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Topic Title: Danger of installing a C type MCB instead of a B type
Topic Summary: Is there an issue with installing a C type mcb in the place of a B type apart from Earth Fault Loop Impedance
Created On: 10 December 2012 10:05 PM
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 10 December 2012 10:05 PM
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Frankmul

Posts: 20
Joined: 10 December 2012

I always thought that an MCB should be selected according to the type of load but a discussion started and it was stated that C type MCB's are generally installed in commercial boards for convenience instead of B or C depending on the load type as I thought it should be done. Apart from a lower EFLI, is there any other issue with doing this
 10 December 2012 10:12 PM
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leckie

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I don't understand the post? You mean is there any other reason a B/C selection is made other than earth loop impedance values?

There are inrush and start current values to consider
 10 December 2012 11:04 PM
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Frankmul

Posts: 20
Joined: 10 December 2012

Sorry about the confusion,I know the reasoning behind the different types of MCB's. I am wondering if there is any issue with putting a type C mcb on a resistive load say an immersion apart from the lower EFLI allowed.
 10 December 2012 11:24 PM
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alancapon

Posts: 5761
Joined: 27 December 2005

You also need to remember the phase - neutral loop too (many don't), as it is only the mcb will disconnect this type of fault. In most cases it will be lower than the phase - earth loop, but you need to satisfy yourself that that is true.

Regards,

Alan.
 11 December 2012 10:09 AM
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Parsley

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You may need to refer to 434.5.2 and take look at osg table B7.

You don't mention if your using reduced sized CPC's.

Regards
 11 December 2012 11:52 AM
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Frankmul

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Originally posted by: Parsley

You may need to refer to 434.5.2 and take look at osg table B7.



You don't mention if your using reduced sized CPC's.



Regards

Thanks Parsley

I never looked into it but does the let through energy vary greatly between different type of mcb of the same model and manufacturer. I have to dig out my onsite guide for table b7
 11 December 2012 12:41 PM
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Parsley

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From my own experience, the manufactures energy let through data for the class 3 current limiting devices has shown that the next size CPC down may be OK compared to OSG B7.

You need to check with each manufactuer.

Regards
 12 December 2012 10:36 PM
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Frankmul

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Can't locate my OSG but I came across this
http://www.aerco.co.uk/images/...-technical-MCB(1).pdf

It can be a discrimination issue with high short circuit current but I guess that would be only in an industrial situation.
 12 December 2012 10:57 PM
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Westonelectrical

Posts: 85
Joined: 26 November 2012

I've just fitted out a load of hf lights with a type c on a 100a single phase supply... you use them on 1 example when you have inductive circuits, long as the kept within the max zs is there a problem??
 13 December 2012 09:45 AM
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AJJewsbury

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long as the kept within the max zs is there a problem??

One possible problem is the energy let-though of MCBs a higher fault currents - and whether to conductors can survive the fault current/duration (or more correctly, whether the conductor's insulation will survive).

As parsley says, the regs (434.5.2 in general and 543.1.3 specifically for c.p.c.s - same equation, just re-arranged) say we need to check that where the conductor is rated at less than the protective device (as reduced c.p.c.s often are).

There are tables (another is A5 in the current GN3) that list minimum c.p.c. size for MCB type and fault current for any-brand MCBs - e.g. for a C type (all ratings up to 40A) minimum c.p.c. size is 2.5mm2 for faults up to 6kA and 1.5mm2 for faults 3kA & below - so T&E could well have a problem.

If you can specify a given brand of MCB then you can use manufacturer's data, which usually gives you a fair bit more wiggle room, but not always a lot.

Note that the worst-case for energy let-through for MCBs (unlike fuses) is at high currents - hence low Zs - so PFC at the DB rather than Zs at the far end of the circuit is the critical factor in this case. (You still need to check Zs of course to ensure ADS - but that's not the whole story any more).

- Andy.
 13 December 2012 07:41 PM
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Westonelectrical

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Adiabatic equation then
Statistics

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