The B,C,D,K,Z indicates the instantaenous tripping current.
It means the minimum value of current causes the circuit to
trip without intentional time delay. approximately < 100ms.

Consider Rated Current be : (In)
Instantaenous tripping current for C Curve is 5-10 times
the rated current(In).
Instantaenous tripping current for D Curve is 10-20 times
the rated current(In).

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!.

the main thing is to selection of proper MCB which matches
with your equipments and instruments

for that i'd like to give you an case study...

let's Assume one Heating Control Action

suppose i am using a 1ph. heater of 1200 Watt (5.2 Amps)
now i am controlling heating action using an SSR or
relay/contactor

now if i will select SSR (relay/contactor)'s rating =6 amps
and i also impliment MCB of 6 Amps.b-curve

then in case of heater failer or implimenting different
rating's heater,

assume that circuit is taking some higher current say, 10
Amps in such my SSR(relay/contactor)will damage due to high
load current which is higher then their rating of 6 Amps

but MCB will not trip instently ( it trips after 5-10 min.
because load current is not more then 12 Amps)

in this case MCB will not protect your instruments.

now if i'll decrease MCB's rating it will starts tripping
on and on.....

if i'll select MCB more then 6 Amps again in this case MCB
will not trip

so that, if i m using b curve 6 Amps MCB then i must select
SSR(relay/contactor)'s rating more then 12 Amps say 15Amps.
then and then problem will be solve.

also if i will use MCB of C or D curve then Again i have to
choose instruments which has atleast 3-5 times higher
rating of MCB.

Basically B C D and K are the curves
Bis for lighting duty Very low thermal withstading
C is for motor duty Medium Thermal withstanding 6-10In
D is for the loads which is taking high in rsh Current
Ex:Capacitors,Computers and Electronic Equipments 10-20In
K is for Cable protection

The magnetic trip for a B-curve MCB will not operate below 5
times rated thermal current but will surely operate
instantaneously above 10 times rated thermal current.
The magnetic trip for a D-curve MCB is will not operate
below 10 times rated thermal current but will surely operate
instantaneously above 20 times rated thermal current.
The choice between B-curve and C-curve depends on the loads
ratio between load inrush current and load running steady
state current.
For the same running load current if the cable is long then
a D-type MCB may need a larger cable cross sectional area,
compared to a C-type MCB, just to ensure that sufficient
fault current always is available for operation of the
instantaneous magnetic trip.

Awesome thread with a lot of almost correct and mostly
correct answers.

The challenge is knowing to which standard a particular
breaker was manufactured.

AS/NZS 4898 specifies:
B Curve between 3 In and 5 In
C Curve between 5 In and 10 In
D Curve between 10 In and 14 In

This was an update to AS 3947.2 which specified:
B Curve between 3.2 In and 4.8 In
C Curve between 7 In and 10 In
D Curve between 10 In and 14 In

The application notes in #11 and #13 are pretty much spot on.

Now lets assume we're only using AS/NZS 4898 breakers.
Lets assume we're using a Clipsal C60 10A B curve breaker.

The breaker should operate electromagnetically (<0.2s)with
an inrush current greater than 30A (3 x 10A).
The same 10A breaker however with 15A of current will not
trip electromagnetically but will trip thermally between 30
and 600 seconds after load is applied.

A C curve breaker of the same type should not operate
electromagnetically at either 15A or 30A. It should operate
thermally between 30 and 1000 seconds after a 15A load is
applied and between 6 and 50 seconds after a 30A load is
applied.

Note the distinct difference between and instantaneous /
electromagnetic trip and a delayed / thermal trip.

B curve 4x In
c curve 7.5x In
d curve 10x In
c curve is used in most house switchbords. d curve is used where in rush currents are a problem ie motors that do not have soft starting.