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