Selection of MCB

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How we will have to select the MCB based on thier Grades (A, B, C, D...)


Earlier of any one your response is appreciated.,

K.Jeyabalan
 
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pdcelec said:
What is it going to be used for?


For COmputer applications.

2. How the grades are to be seen before selection of MCB's for any applications?
 
The things that affect the choice are tolerance of surges, disconnection time needed and earth-fault-loop impedance. You probably shouldn't be making the selection if you don't understand the factors involved.

Also, I doubt if anyone here is familiar with Indian supply systems and regulations...
 
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1: as already mentioned it depends on the load
2: you mention A B C and D in our system of breakers there is no such thing as type A are you using kit made to british standards and are you actually in india
3: assuming you are using uk breakers you generally wan't to use type B unless you have a good reason not to (such as equipment with large switch on surges) with type C and D you need far lower impedances to ensure fast fault disconnection so only use theese if you know what your doing.
 
India follows the old 'round pin' radial British system, in red and black PVC, still has a lot of class zero (metal case no earth, not double insulated)gear. Some of us are remarkably familiar with it. :confused:
The Indian wiring regs are based on BS7671 (like ireland, like Malta - its the old colonial influence what-ho)
now i've finished berating my colleagues, to the question,
If you know the live-earth fault resistance, which sets the prospective maximum fault current, and the type of load you can determine what you need.
Type a (instant trip) is never used, and isn't readily available in the UK
Type B, C D all have two mechanisms for tripping,
1) a thermal part which (like a fuse) does not fire for very brief overloads, but will trip before the cables get dangerously hot. This sets the rating of the trip, and the working load and cable size determine that.
2) a magnetic part, with a much higher tripping current, that operates near instantly. The secret is to allow normal cold filament, start motor type surges, without tripping the breaker, and yet still operate near instantly if the cable is cut or a dead-short is applied.

Most of the 230V world ( Russia, China and the EU) uses 16A radials with type C breakers (5 to 10X transient rating), we tend to use 32 A breaker with a type B (2 to 5 X transient rating) characteristic -the effect is the same, the surges for domestic equipment like TV or fridge are in the range 50-80A, but a 200A dead short fault guarantees instant tripping.
Opinion is divided over type B or C at 6A for separated lighting circuits, actually both protect the wire against overload, but B can be a bit too frisky on installations with many lamps switching together (a cold tungsten filament is about 1/12 the resistance of a hot one.)
Type D is normally reserved for heavy machines and is best decided only in consultation with the makers.
Hope that gets you started
 
mapj1 said:
Type a (instant trip) is never used, and isn't readily available in the UK
Dunno about a Type A - I did find reference to a Type Z recently (counter-intuitively it trips faster than a B, not as you might expect from the letter progression only after several days at 1000x In...)

CURVE.GIF


[code:1] ------- Thermal Trip ------- ------ Magnetic Trip ------
Must not trip Must Trip Must not trip Must trip
over 1 hour under 1hr over 100ms at 100ms

TYPE-B 1.13xRC 1.45xRC 3xRC 5xRC

TYPE-C 1.13xRC 1.45xRC 5xRC 10xRC

TYPE-D 1.13xRC 1.45xRC 10xRC 16xRC

TYPE-E 1.05xRC 1.35xRC 14xRC 18xRC

TYPE-G 1.05xRC 1.35xRC 8xRC 10xRC

TYPE-Z 1.05xRC 1.35xRC 2xRC 3xRC
[/code:1]

They're American breakers though - don't know how much the curves differ from European ones.
 
Jeyabalan.K said:
pdcelec said:
What is it going to be used for?


For COmputer applications.

2. How the grades are to be seen before selection of MCB's for any applications?

If it is for a computer installation then section 607 will also come into effect
 

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