!" post. i have tremendous respect for you.

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PWSlack
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#8
In reply to #5

Re: Current Liminting MCCB Coordination

06/02/2011 7:11 AM

The application of BS7671 can answer a great many postings in this forum.

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kvsridhar
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#9
In reply to #8

Re: Current Liminting MCCB Coordination

06/02/2011 8:46 AM

Of course. i found this article from Eaton MEM on BS7671 and discrimination.

http://edmundson-electrical.voltilink.co.uk/news/3544/cm/guide-to-discrimination.html

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jonald
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#10

Re: Current Liminting MCCB Coordination

06/03/2011 5:05 AM

To all,

Thanks for the information that you have shared.

It helps me a lot in my design.
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Vinu_Answers
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#11
In reply to #10

Re: Current Liminting MCCB Coordination

06/03/2011 6:49 AM

Here are attached the time-current characteristics of various fuses and circuit breakers.
May this will help you.

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