What are the difference between C-Curve and D-Curve for
MCB's.
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Answer Posted / bright spark
There is no A type.
B C and D types all consist of 2 similar parts:
The thermal trip, this is the same for all MCBs of the same
rating eg 6A.
The Magnetic trip, this is where the difference is.
B C and Ds have an increasing degree of initial "tolerance"
to overload.
B types have a limit of 3 - 5 times the rating.
e.g. a 6A MCB would trip with a surge of 18-30A.
C types have a limit of 5 - 10 times the rating.
e.g. a 6A MCB would trip with a surge of 30 - 60A.
D types have a limit of 10 - 20 times the rating.
e.g. a 6A MCB would trip with a surge of 60 - 120A.
Now you would think that the B curve would be the safest,
and it is, but not very tolerant of power surges. If you
have quite a few lights on in the house and switch on
another, this can be enough for that initial start up surge
to trip you whole lighting ring, and a reset at you consumer
unit. B types are notorious for this, but ideal for a ring
final where you don't have too many outlets, gives ideal
protection.
As you can see a type D 6A MCB can tolerate a 120A surge, so
a 32A Type D would tolerate an initial surge of 320 - 640A
before tripping!. Domestic Consumer Units will have a master
trip of @60 - 100A (80A is most common), hence why there is
never a need for type D in the domestic environment, not
unless you have a 3 phase incomer and have a car production
line in your back garden instead of a washing line!.
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